The spectroscopy of alkali fluorides containing hexavalent uranium

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CONTAINING HEXAVALENT URANIUM

A thesis submitted for the

degree of

DOCTOR OF PHILOSOPHY

in the

Australian National University

by

BHAVANI SRINIVASAN

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STATE M E N T

The studies described in this thesis were carried out while I was a full-time research scholar at The Australian National University, Canberra A.C.T.

Except where mentioned in the text, the research described in this thesis is my own.

This thesis has never been submitted to another university or similar institution.

B. Srinivasan

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A C K N O W L E D G E M E N T S

I am indebted to my supervisors Professor W.A. Runciman and Dr. N.B. Manson for their meticulous supervision and support throughout the course of this work.

I am grateful to Dr. D.D. Richardson and Dr. S. Saboe for their assistance with the computer systems used in this work and

valuable discussions. I wish to extend my thanks to Dr. A. Edgar

for additional supervision and to Dr. E.A. Magnusson for helpful

discussions. I am grateful to Mr. G. Sampietro for preparation

of the crystals used in this study.

I wish to thank my colleague Dr. J. Pörsch for his help. The

assistance of the technical staff of the Department of Solid State

Physics is acknowledged. Acknowledgement is also due to Mrs. M.

Turner for typing the initial draft of this thesis.

I would like to express my gratitude to Miss S. Turpie for her able typing of this thesis.

I wish to thank the Australian National University for granting me a research scholarship during the course of this project.

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6+

The spectral features of LiF:U crystals are studied experimentally and theoretically. Absorption spectra in the visible-near UV region are measured at low temperatures for

6 + 6 +

various crystals of LiF:U and NaF:U differing in growth conditions and chemical compositions. The crystal preparation conditions under which different centres are substantially present and hence the restrictions placed on the physical nature of these centres are discussed. A simple Coulomb point charge calculation is made of the defect formation energies for the-various defect clusters expected in these crystals in order to identify the energetically favourable defect complexes.

A detailed study of the spectroscopic properties of the principal centres in LiF:U and NaF:U crystals grown in

oxidising atmospheres is made using polarized selective excitation spectroscopy. For some degenerate electronic level features these measurements are complemented by magnetic circular dichroism (MCD) measurements. Using these results energy level diagrams for the principal centres in LiF:U^+ and NaF:U^+ are derived.

Energy level calculations are made for a UO^-F complex believed to be the configuration of these principal centres

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The visible region absorption and the MCD spectra of some of the centres created by x-irradiation in LiF:U and NaF:Ü are

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e l e c t r o n s , h a s b e e n c a r r i e d o u t s u c c e s s f u l l y , t h e o p t i c a l s p e c t r a o f compounds c o n t a i n i n g a c t i n i d e i o n s , h a v i n g 5 f n e l e c t r o n i c c o n f i g u r a t i o n s , a r e n o t n e a r l y so w e l l u n d e r s t o o d . For t h e

a c t i n i d e i o n s i n c o r p o r a t e d as i s o l a t e d i o n s i n c r y s t a l s , t h e f r e e io n s p l i t t i n g s due t o i n t e r e l e c t r o n i c r e p u l s i o n and s p i n - o r b i t c o u p l i n g a r e f u r t h e r augmented by t h e e l e c t r o s t a t i c f i e l d

g e n e r a t e d by t h e p r e s e n c e o f o t h e r i o n s i n t h e en v iro n m e n t which d i s t o r t s t h e f r e e i o n w a v e f u n c t io n s and g iv e s r i s e t o l a r g e c r y s t a l f i e l d s p l i t t i n g s , w h e r e a s , f o r t h e l a n t h a n i d e s e r i e s t h e 4 f

e l e c t r o n s a r e v e r y h i g h l y l o c a l i z e d and h e n c e t h e l a n t h a n i d e io n s i n c r y s t a l s e x p e r i e n c e much s m a l l e r c r y s t a l f i e l d s p l i t t i n g s . Th u s, u n l i k e t h e s i t u a t i o n f o r l a n t h a n i d e s , t h e e n e r g i e s o f t h e

c r y s t a l f i e l d , i n t e r - e l e c t r o n r e p u l s i o n and s p i n - o r b i t i n t e r a c t i o n s a r e a l l co m p arab le f o r t h e a c t i n i d e s so t h a t o p t i c a l t r a n s i t i o n s a r e n o t o b s e r v e d as c l o s e l y s p a c e d l i n e groups w i t h e n e r g i e s m a in ly d e t e r m in e d by one ty p e o f i n t e r a c t i o n .

Most o f t h e a c t i n i d e s a r e h i g h l y r a d i o a c t i v e r e q u i r i n g s p e c i a l p r o c e d u r e s d u r i n g e x p e r i m e n t a l w ork. In a d d i t i o n t h e t r a n s u r a n i u m e le m e n ts i n t h e a c t i n i d e s e r i e s a r e n o t a v a i l a b l e i n l a r g e q u a n t i t i e s and t h e h a l f - l i v e s o f t h e s e e l e m e n t s may be v e r y

238

s h o r t . U i s one o f t h e few a c t i n i d e s r e a d i l y a v a i l a b l e and does n o t h av e t h e p ro b lem s a s s o c i a t e d w i t h r a d i o a c t i v i t y . The u ran iu m i o n i s known t o e x i s t i n e v e r y v a l e n c e s t a t e from 6+ t o 3+, 2+ h a s a l s o b e e n s u g g e s t e d . The most s t u d i e d a c t i n i d e i o n h as

4+ 2

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r e s o n a n c e r e s u l t s have b e e n p u b l i s h e d and s u c c e s s f u l c r y s t a l f i e l d c a l c u l a t i o n s o b t a i n e d . However, t h e r e i s a l a c k o f u n d e r s t a n d i n g o f t h e s p e c t r a o f t h e l/*+ i o n h a v i n g t h e c l o s e d s h e l l e l e c t r o n i c c o n f i g u r a t i o n o f r a d o n .

The u r a n y l i o n i s p r o b a b l y t h e b e s t known ch em ica l e n t i t y a s s o c i a t e d w i t h a U^+ i o n . A lth o u g h i t h as b een s t u d i e d e x t e n s i v e l y f o r a lo n g p e r i o d (D ieke and Duncan, 1949: R ab in o w itch and B e l f o r d , 1964: G o r l l e r - W a l r a n d and V an q u ick en b o rn e 1972a, 1972b Denning e t a l . , 1976: J o r g e n s e n , 1977) t h e s p e c t r o s c o p i c

b e h a v i o u r o f t h e u r a n y l i o n was n o t v e r y w e l l u n d e r s t o o d .

S i g n i f i c a n t p r o g r e s s h a s b e e n made by Denning and c o l l a b o r a t o r s (1976, 1979a, 1979b) i n e x p l a i n i n g t h e e x p e r i m e n t a l s p e c t r a o f t h e u r a n y l i o n i n t e t r a g o n a l and t r i g o n a l e q u a t o r i a l f i e l d s , by

com paring t h e e x p e r i m e n t a l d a t a w i t h an e m p i r i c a l e l e c t r o n i c c o n f i g u r a t i o n model where t h e e x c i t e d s t a t e s a r i s e from f o u r p o s s i b l e c o n f i g u r a t i o n s o f t h e i s o l a t e d i o n . I t was shown t h a t t h e d a t a i s e x p l i c a b l e i n te rm s o f t h e h i g h e s t e n e r g y o c c u p ie d o r b i t a l i n t h e ground s t a t e h a v i n g G symmetry w i t h t h e e x c i t e d s t a t e s d e r i v e d from o 6 and G (b s t a t e s where t h e d>

u u u u u

o r b i t a l l i e s above t h e 6^ o r b i t a l . S u b s c r i p t s g and u r e f e r t o s t a t e s w i t h even and odd p a r i t y r e s p e c t i v e l y . T h is model i s d i f f e r e n t from t h e e a r l y model o f McGlynn and Sm ith (1961) where t h e lo w e s t e x c i t e d s t a t e s w ere assumed t o be d e r i v e d from e i t h e r

3 3

Hg (j)^ o r 7Tg 6^ and t h e a n a l y s i s o f B r i n t and M cC afferty (1973) and J o r g e n s e n and R e i s f e l d (1973) who i d e n t i f i e d t h e e x c i t e d

3 3

s t a t e s as d e r i v e d from tt 6 o r tt <b s t a t e s , b u t a g r e e s on

u u u u

t h e q u e s t i o n o f t h e o r d e r i n g o f t h e h i g h e s t o c c u p i e d l e v e l s w ith t h e c o n c l u s i o n s o f Walch and E l l i s ( 1 9 7 6 ) . P r o g r e s s w i t h t h e

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6 +

complexes, for example, other U oxygen complexes, may be

attainable.

It has been claimed for a long time that LiF and NaF crystals doped with hexavalent uranium and grown in the presence of oxygen

give rise to some form of uranium oxygen complexes. There is

strong fluorescence associated with these complexes. In the

present work this emission and the associated absorption are used to try and establish the nature of the complex centres and to determine the electronic configurations involved.

Although some models for these centres h a v e ,been proposed there is still a lot of uncertainty about the nature of these

centres. In the present work various methods have been used in

an attempt to obtain more information to establish the nature of these centres.

The various different configurations expected in these

complexes are given in Chapter 2 and simple calculations of defect formation energies to find the energetically favourable config

urations are discussed. The spectral features of crystals grown

from different starting materials and under different atmospheric

conditions are reported in Chapter 3. The spectroscopic properties

of the principal centre are described in Chapter 4. The information

in this instance is not solely directed at establishing the nature of the centre, as using the experimental data only the symmetry

can be determined. This data, however, if taken together with

model calculations of possible electronic configurations can yield

information about the nature of the centres. In Chapter 5 attempts

have been made to model the electronic configuration of a UO^F complex which is considered to be the most probable arrangement of

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the calculations of Denning et a l . (1979b) on the uranyl ion. In a separate study attempts were made to change the nature

of the centres (perhaps the valence) by radiation damage. This

was undertaken as it was thought that it may be easier to try to establish the nature of the radiation damage centres and hence deduce the nature of the undamaged centres from which they are

formed. This work is reported in Chapter 6.

1,2

S U R V E Y O F S T U D I E S O N L i F :U 6+ A N D N a F : U 6+ D O M I N A N T

C E N T R E S

6 +

The study of the bright blue-green luminescence in LiF:U and green-yellow luminescence in NaF:U^+ has been the subject of a number of investigations (Slattery 1929: Runciman 1955, 1956:

Feofilov 1959, 1960: Kaplyanskii et a l . 1962, 1963, 1964, 1970:

Bleijenberg et a l . 1978, 1980). The low temperature luminescence

spectra consists of numerous lines and narrow bands of electronic and vibronic nature, showing variability in relative intensity in different samples depending on the conditions of their preparation. This suggests the presence of different centres containing U^*+ and

giving overlapping spectra. However, in all the samples at 77 K,

there is a pair of intense luminescent lines at 518.5 and 527.8nm for LiF:U^+ and at 552.8 and 563.6nm for NaF:U^+ (Kaplyanskii et

al., 1964), and associated electronic vibrational series. These

dominant line series in LiF:U^+ and NaF:l/*+ are similar and it is highly likely that the associated defect centres are perfectly

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E a r l y w o r k e r s who d e s c r i b e d t h e e m i s s i o n ( N ic ho ls and

S l a t t e r y , 1926: S l a t t e r y , 1929) c o n c l u d e d t h a t t h e u r a ni u m i s i n s o l i d s o l u t i o n w i t h t h e f l u o r i d e , h e n c e , a t t r i b u t i n g t h e

f l u o r e s c e n c e t o UF^ g r o u p s . However, t h i s a s s i g n m e n t c a n n o t

6+ 6 +

a c c o u n t s a t i s f a c t o r i l y f o r t h e c o l o u r o f NaF:U and LiF:U e m i s s i o n as s o l i d UF, h a s a b r i g h t v i o l e t f l u o r e s c e n c e (Runciman, 1955) .

Runciman (1955) was t h e f i r s t t o a s c r i b e t h e l u m i n e s c e n c e o f NaF:U^+ t o d i f f e r e n t u r a n i u m - o x y g e n c ompl exes . I n R unci man 's model t h e l u m i n e s c e n c e was a t t r i b u t e d t o UO^ c ompl exes , w i t h t h e e x c e s s n e g a t i v e c h a r g e o f t h i s d e f e c t complex b e i n g c h a r g e

c om pe ns a te d i n d i f f e r e n t ways t o p r o d u c e v a r i o u s d i f f e r e n t c e n t r e s . The main l u m i n e s c e n t c e n t r e was a s s i g n e d t o be a UCL c l u s t e r c h a r g e

c om pe ns a te d by an a ni o n v a c a n cy a l o n g < 1 1 1 > ( F i g u r e 1 . 1 a ) , s u g g e s t i n g t h e p r e s e n c e o f a t h r e e - f o l d a x i s (Runciman, 1 956 ). The p o s s i b i l i t y o f i so mor phous s u b s t i t u t i o n o f 0^ f o r F and U^+

f o r L i + o r Na+ i s a s s u r e d due t o t h e s i m i l a r i t y i n t h e r a d i i o f t h e 0^ i o n ( r = 1. 39 Ä ) and F i o n ( r = 1 . 3 3 Ä ) , and i n t h e r a d i i o f t h e U^+ i o n ( r = 0 . 8 0 Ä ) and t h e L i + i o n ( r = 0 . 6 8 Ä ) and t h e Na+ i o n ( r = 0 . 9 7 Ä ) .

I n v e s t i g a t i o n s o f t h e p o l a r i z e d l u m i n e s c e n c e by F e o f i l o v (1959, 1960, 1 9 61 ) , B e l y a ev e t a l . ( 1 9 6 1 ) , K a p l y a n s k i i e t a l . (1964) r e v e a l e d t h a t t h e main c e n t r e s , a s s o c i a t e d w i t h t h e l u m i n e s c e n t l i n e s m e n t i o n e d e a r l i e r , a r e a n i s o t r o p i c f o r m a t i o n s o r i e n t e d i n t h e c r y s t a l l a t t i c e a l o n g a f o u r f o l d symmetry a x i s w i t h t h e t r a n s i t i o n i n t h e l o n g and s h o r t w a v e l e n g t h f l u o r e s c e n t

l i n e s c o r r e s p o n d i n g t o m a g n e t i c and i n d u c e d e l e c t r i c d i p o l e r a d i a t i o n s r e s p e c t i v e l y . F e o f i l o v (1959) was t h e f i r s t t o

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(a) UO^ c l u s t e r w i t h n e g a t i v e i o n v a c a n cy a l o n g < 1 1 1 >

(b) UO^ c l u s t e r

FIGURE 1 . 1 Uraniurn-oxygen c l u s t e r s

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for these centres. Piezospectroscopic studies by Kaplyanskii and Moskvin (1962, 1963a) provided further evidence for these centres

having tetragonal symmetry. The existence of a linear pseudo-

Stark effect in the centres (Kaplyanskii et al. 1970) indicated

the absence of inversion symmetry. The splittings of these

centres in an external electric field is termed 'pseudo-Stark' splitting, because it is due not to a splitting of degenerate

levels of individual centres but only to a difference in the shifts of the levels of centres with different orientations with

respect to the external field. Based on group theoretical

arguments concerning the nature of the associated transitions Kaplyanskii et al. (1970) concluded these principal centres to

have symmetry and proposed a scheme of levels and transitions

in these centres. All these symmetry conditions are satisfied by

the model proposed by Feofilov.

These centres have been claimed to be uranyl-like (Figure 1.1c) even as recently as 1963 - 1966 (Kaplyanskii et al. 1962,

1963a: Risgin and Becker, 1966). To obtain an inversionless centre

with tetragonal symmetry one will have to assume some non-centrally

symmetric deformation of the linear ion. Moreover, the

emission spectra of the linear uranyl molecule, which is similar in its essential features for many salts, consist of a number of equally spaced bands due to the strong coupling of the totally symmetric mode with frequency 860 cm ^ and usually at least four

quanta of it are observed. One quanta of the antisymmetric mode

with frequency 920 cm ^ and one quanta of the bending mode with frequency 210 cm ^ are also present in the uranyl spectrum (Dieke

and Duncan, 1949: Rabinowitch and Belford, 1964). In the emission

6 + 6 +

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( f r e q u e n c y 800cm ^ f o r LiF:U ^+ and 708cm ^ f o r NaF:U^+) i s weak and h a s low i n t e n s i t y even f o r two phonon p r o c e s s e s . F u r th e r m o r e , no v i b r a t i o n a l f e a t u r e s e q u i v a l e n t t o t h e a n t i s y m m e t r i c and

b e n d in g modes o f t h e u r a n y l io n a r e p r e s e n t . Thus t h e c h a r a c t e r

-6 + 6 +

i s t i c s o f t h e LiF:U and NaF:U e m i s s i o n a r e t o t a l l y u n l i k e t h o s e a s s o c i a t e d w i t h u r a n y l c e n t r e s i n o t h e r m a t e r i a l s and i f t h i s i s t a k e n t o g e t h e r w i t h t h e symmetry c o n s i d e r a t i o n s , i t can be s e e n t h a t i t i s d i s t i n c t l y u n l i k e l y t h a t t h e LiF:U ^+ and NaF:U^+ e m is s i o n a r e due t o u r a n y l c e n t r e s .

K a p l y a n s k i i ’ s model w here t h e t r a n s i t i o n s a s s o c i a t e d w ith t h e p r i n c i p a l c e n t r e were a s s i g n e d t o e l e c t r i c and m a g n e tic d i p o l e t r a n s i t i o n s from two l e v e l s i n t h e e x c i t e d s t a t e , was c r i t i c i z e d by P ant e t a l . (1969) b a s e d on t h e i r o b s e r v a t i o n s o f some

lu m in e s c e n c e b ands o f t h e e l e c t r i c d i p o l e s e r i e s even a t 4 K and t h e l u m in e s c e n t l e v e l s h a v i n g two d eca y tim e s a t 80 K. T hese arg u m e n ts a r e b e l i e v e d t o be i n v a l i d as t h e lu m in e s c e n c e b an d s found a t 4 K by P ant e t a l . do n o t b e l o n g t o t h e e l e c t r i c d i p o l e s e r i e s o f t h e p r i n c i p a l c e n t r e b u t t o some o t h e r d i f f e r e n t c e n t r e s a c c i d e n t a l l y p r e s e n t i n t h e same r e g i o n (Runciman, 1 9 5 9 ). The m easurem ents o f Runciman and Wong (1979) p r e d i c t a s i n g l e decay ti m e f o r t h e lu m in e s c e n c e l e v e l s o f t h e p r i n c i p a l c e n t r e . The

2

-model s u g g e s t e d by P an t e t a l . , o f a UO^ i o n f o r t h e p r i n c i p a l c e n t r e , can be d i s c a r d e d as i t c o n t r a d i c t s t h e a b s e n c e o f i n v e r s i o n symmetry found from t h e e l e c t r i c f i e l d s p l i t t i n g s by K a p l y a n s k i i e t a l . ( 1 9 7 0 ) .

S in c e t h e U^+ i o n i s d i a m a g n e t i c , t o u s e e l e c t r o n p a r a m a g n e t i c r e s o n a n c e (EPR), i t h a s t o be c o n v e r t e d i n t o p a r a m a g n e t i c \ f > + s t a t e by x- o r y - i r r a d i a t i o n . The s t r u c t u r e o f t h e s e lT*+ c e n t r e s

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structure of the parent U^+ centres.

Previous electron paramagnetic resonance experiments of Lupei and coworkers (1974, 1976a, 1976b, 1977, 1979) on irradiated LiF:U and NaF:U crystals revealed in each case a prevailing centre with tetragonal symmetry having low g-factors and showing superhyperfine

splitting due to a fluorine nucleus. The hyperfine structure of

235

this centre in samples with U proved this centre to be associated

with uranium, which has to be in the form of IT*+ to give the

required paramagnetic centre. This result is significant as it

indicates the presence of a F in the first co-ordination sphere

of the uranium complex. Lupei et a l . (1976b) proposed a model,

6 + 2 —

where this centre is created from a U centre with five 0 and

one F in the first co-ordination sphere upon irradiation, by the

l/*+ converting into lf’ + by trapping one electron supplied by the

2_

0 ion situated along the fourfold axis. This explanation for

the origin of the electron is not very satisfactory as in view of

the strong interaction of the U^+ and 0^ orbitals (Chapter 5)

2

-transfer of an electron from an 0 ion in the first co-ordination

sphere to the U^+ ion would not be expected to give a paramagnetic

centre as observed. The electron needed to form U^+ is much more

likely to be supplied by distant ions in the lattice.

Parrot and coworkers (1977) have also studied irradiated

LiF:U crystals. They studied the optical spectra of irradiated

LiF:U, determining the energy levels for two of the centres

created by irradiation. From optical and EPR studies of the

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Bagai and Warrier (1977) obtained the absorption spectrum of a uranium doped LiF crystal. In the near UV region this is less well resolved than the spectra reported in Chapter 4 but they have extended the spectrum to higher energy regions. However, their analysis is in terms of the linear uranyl complex and this is believed to be invalid.

Using luminescence and electrical conductivity measurements, Bleijenberg and Timmermans (1978) attributed the main luminescent

6+

centre in NaF:U to be a UO^ complex associated with a fluorine ion vacancy situated at a site along a tetragonal axis of the UO,

<■ 6

group (Figure l.ld). This model is fully consistent with all symmetry considerations.

Aleshkevich et a l . (1980) calculated the oscillator strengths

6+

of some experimental absorption bands in LiF:U , assigning a -4

value of 2.8 x 10 for the oscillator strength of the band at 522nm. The relation between oscillator strength f and decay rate constant t is given by (Birnbaum, 1964)

, ,2

m u g A

fx = — 2 1 — ■ 0 . 1 )

8tt e g1

where §2^1 are uPPer anc* lower state degeneracies, m, e

are the electronic mass and charge, t) is the velocity of light in the medium and A is the wavelength associated with the transition. The decay rate constant given by Runciman and Wong (1979) for the main centre in LiF:U for the 518.5nm transition is 123.4ys . This should be corrected to 123.4ms ^ . Using this corrected value together with equation 1.1 yields an oscillator strength of 3.7 x

-4

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a s s ig n m e n t o f t h e e l e c t r o n i c t r a n s i t i o n and v i b r a t i o n a l f e a t u r e s by A le s h k e v ic h e t a l . (1980) a r e n o t c o n c l u s i v e as t h e a b s o r p t i o n ban d s g iv e n i n t h i s work need n o t a l l b e l o n g t o . t h e same c e n t r e

and t h e f e a t u r e s a s s i g n e d t o be v i b r a t i o n a l components a r e n o t w e l l r e s o l v e d . R e s u l t s f o r p a r t o f t h e s p e c tr u m r e p o r t e d i n C h a p t e r 4 show t h a t t h e bands i n t h e UV r e g i o n c o n t a i n e l e c t r o n i c l e v e l s t o g e t h e r w ith v i b r a t i o n a l f e a t u r e s and n o t a s im p le

v i b r a t i o n a l s e r i e s as a s s i g n e d by A le s h k e v ic h e t a l .

By u s i n g t h e v a r i o u s f i x e d f r e q u e n c y e x c i t a t i o n s a v a i l a b l e from an arg o n io n l a s e r , Runciman and Wong (1979) were a b l e t o show some f e a t u r e s o f t h e d i f f e r e n t c e n t r e s b e i n g e x c i t e d . T h is t y p e o f t e c h n i q u e i s s i g n i f i c a n t l y im proved i f a t u n a b l e dye l a s e r can be u s e d . T h is was done by B l e i j e n b e r g and B r e d d e l s (1980) who u sed a t u n a b l e dye l a s e r t o e x c i t e s e l e c t i v e l y t h e d i f f e r e n t

c e n t r e s i n NaF:l/*+ . They s e p a r a t e d t h e s p e c t r a l f e a t u r e s o f t h e p r i n c i p a l c e n t r e i n NaF:U^+ i n e m is s i o n and e x c i t a t i o n s p e c t r a and i n t e r p r e t e d them i n te rm s o f a UO^ v aca n c y model d e p i c t e d i n

F ig u r e l . l d . They a s s i g n e d t h e zerophonon l i n e s a t 5 6 3 . 6nm and 5 5 2 . 8nm i n NaF:U t o A^ ■ + A^ and E -* A^ t r a n s i t i o n s r e s p e c t i v e l y . B l e i j e n b e r g and B r e d d e l s , a l t h o u g h u s i n g s e l e c t i v e

e x c i t a t i o n , h av e n o t c o n s i d e r e d t h e p o l a r i z a t i o n o f t h e e x c i t i n g and e m i t t e d l i g h t . Hence d e t a i l e d i n f o r m a t i o n i s l o s t and

c o n s e q u e n t l y t h e y make wrong a s s i g n m e n t s , t h e most n o t a b l e o f which i s t h e 5 5 2 . 8nm l i n e t o a E -* A^ t r a n s i t i o n . The m e a s u re ments d e s c r i b e d i n C h a p t e r 4 and a l r e a d y p u b l i s h e d i n o u t l i n e

(Runciman e t a l . , 1981) h av e shown t h a t no E d o u b l e t i s in v o l v e d i n t h e 5 5 2 . 8nm t r a n s i t i o n and g iv e s u p p o r t t o t h e a s s ig n m e n t o f K a p ly a n s k ii e t a l . (1970) o f t h e 5 5 2 . 8nm l i n e t o a A^ -*■ A^

(22)

b u t i n an e n t i r e l y d i f f e r e n t e n e r g y p o s i t i o n .

D e s p i t e t h e e x t e n s i v e work on t h i s s u b j e c t , t h e e x a c t

p h y s i c a l n a t u r e o f t h e s e c e n t r e s was n o t c l e a r . . The symmetry a s s o c i a t e d w i t h t h e s e c e n t r e s i s p o s s e s s e d by b o t h t h e UO^F

( F i g u r e 1 .1 b ) and t h e UO^ v aca n cy ( F i g u r e 1 . Id) m o d els. The r e m o te n e s s o f t h e v aca n cy from t h e c e n t r e o f t h e complex i n t h e l a t t e r model would make i t d i f f i c u l t t o a c c o u n t f o r t h e

c o n s i d e r a b l e u n i a x i a l s t r e s s and e l e c t r i c f i e l d s p l i t t i n g s which h a s b een o b s e r v e d . The EPR work o f L upei e t a l . s u g g e s t s t h e

e x i s t e n c e o f UO,_F c e n t r e s i n u n i r r a d i a t e d c r y s t a l s . Hence a UO,_F model f o r t h e p r i n c i p a l c e n t r e i n L iF :U ^+ and NaF:U^+ c r y s t a l s grown i n o x i d i s i n g atm o s p h e re seems t o be more p l a u s i b l e .

The combined r e s u l t s o f EPR and lu m in e s c e n c e m easurem ents on s i n g l e c r y s t a l s grown u n d e r v a r i o u s c o n d i t i o n s , v e r y r e c e n t l y , o f Lupei e t a l . (1982) g iv e f u l l s u p p o r t f o r t h e n a t u r e o f t h e main h e x a v a l e n t u ra n iu m c e n t r e i n NaF b e i n g a HO^F com plex.

I t i s s t i l l n o t c l e a r what a r e t h e e l e c t r o n i c t r a n s i t i o n s r e s p o n s i b l e f o r t h e lu m in e s c e n c e and a b s o r p t i o n f e a t u r e s o f t h e s e c e n t r e s . P a r r o t e t a l . (1981) a r e t h e o n ly a u t h o r s t o t a c k l e t h i s q u e s t i o n on t h e b a s i s o f a UO^ c l u s t e r . They a n a l y z e d t h e

p o l a r i z e d e m i s s i o n and e x c i t a t i o n s p e c t r a o f t h e L i F : l / > + main c e n t r e and p ro p o s e d an e l e c t r o n i c c o n f i g u r a t i o n model a s s o c i a t i n g t h e l i n e s i n t h e v i s i b l e r e g i o n t o t r a n s i t i o n s b etw een t h e

fu n d a m e n ta l s t a t e 6p^ and t h e s t a t e s o f t h e e x c i t e d c o n f i g u r a t i o n 6p^ 7 s . One would e x p e c t t h e s e t o be a llo w e d t r a n s i t i o n s and h en ce t h e s m a l l v a l u e f o r t h e o s c i l l a t o r s t r e n g t h o f t h e s e

t r a n s i t i o n s (m e n tio n ed i n a p r e v i o u s s e c t i o n ) i s i n c o n t r a d i c t i o n w i t h t h i s model. P a r r o t ' s model would s u g g e s t s i m i l a r i n t e n s i t i e s

(23)

r i (6p6 , l S ) r i ( J = l ) and r ^ ( 6 p ^ , l S ) -> r5( J =l ) r e s p e c t i v e l y b u t t h e 5 0 3 . 3nm l i n e i s v e r y much w eak er t h a n t h e 5 1 8 . 5nm l i n e

( C h a p te r 4 ) . M oreo v er, P a r r o t ' s model would p r e d i c t a g - v a l u e o f g r e a t e r t h a n u n i t y and l e s s t h a n 1 .5 f o r t h e s t a t e w ith J = 1 , i n c o n t r a d i c t i o n t o t h e o b s e r v e d g - v a l u e o f - 0 . 0 8 . I t i s much more l i k e l y t h a t t h e p h y s i c a l s i t u a t i o n i s a n a lo g o u s t o t h a t i n t h e u r a n y l complex (Denning e t a l . , 1979b) where t h e e x c i t e d s t a t e c o n f i g u r a t i o n s a r e due t o a e l e c t r o n t r a n s f e r from t h e oxygen o r b i t a l t o t h e empty u ra n iu m 5 f o r b i t a l .

1 , 3

SURVEY OF S T U D I E S ON L i F : U 6 + AND N a F : U 6 + SECONDARY

CENTRES

A ll o f t h e above m e n tio n e d d i s c u s s i o n i s d i r e c t e d a t t h e dom inant c e n t r e i n L iF :U ^+ and NaF:U^+ . Most w o rk e rs r e p o r t e m is s i o n from v a r i o u s c e n t r e s i n t h e s e c r y s t a l s . The r e l a t i v e i n t e n s i t y o f t h e l i n e s a s s o c i a t e d w i t h t h e s e c e n t r e s depend t o a g r e a t e x t e n t upon sam ple p r e p a r a t i o n c o n d i t i o n s and h en ce t h e s p e c t r a r e p o r t e d can d i f f e r from a u t h o r t o a u t h o r (B e ly a e v e t a l . 1959, 1960, 1961: B l e i j e n b e r g and Timmermans, 1978: Runciman and Wong, 1979: Lupei e t a l . , 198 2 ).

The symmetry o f some o f t h e s e a d d i t i o n a l c e n t r e s h av e b een e s t a b l i s h e d by K a p l y a n s k i i e t a l . (19 6 2 , 1963a, 1963b, 1970) u s i n g t h e u n i a x i a l s t r e s s and e l e c t r i c f i e l d s p l i t t i n g s o f t h e a s s o c i a t e d s p e c t r a l l i n e s . The s p e c t r o s c o p i c d e t a i l s o f t h e s e c e n t r e s can be e s t a b l i s h e d u s i n g s e l e c t i v e e x c i t a t i o n t e c h n i q u e s .

(24)

centres. The variation in luminescence from sample to sample can also be used to establish the spectra associated with separate

centres. Using this method together with electrical conductivity

measurements Bleijenberg and Timmermans (1978) suggested assignments

6 +

of models for some of these additional centres in NaF:U . These

assignments are in question and will be discussed in Chapter 3 where the centres observed in different crystals in the current work are reported.

Lupei et a l . (1979, 1982) have obtained several new U^+

centres in x or y irradiated LiF:U and NaF:U ,crystals and from the analysis of the EPR spectra proposed tentative models of charge

compensation. An unambiguous connection of these U^+ centres with

6+

the U centres in unirradiated crystals requires good isolation

of one U^+ centre in a given sample. It can be seen from the data

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CALCULATION IN L i F ( U . O ) •

2.1 INTRODUCTION

2.2 THE METHOD OF CALCULATING THE DEFECT ENERGY

2.3 NON-COULOMB POTENTIAL

2.4 COULOMB CONTRIBUTIONS

2.5 LATTICE RELAXATION

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2 , * 1 I N T R O D U C T I O N

T h i s c h a p t e r d e s c r i b e s a d e f e c t f o r m a t i o n e n e r g y c a l c u l a t i o n f o r v a r i o u s p o s s i b l e models o f h e x a v a l e n t u r a ni u m c e n t r e s i n l i t h i u m f l u o r i d e . T h i s c a l c u l a t i o n was b a s e d on t h e c omput er p ro gr am o f R i c h a r d s o n (1982) which i s i n FORTRAN and c a l c u l a t e s t h e f o r m a t i o n e n e r g i e s o f v a c a n c i e s and i n t e r s t i t i a l s i n c u b i c i o n i c c r y s t a l s . The o r i g i n a l p r og r am o f R i c h a r d s o n i mpl ement s l a t t i c e r e l a x a t i o n e f f e c t s , Coulomb p o t e n t i a l and s h o r t r a n g e non- Coulomb p o t e n t i a l e f f e c t s . For r e a s o n s e x p l a i n e d l a t e r , t h e

progr am was s i m p l i f i e d t o c a l c u l a t e f o r m a t i o n e n e r g i e s o f v a c a n c i e s and i n t e r s t i t i a l s i n a r i g i d i o n l a t t i c e c o n s i d e r i n g o n l y t h e Coulomb i n t e r a c t i o n b e t w e e n t h e i o n s and n e g l e c t i n g r e l a x a t i o n e f f e c t s .

2 , 2 T H E M E T H O D O F C A L C U L A T I N G T H E D E F E C T E N E R G Y

The d e f e c t f o r m a t i o n e n e r g y i s t h e change i n e n e r g y b et we e n t h e b i n d i n g e n e r g i e s o f t h e d e f e c t c r y s t a l and t h e i d e a l c r y s t a l . T h e r e f o r e , i n g e n e r a l we r e q u i r e t o c a l c u l a t e t h e e n e r g y due t o

( i ) t h e p r e s e n c e o f t h e d e f e c t ( ch a n g e s i n i n t e r a c t i o n s w i t h n e i g h b o u r s )

( i i ) t h e l a t t i c e r e l a x a t i o n s a r o u n d t h e d e f e c t and

( i i i ) t h e changes i n e l e c t r o n i c s t r u c t u r e due t o t h e d e f e c t ( s c r e e n i n g and e l e c t r o n r e l a x a t i o n ) .

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In an ionic system like LiF we will have two interaction

potentials, the Coulomb potential and the non-Coulomb short-range

potential, contributing to (1).

2,3

NON-COULOMB POTENTIAL

To include non-Coulomb potentials in our calculation we

require anion-anion, cation-anion and cation-cation potentials for

the ideal host lattice ions and interstitial-host anion,

interstitial-host cation potentials and as there are more than one

interstitial in the defect complexes considered (figures 2.1 - 2.8),

then all the interactions between interstitials have to be

described as well.

It is a difficult task to find suitable non-Coulomb potentials

and there is no absolute validity of these. This problem has been

discussed for alkali halides by Catlow et al. (1977). As an

example even in the simplest case of a UO^ model, we require

suitable potentials to describe the U-0, U-F, 0-F, 0-0 and F-F

non-Coulomb interactions. A simple approximation would be to

treat the oxygen and fluorine ions as equivalent and to use the

potentials available for UO^ (Catlow, 1977a). However, these UO^

potentials are appropriate for a fluorite structure whereas our

defect complexes have a rocksalt structure, hence, adopting these

potentials will not give very reliable results.

Moreover, the present calculation was mainly aimed at finding

the plausible configurations for a given number of specified defect

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physical nature of the defect centres in LiF(U,0). In all cases there are approximately the same number of nearest neighbour interactions and these dominate the non-Coulombic interactions. Hence as the interest was only in the differences in energy between configurations having identical constituent ions and

vacancies, the non-Coulomb potential was not included in the present work.

2,4 COULOMB CONTRIBUTIONS

The Coulomb potential between ions i and j is described by

q . q .

* (r..) = , (2.4.1)

J ij

for ion charges q^ , q^ and

r . . = I r . - r . I , (2.4.2)

where r^ represents an ion position in the crystal. The present work considers a cluster of discrete ions in a spherical region with the presence of the defect at the centre of this region, S . The Coulomb energy of ions in the spherical region S is given by the lattice sums

Ec S

i,jes r ij i<3

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The Coulomb p o t e n t i a l c o n t r i b u t i o n t o t h e d e f e c t f o r m a t i o n e n e r g y w i l l be g i v e n by t h e d i f f e r e n c e i n t h i s sum f o r t h e d e f e c t c r y s t a l and t h e i d e a l c r y s t a l . T h i s d i f f e r e n c e , c o u l d be s e p a r a t e d i n t o two p a r t s as f o l l o w s .

(a) S i n g l e d e f e c t c o n t r i b u t i o n , which i s t h e change i n Coulomb sum when a d e f e c t i s i n t r o d u c e d i n t h e i d e a l l a t t i c e and i s g iv e n by

Ec

j e s r i j

**i*j**

( 2 . 4 . 4 )

N e g l e c t i n g l a t t i c e r e l a x a t i o n e f f e c t s , e q u a t i o n ( 2 . 4 ) can be r e w r i t t e n as

Ec

_(q^-q^

( 2 . 4 . 5 )

where q!^ , q^ a r e t h e i o n c h a r g e s a t i o n s i t e i f o r t h e d e f e c t and i d e a l c r y s t a l s r e s p e c t i v e l y , i s t h e Madelung c o n s t a n t and a i s t h e l a t t i c e p a r a m e t e r .

I f i s t h e r e g i o n d e f i n e d by t h e d e f e c t complex, t h e

t o t a l s i n g l e d e f e c t c o n t r i b u t i o n f o r an u n r e l a x e d l a t t i c e i s g i v e n by

4 K-V

* T

16S1

( 2 . 4 . 6 )

where i i s a sum o v e r t h e d e f e c t i o n s o n l y .

(30)

Ec

2

l

i>jG S 1

i£j

cq±"qi.)

r. . iJ

l

i,ies

1 i/j

(qj-q^

(2.4.7)

where i and j refer to only the defect ions. The first term

in equation (2.4.7) is the defect-defect contribution for the

defect lattice. The second term removes the redundant term

already calculated in the Madelung sum.

The total defect formation energy will be a sum of the two

contributions

Ec defect

„c „c

E1 + E2 (2.4.8)

2.5 LATTICE RELAXATION

It should be noted that equations (2.4.5) and (2.4.6) hold

true only if relaxation of ions about the lattice sites is

ignored. If relaxation effects, which would violate translational

invariance, were to be included it would require calculating the

v q i

sum 2. — ~ equation (2.4.4) explicitly and using it in equation

Ti;i “m

(2.4.6) to replace — . This can be done using Ewald's method

(Nijboer and de Wette, 1957: Richardson, 1982). To obtain a good

convergence of the Coulomb sum for large defect clusters, as is

the case in the present study a large sphere of ions around the

defect cluster (about 600 ions in this case) will have to be

considered.

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e f f e c t s w ere im p lem en ted , would n eed t o s t o r e th e seco n d d e r i v a t i v e s o f t h e e n e r g y w i t h r e s p e c t t o d i s p l a c e m e n t s ' as a m a t r i x o f s i z e 3N (3N +l)/2 f o r N io n s c o n s i d e r e d i n t h e s p h e r e . For N ~ 600 , as i n t h i s c a s e , t h i s would have r e q u i r e d a v e r y l a r g e co m p u ter s t o r a g e w hich was n o t f e a s i b l e . T h is s t o r a g e can be r e d u c e d by m i n im i s i n g t h e e n e r g y u s i n g lo w er d e r i v a t i e s , b u t a t t h e c o s t o f g r e a t l y i n c r e a s i n g t h e com puting ti m e r e q u i r e d .

A lso non-Coulomb p o t e n t i a l s h a v e a l a r g e e f f e c t on t h e r e l a x a t i o n o f t h e io n s and r e a l i s t i c r e l a x a t i o n c a l c u l a t i o n s a r e n o t f e a s i b l e i f non-Coulomb p o t e n t i a l s a r e n o t i n c l u d e d . Hence r e l a x a t i o n e f f e c t s w ere n o t i n c l u d e d i n t h e p r e s e n t c a l c u l a t i o n . T h e r e f o r e , f u l l l a t t i c e sums were n o t s t r i c t l y r e q u i r e d as o n ly sums o v e r d e f e c t io n s and v aca n cy s i t e s as p e r e q u a t i o n ( 2 . 4 . 6 ) and ( 2 . 4 . 7 ) were n eed ed t o g e t h e r w i t h a l r e a d y a v a i l a b l e Madelung c o n s t a n t s . The Madelung c o n s t a n t was c a l c u l a t e d u s i n g t h e

Ew ald’ s method a l r e a d y im p lem en ted i n t h e program b u t t h i s i s n o t c r u c i a l t o t h e r e s u l t .

2 . 6 RESULTS AND DISCUSSION

F i g u r e s 2 .1 - 2 . 8 show t h e p o s s i b l e c o n f i g u r a t i o n s c o n s i d e r e d i n t h i s s t u d y and g i v e t h e c a l c u l a t e d d e f e c t f o r m a t i o n e n e r g i e s o f t h e s e i n a LiF c r y s t a l . The main c e n t r e i n LiF(U) i s b e l i e v e d t o be a UO^ complex ( C h a p t e r 1) and i s p r e s e n t i n a l l t h e c r y s t a l s

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(a) U05 + OM = -292.45 eV

-301.82 eV -3Q1.03 eV

CONCLUSION - Clusters (b) and (c) are energetically favourable

FIGURE 2.1 UOgM complexes. M is a divalent ion.

(a) U05 + o - L i - □ = -263.35 eV

CONCLUSION - Clusters (c) and (d) are energetically favourable

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(a) UO5 + UO5 = -513.65 eV

CONCLUSION - Clusters Co), (c) and (d) are energetically favourable

FIGURE 2 . 3 _^

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(a) UO5 + 0 — R — 0 = - 3 3 5 . 9 2 eV

(b) U05 + 0 — R = - 3 3 ^ . 4 2 eV

I

0

- 3 4 3 . 6 7 eV

o

R ---O

- 3 4 2 . 8 8 eV

- 3 4 1 . 2 8 eV

FIGURE 2 , 4 UOyR complexes

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o

-332.78 eV

CONCLUSION - C l u s t e r s ( c ) , ( d ) , ( e ) , ( f ) , ( g) a n d ( h) a r e e n e r e e t i c a l l v f a v o u r a b l e

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(a)

UO

5

+ TO

3

- ~337,29 eV

-399.81 eV

-599.10 eV

O

-392.« eV

CONCLUSION - All clusters (b) - (O' are energetically favourable

FI GURE 2 , 5 UOgT com plexes

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(a) _{-2 4 1 . 7 3 eV}

- 2 4 4 . 9 4 eV _{-2 4 3 . 4 5 eV}

- 2 4 2 . 5 5 eV

a

- 2 4 3 . 3 7 eV

U05nLiüp complexes

(38)

- 2 4 2 . 5 6 eV

a

o

D u

F

- 2 4 2 . 2 7 eV

- 2 4 2 . 0 0 eV

U0^ü|jüp complexes (c o n t.)

(39)

□u

-233.13 eV

COftCLUSION - C l u s t e r s ( b ) - ( j ) a r e e n e r g e t i c a l l y f a v o u r a b l e

U0,£}.Dp complexes (cont.)

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( a ) UO5

+[ p Li- a 3 - [ü f-öj

-238.10 eV

-223.88 eV

-223.20 eV

U04nLi complexes

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-218.37 eV

CGiiCLUSlOIl - A l l c l u s t e r s { b ) - (k) a r e n o t e n e r g e t i c a l l y f a v o u r a b l e

(42)

(e)

o---- □

- 1 8 2 . 9 8 eV

•187.16 eV

(f)

o

u----o---- □

Ö

-181.80 eV

Li

( O

□u

- F —

-179.70 eV

u— o— a.

o -171.61 eV

(43)

( i )

O

C O N C L U S I O N

--178.97 eV

A l l c l u s t e r s ( b) - ( k ) a r e n o t e n e r g e t i c a l l y f a v o u r a b l e

U0,n. complexes ( c o n t . )

(44)

as b a s e d on a UO^ complex and some o t h e r d e f e c t s . T h e r e f o r e , t h e c o n f i g u r a t i o n s which h av e lo w er e n e rg y t h a n t h e sum o f t h e e n e r g i e s o f t h e components n e c e s s a r y t o form t h e s e complexes from a UO,. complex ( t h e s e a r e a l s o g iv e n i n t h e f i g u r e s ) would be p l a u s i b l e .

I t i s h i g h l y l i k e l y t h a t t h e v a r i o u s c e n t r e s i n LiF(U) and NaF(U) a r e u ran iu m oxygen complexes o f d i f f e r e n t s y m m e tr ie s ,

t o g e t h e r w i t h o t h e r d e f e c t o r d e f e c t s t o g iv e c h a r g e c o m p e n s a tio n . The p a r t i c u l a r c l u s t e r s ch o sen f o r t h i s s t u d y a r e t h e ones which

a r e more l i k e l y t o be p r e s e n t . F o r exam p le, p r e s e n c e o f Mg^+ i n t h e m e lt o f LiF(U) en h an c es t h e r e l a t i v e i n t e n s i t y o f a c e n t r e w ith a b s o r p t i o n a t 4 8 2 . 8nm ( C h a p t e r 3 ) . T h e r e f o r e i t i s d e s i r a b l e t o compare t h e f o r m a t i o n e n e r g i e s o f d i f f e r e n t UO^M (M i s a

d i v a l e n t io n ) , and UO ^111^411^ com plexes as Mg^+ can e i t h e r be d i r e c t l y i n v o l v e d i n t h e d e f e c t c e n t r e o r can p ro d u c e p o s i t i v e io n v a c a n c i e s t o c r e a t e t h e c e n t r e .

A d i s t i n c t f e a t u r e i n t h e s e r e s u l t s i s t h e s m a l l e n e rg y d i f f e r e n c e b etw een t h e c o n f i g u r a t i o n s o f s i m i l a r com plexes compared t o t h e a b s o l u t e m a g n itu d e s o f t h e Coulomb d e f e c t f o r m a t i o n e n e r g i e s . But a lth o u g h t h e s e a r e o n ly a s m a ll

p e r c e n t a g e o f t h e a b s o l u t e e n e r g i e s , t h e e n e rg y d i f f e r e n c e s i n t h e o r d e r o f 1-10 eV a r e s i g n i f i c a n t as t h e y a r e c o m p a ra b le t o t h e c o h e s i v e e n e r g i e s o f LiF and NaF w hich a r e 1 0 .7 eV and 9 . 5 eV r e s p e c t i v e l y (Hardy and K aro, 1 9 7 9 ). I t s h o u ld be p o i n t e d o u t t h a t t h e d e f e c t f o r m a t io n e n e r g i e s o f t h e s e com plexes i n a NaF h o s t l a t t i c e ( p o s s i b l e c o n f i g u r a t i o n s f o r NaF(U) c e n t r e s d e s c r i b e d

(45)

The e x c l u s i o n o f t h e s h o r t r a n g e c o n t r i b u t i o n t o t h e en e rg y i s n o t e x p e c t e d t o a f f e c t t h e r e s u l t s , as i t i s e s s e n t i a l l y a n e a r e s t n e i g h b o u r i n t e r a c t i o n . Most o f t h e d e f e c t s i n t h e c o n f i g u r a t i o n s i n each s e t h av e s i m i l a r n e i g h b o u r s i n t h e f i r s t

2

-c o o r d i n a t i o n s p h e r e w i t h t h e i n t e r -c h a n g i n g o f F and 0 i n some c a s e s . The s i m i l a r i t y b etw ee n t h e e l e c t r o n i c c o n f i g u r a t i o n and

2

-t h e r a d i i o f F and 0 i o n s i n d i c a t e s s i m i l a r p o t e n t i a l s f o r t h e two i o n s (C a tlo w , 1 9 7 7 b ).

For example l e t us c o n s i d e r t h e two d i f f e r e n t UO^M co m p lex es. T hese a r e shown a g a i n i n F ig u r e 2 .9 i n g r e a t e r d e t a i l . The u ran iu m

i o n and ^ 0 , ^ 0 and ^ 0 i o n s h av e t h e same n e a r e s t n e i g h b o u r i n t e r a c t i o n s i n b o t h c a s e s . One o f t h e M2 + - F i n t e r a c t i o n i n F ig u r e 2 . 9 a i s changed t o a M2* - 0 2 i n t e r a c t i o n i n F ig u r e 2 . 9 b , and one o f t h e L i + - 0 2 i n t e r a c t i o n i n F i g u r e 2 . 9 a i s changed t o a L i+ - F i n t e r a c t i o n i n F i g u r e 2 .9 b b u t assum ing s i m i l a r

2

-p o t e n t i a l s f o r F and 0 r e s u l t s i n t h e s e i n t e r a c t i o n s b e i n g e q u i v a l e n t . The ^ O 2" - M2 + , ^ O 2' - L i + , ^ O 2" - L i + i n t e r a c t i o n s i n F ig u r e 2 . 9 a a r e r e p l a c e d by ^ 0 2 - Li , ^2^ 0 2 - M2 ,

131 2- 2+

K J0 - M i n t e r a c t i o n s i n F i g u r e 2 . 9 b . H ence, t o a f i r s t

a p p r o x im a ti o n t h e o n ly n e a r e s t n e i g h b o u r i n t e r a c t i o n change b e tw ee n 2- +

t h e two UO^M com plexes i s t h e r e p l a c e m e n t o f a 0 - Li i n t e r -2- 2+

a c t i o n by a 0 - M i n t e r a c t i o n .

I n c l u d i n g t h e e f f e c t s o f l a t t i c e r e l a x a t i o n would make a s i g n i f i c a n t r e d u c t i o n i n t h e m a g n itu d e o f t h e d e f e c t f o r m a t io n e n e r g y . For ex am p le, t h e m a g n itu d e s o f t h e u n r e l a x e d and r e l a x e d e n e r g i e s i n NaCl a r e 7 .2 eV and 4 .7 eV f o r a c a t i o n v aca n cy and 7 . 3 eV and 5 .1 eV f o r an a n io n v aca n cy r e s p e c t i v e l y ( R ic h a r d s o n , p r i v a t e c o m m u n ic a tio n ) . As i n t h e c a s e o f t h e s h o r t r a n g e

(46)

(47)

affect the magnitudes of the formation energies substantially one would not expect it to change the energy differences to-a great

extent, since nearest (and possibly next nearest) neighbour ions have the most significant relaxations. This could be verified

only by implementing lattice relaxation effects'in the calculation. The present problem with the large storage space required could be reduced by using the point group symmetry about the defect to block diagonalise the required matrix (Richardson, 1982), but this has not been attempted in the present work.

Although this calculation is only an approximation of the real situation, due to the limitations mentioned above, it provides a basis for predicting the physical nature of possible centres. For example, this indicates that formation of UO .CL . or U O X l .

r 4 Li 3 Li Li

complexes are energetically unfavourable in LiF and NaF, compared with a UO^ centre (Figures 2.7 and 2.8). Which of the two

(48)

(example F i g u r e 2 . 9 a ) w i l l be l ow er i n e n e r g y t h a n a more c l o s e l y p ac ke d c o n f i g u r a t i o n ( example F i g u r e 2 . 9 b ) .

These c a l c u l a t i o n s o n l y depend on t h e Madelung c o n s t a n t s and h en ce t h e c o n c l u s i o n s a p p l y t o a l l t h e a l k a l i h a l i d e h o s t l a t t i c e s a l t h o u g h t h e f o r m a t i o n e n e r g i e s h a v e t o be s c a l e d by t h e

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L iF (U ) AND NaF(U)

3 . 1 INTRODUCTION

3 . 2 EXPERIMENTAL DETAILS

3 . 3 ABSORPTION SPECTRA OF L i F ( U )

3 . 3 . 1 EFFECTS OF URANIUM CONCENTRATION

3 . 3 . 2 EFFECTS OF CRYSTAL GROWTH ATMOSPHERE

3 . 3 . 3 EFFECTS OF IMPURITY ADDITION TO THE MELT

3 . 4 DISCUSSION OF THE L i F ( U ) CENTRES

3 . 5 ABSORPTION SPECTRA OF NaF( U)

3 . 5 . 1 EFFECTS OF URANIUM CONCENTRATION

3 . 5 . 2 EFFECTS OF CRYSTAL GROWTH ATMOSPHERE

3 . 5 . 3 EFFECTS OF ADDED IM PU R ITIES IN THE MELT

3 . 6 DISCUSSION OF THE NaF( U) CENTRES

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3,1

INTRODUCTION

It has been known for a long time that hexavalent uranium doped LiF and NaF have very strong luminescence in the blue-green

and green-yellow regions respectively (Slattery, 1929). The

presence of oxygen during sample preparation is essential for this

emission to occur. The low temperature luminescence spectra

consists of numerous narrow bands and lines, showing variability in relative intensity from sample to sample depending on the

different sample preparing conditions. This suggests the presence

of several types of luminescence centres. This emission has been

attributed to uranium-oxygen complexes with the diversity of the centres arising from different methods of local charge compensation (Runciman, 1956: Feofilov, 1959: Kaplyanskii et al. 1963a, 1963b, 1964, 1970: Bleijenberg and Timmermans, 1978: Runciman and Wong, 1979: Bleijenberg and Breddels, 1980).

The low temperature absorption spectrum of LiF(U) was investigated by Runciman and Wong (1979) and the relative line

intensities were found to be very specimen dependent. Bleijenberg

and Breddels (1980), using selective excitation techniques,

obtained the individual absorption spectra due to three different luminescent centres in NaF(U) whose relative intensities varies in samples grown under various conditions (Bleijenberg and Timmermans, 1978) .

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3 , 2 EXPERIMENTAL DETAILS

The crystals used in this study, unless mentioned otherwise,

were grown by the Czochralski method using r-f induction heating

of a platinum crucible. These crystals were grown with different

uranium concentrations which was introduced into the melt in the

form of uranyl nitrate or oxide. These different forms of the

activators will lead to the same results as uranyl nitrate will

decompose to the oxide at temperatures lower than the melting

point of the host crystals (870°C for LiF and 980°C for NaF)

(Belyaev et a l ., 1960). It has also been found '(Belyaev et al.,

1961) that LiF(U) crystals show similar luminescence spectra

whether the activator is introduced as uranyl nitrate or uranyl

sulfate. But some chemical compositions of the added uranium

compound are found to affect the luminescence spectrum as LiF

activated with uranyl acetate has an intense additional luminescence

line series with the main line at 522.5nm (Kaplyanskii and Moskvin,

1962) .

The crystals were greenish in color for LiF(U) with the

concentrated uranium crystals fluorescing green and less uranium

concentrated crystals showing blue fluorescence. For NaF (U), the

uranium concentrated crystals had an orangish color and fluoresced

yellow and the low uranium concentrated crystals were of a pale

yellow color and fluoresced yellow. The maximum uranium

concentration in single crystals obtained was 0.03 at.% for

LiF and 0.2 at.% for NaF. At higher concentrations precipitated

crystals were formed on the melt surface. These percentages refer

to the amounts added to the melt. The uniform coloring of most of

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same u ran iu m c o n c e n t r a t i o n f o r t h e c r y s t a l . Some o f t h e LiF(U) c r y s t a l s , grown i n a l i m i t e d oxygen a tm o s p h e r e , showed a marked v a r i a t i o n i n f l u o r e s c e n c e w ith t h e t o p p a r t o f t h e c r y s t a l which was f i r s t p u l l e d o u t o f t h e m e lt f l u o r e s c i n g g r e e n and t h e b o tto m p a r t f l u o r e s c i n g b l u e .

For w a v e le n g th m easurem ents t h e c r y s t a l s w ere c o o le d by th e r m a l c o n t a c t w ith l i q u i d n i t r o g e n o r l i q u i d h e liu m i n a g l a s s dewar and p h o t o g r a p h i c p l a t e s o f a b s o r p t i o n and f l u o r e s c e n c e were t a k e n w i t h a H i l g e r s p e c t r o g r a p h . C a l i b r a t i o n was made u s i n g i r o n a r c r e f e r e n c e s p e c t r a .

For a b s o r p t i o n m easurem ents c r y s t a l s w ere c o o l e d t o a b o u t 15K by h e l iu m gas flow i n a flow t u b e and t h e s p e c tr u m was r e c o r d e d u s i n g a Cary 17 s p e c t r o p h o t o m e t e r .

3 , 3 A B S O R P T I O N S P E C T R A OF L i F ( U )

The low t e m p e r a t u r e a b s o r p t i o n s p e c t r a o f LiF(U) i s v e r y complex due t o t h e o v e r l a p p i n g s p e c t r a o f d i f f e r e n t c e n t r e s .

F i g u r e 3 .1 shows a p o r t i o n o f t h e a b s o r p t i o n and e m i s s i o n s p e c t r a o f a LiF(U) c r y s t a l grown i n an o x i d i s i n g a tm o s p h e r e . The r e l a t i v e i n t e n s i t i e s o f t h e s e c e n t r e s and h e n c e t h e l i n e s e r i e s a s s o c i a t e d w i t h them a r e v e r y sam p le d e p e n d e n t. The a b s o r p t i o n s p e c t r a o f some LiF(U) c r y s t a l s a r e shown i n a p a p e r by Runciman and Wong

(53)

In

tensity

A b s o r p t i o n

Emission

W a v e l e n g t h (nm)

(54)

S e l e c t i v e e x c i t a t i o n t e c h n i q u e u s i n g a dye l a s e r was

a t t e m p t e d f o r t h i s p u r p o s e . The p a r t i c u l a r e x p e r i m e n t a l s e t - u p u s e d i s d e s c r i b e d i n C h a p te r 4. A lth o u g h t h i s e n a b le d t h e l i n e s b e l o n g i n g t o t h e dom inant 5 1 8 . 5nm a b s o r p t i o n l i n e s e r i e s t o be

found ( C h a p t e r 4 ) , i t was d i f f i c u l t t o s e p a r a t e t h e i n d i v i d u a l s p e c t r a o f t h e o t h e r c e n t r e s . F i g u r e 3 .2 shows t h e e m is s i o n s p e c t r a o f some o f t h e lo n g w a v e le n g th c e n t r e s p r e s e n t i n L iF (U ). I t can be s e e n , as an ex am ple, t h a t i t i s d i f f i c u l t t o m o n i to r t h e e m i s s i o n o f t h e c e n t r e shown i n F i g u r e 3 .2 b w i t h o u t i n t e r f e r e n c e from t h e c e n t r e shown i n F ig u r e 3 . 2 a , b o t h o f w hich a r e co m p arab le i n i n t e n s i t y i n most s a m p le s . The 5 1 8 . 5nm l i n e i s lo w er i n e n e rg y t h a n l i n e s due t o o t h e r s t r o n g l y p r e s e n t c e n t r e s , and h e n c e can be e x c i t e d w i t h o u t e x c i t i n g o t h e r c e n t r e s t o g iv e t h e e m is s i o n s p e c t r a o f t h i s c e n t r e ( F i g u r e 3 . 2 c ) . The c e n t r e s t o t h e l o n g e r w a v e le n g th s i d e o f t h i s p r i n c i p a l c e n t r e , ( e m i s s i o n o f two o f t h e s e a r e shown i n F i g u r e s 3 .2 d and 3 .2 e ) a r e v e r y weak i n r e l a t i v e i n t e n s i t y and h en ce i n t e r f e r e n c e from t h e s e c e n t r e s i n t h e e m is s i o n and e x c i t a t i o n s p e c t r a o f t h e p r i n c i p a l c e n t r e was n e g l i g i b l e .

M o n ito r i n g t h e peak o f t h e e m is s i o n z e ro -p h o n o n l i n e and

sw e ep in g t h e e x c i t i n g l a s e r w a v e le n g th g iv e s t h e a b s o r p t i o n s p e c t r a a s s o c i a t e d w i t h t h e c e n t r e . T h is can be ch eck ed by m o n i t o r i n g t h e e m is s i o n i n t h e wings o f t h e z e r o - p h o n o n e m is s i o n l i n e and a g a i n s w e ep in g t h e e x c i t i n g w a v e le n g th . A d i s t i n c t r e d u c t i o n i n t h e i n t e n s i t y o f t h e a b s o r p t i o n l i n e s i n t h e seco n d i n s t a n c e i s an i n d i c a t i o n o f t h e l i n e s b e l o n g i n g t o t h a t p a r t i c u l a r c e n t r e and n o t t o some o t h e r c e n t r e w hich m ight a c c i d e n t a l l y h av e a n o n - z e r o

The spectroscopy of alkali fluorides containing hexavalent uranium

CONTAINING HEXAVALENT URANIUM

A thesis submitted for the

degree of

in the

Australian National University

by

STATE M E N T

A C K N O W L E D G E M E N T S

TABLE OF CONTENTS

1,2

S U R V E Y O F S T U D I E S O N L i F :U 6+ A N D N a F : U 6+ D O M I N A N T

C E N T R E S

1 , 3

SURVEY OF S T U D I E S ON L i F : U 6 + AND N a F : U 6 + SECONDARY

CENTRES

CALCULATION IN L i F ( U . O ) •

2.1

INTRODUCTION

2.2

THE METHOD OF CALCULATING THE DEFECT ENERGY

2.3

NON-COULOMB POTENTIAL

2.4

COULOMB CONTRIBUTIONS

2.5

LATTICE RELAXATION

2,3

NON-COULOMB POTENTIAL

i*j

(q^-q^

4

K-V

* T

l

i£j

cq±"qi.)

l

i,ies

1

i/j

(qj-q^

2.5

LATTICE RELAXATION

I

o

R ---O

o

-332.78 eV

(a)

UO

+ TO

- ~337,29 eV

-399.81 eV

-599.10 eV

O

-392.« eV

a

U05nLiüp complexes

a

o

D u

F

□u

U0,£}.Dp complexes (cont.)

+[ p Li- a 3 - [ü f-öj

o---- □

(f)

o

u----o---- □

Ö

□u

- F —

u— o— a.

O

L iF (U ) AND NaF(U)

3,1

INTRODUCTION

In

tensity

**i*j**

_(q^-q^