P h o toem ission M easurem ents

X-ray photoem ission studies were performed on a-BijSnjOy by our collaborators at O xford University. The valence XPS is shown in Figure 6.15 along with the ED O S.

Al Ka XPS

2 0

14 12 10 8 6 4

Binding energy (eV)

DFT DOS

2 0

12 10 8 6 4

Binding energy (eV)

Figure 6.15 Com parison between valence band XPS o f BijSnjO^ and the density o f states from our D F T calculations.

T here is a ver\' go o d correspondence betw een the theoretical density o f states and the experim ental spectrum . In particular, six com ponents, labelled I-V I, can b e id en tified in b o th cases. T h e tw o bands V and V I w hich are found at the b o tto m o f the valence b an d corresp o n d to the bonding Sn 5j' and Bi 6s states, b o th o f w hich are hybridised to som e extent w ith the O 2p states. T he next three bands II, III and I\^ c o rre sp o n d to th e m ain O 2p valence b a n d state, w hich hybridise mainly w ith Sn 5p and Bi 6p states to give m etal-oxygen b o n d in g states. T he extent o f hybridisation is m o st p ro n o u n c e d in the relatively sharp ban d IV.

Finally, the relatively weak b and I, w hich appears in the experim ental sp ectru m as a well defined shoulder at the top o f the valence band, corresp o n d s to the Bi 6j—O 2p

antibonding levels, w hich in m m hybridise w ith Bi 6p states to give the asym m etric electron densit)" discussed above. T he only m ajor discrepancy betw een the experim ental and theoretical data is that the overaO spread o f energies is greater in the form er. T h e close correspondence betw een the calculated total density o f states and the experim ental photoem ission spectrum suggest that cross sections for ionisation o f O 2p, Sn 5s and Sp

and Bi 6j' and 6p states are all rather similar. This is n o t quite in accord w ith the calculated ionisation cross sections o f Yeh and Lindau'*"’ w hich w ould give m u ch greater w eight to the m etal s and p states than to the O 2p states. This discrepancy has b een noted before in com paring X-ray photoem ission data for PbO j'*’^ P b O and 6 1 2 0 3’"’ w ith theoretical densities o f states and suggests that the previously calculated cross sections are n o t entirely reliable.

6.4.6 D iscu ssio n

B oth the theoretical electronic densit}' o f states and the experim ental X PS spectra indicate a high degree o f covalency in the system. Bader analysis o f th e calculated charge

density results in electron populations o f 13.10 and 12.53 for Bi and Sn respectively. T h e effective charges o f approxim ately + 2.0 for Bi and +1.5 for Sn clearly deviate fro m the idealised Bi(III) and Sn(IV) species. T h e P E D O S show s that b o n d in g interactions occur b etw een oxygen and the valence s states o f b o th Sn and Bi. T he m ain difference b etw een the m etal-oxygen interactions lies in the fact that for Bi the antib o n d in g co m b in atio n o f this interaction is fiUed, while for Sn the antibonding co m b in atio n Hes above the Ferm i level. T h e Sn ions display the sam e electronic characteristics as the P b ions in P b O j.

BijSnjOy exhibits a n u m b er o f u nique properties relative to o th er Sn based pyrochlores, involving the oxidation o f various organic species and gas sensing w ith selectivity for C O . Key to all these processes is the reduction o f the Sn atom s. W hat is unclear is why Bi2Sn2 0 7 exhibits oxidizing properties exceeding those o f the sim ilar m aterials. Replacing Bi w ith like si2ed m etals results in the form ation o f the sym m etric regular pyrochlore lattice. This can be ascribed to the asym m etric electron densitv’ o f Bi(III) distorting the crystal lattice. A distorted en vironm ent allows for coupling b etw een the Bi 6J and 6p states and the filled antibonding states are stabilized. As such the fo rm atio n o f the lone pair (and hence distorted) strucm re is favoured. A recent D F T study by P ru neda and A rtac h o ’^^ addressed a series o f A jB jO , com p o u n d s w hich a d o p t the regular cubic pyrochlore lattice. Replacing Bi w ith La or Y rem oves the stro n g overlap w ith the valence O 2p states w hich w e observe for Bi2Sn2 0 7. T hese are m o re ionic system s w here L a /Y based states m ake Uttle co n trib u tio n to the valence band.

Sn in its + 2 oxidation state, m uch like Bi(III), is know n to form an asym m etric electron density and have a strong preference for distorted strucm res. In an oxidatitive catalytic process the reduction o f S n (I\^ will produce a Sn(II) species. A d o n atio n o f electrons to

Sn in Bi2Sn2 0 7 will fill the antibonding Sn 5 j - 0 2p orbitals. Similar to Bi, a d isto rted coordination en vironm ent will allow for the additional b o n d in g interaction o f the m etal

p states w ith the antibonding orbitals, thus m aking the reduced configuration m o re

energetically stable. This is m uch akin to the reason w hy SnO adopts the d isto rte d Htharge structure as apposed to cubic rocksalt."’ T h e B i-O layers in this pyrochlore create a distorted structural backbone for Sn-O layers w hich wiU m ake red u ctio n o f the Sn atom s m ore energetically favourable than it w ould be in a regular cubic pyrochlore lattice. This may explain why the inclusion o f Bi in this pyrochlore aids the catalytic activity o f the material. It w ould be o f significant in terest for future studies to address

the reduction o f a Sn term inated surface o f a-B i2Sn;,0 7 in com parison to the regular

pyrochlore lattice and o th er pyrochlore structures. H ow ever, the large size o f the u nit cell is too com putationally dem anding to undertake as p a rt o f this project.

6.5 C onclusion

In this chapter we have exam ined the electronic structure o f P b O j, P b, 0 4 and B ijSnjO ,.

Using the inform ation gained from our study o f the low oxidation state binary^ P b, Sn and Bi oxides, we have extended o u r know ledge o f m etal oxide ceram ics to acco u n t for the underlying interactions and co nduction properties o f the higher oxidation state and m ixed oxidation state m etal oxides. E xperim ental verification was p rovided fo r the

electronic structures o f b o th P b0 2 and Bi,Sn2 0, w ith the m easurem ents in excellent

agreem ent w ith ou r D F T calculations.

F o r P b (IV )0 2 , we found m any o f the covalent interactions betw een P b and O th at w ere present in o ur previous study o f P bO . H ow ever, the increased oxidation state o f Pb

results in the antibonding Pb 6 j - 0 2p com bination, w hich co n trib u ted to the lone pair in

dom inated by strongly hybridized Pb 6s-0 2p states. The apparent metallicity o f P bO j was attributed, in conjunction with experimental data, to the presence o f oxygen vacancies which result in the filling o f these conduction states.

For P b ,0 4, we identified two electronically unique t\^ es o f Pb ions. The electronic

structure o f the ions are indicative of the Pb atoms found in P bO and P bO j respectively. An asymmetric electron density was found on the Pb(II) type atoms which were directed towards the centre o f the hollow in the crystal structure. Strong bonding interactions occurred between O 2p and the s states o f both types o f Pb ion at the botto m o f the valence band. The top o f the P b, 0 4 valence band is dom inated by the Pb(II)

6

j

- 0

2p antibonding states, while the bottom o f the conduction band is

dom inated by the Pb(IV) 6j--0 2p antibonding states. The results infer that electronic conduction in P b, 0 4 can occur through reduction o f the Pb(IV) (oxygen deficiency) or oxidation of the Pb(II) (oxygen excess).

Finally, we examined the electronic structure o f the distorted pyrochlore Bi2Sn20y. The material was found to be a semiconductor with a gap o f approximately 1 eV between the valence and conduction bands. The features observed in the calculated density o f states and the XPS spectra are in excellent agreement and bo th indicate a high degree o f covalency in the system. Strong interactions occur between oxygen and both Sn and Bi. The orbital populations support formation o f this oxide as Bi(III)2Sn(IV) 2 0 7 bu t the

com pound is highly covalent with Bader partial charges o f +1.9 and +1.47 on Bi and Sn respectively. The Bi atoms were found to display the same electronic characteristic as the Bi(III) ions found in Bi20,. An asymmetric electron density was again present due to the filling o f the out o f phase Bi 6 s-0 2p interaction and overlap with the Bi 6/> states at the top o f the valence band. The Sn(IV) atoms displayed the same properties as the Pb(IV)

atoms in P b02, with the antibonding Sn 5 j- 0 2p states dominating the b o ttom o f the

conduction band. This is due to differences in the strength o f the metal s interactions

with O 2p. It is proposed that it is the distorted coordination environm ent produced by

the Bi-O layers which aids the reduction o f Sn(I\') to Sn(II) and as such enhances the catalytic activity o f this material.

7 The Involvement o f Shallow Core

d States in

In document The origin of structural distortions in metal ceramics (Page 160-166)