Composition (m ole fraction o f Si^N^)

Figure 3-6 S pin el lattice param eters as a fu n ction o f starting composition. Each p a ir o f

points, one in the top box a n d the other in the bottom box, correspond to lattice param eters

refin ed f o r the two spinel ph ases identified in each energy dispersive X -ray diffraction

pattern. The so lid line show s V egard’s law (average lattice constant) f o r a random

(SixG ci.JiN4 alloy, drawn between the end m em bers y-Gc3N4 an d y S i3N4, with their lattice

param eters extrapolated to P = 20 GPa an d room temperature.

In order to find out if the newly synthesised ternary spinel is the normal or inverse spinel type, we generated the X-ray diffraction patterns, using PowderCell 2.3, for both cases and com pared them with the measured X-ray diffraction patterns. Figure 3-7 shows the calculated X-ray diffraction patterns for the normal and inverse spinels. One can com pare those with the upperm ost trace o f the experim ental spectra (Figure 3-5) o f the m ixtures, because the Si-rich phase dom inates this pattern with a nominal com position 2:1 Si3N 4:Ge]N4. The calculated pattern for the normal spinel has the (220) reflection much more intense than (400), as it is

observed experim entally (for the pure G e]N 4 spinel, the (400) reflection is approxim ately h a lf as intense as (220)). In the inverse spinel, the (220) reflection is extrem ely w eak com pared to the (400) peak. C learly, the inverse spinel pattern does not m atch the pattern show n in figure 3-5. Furtherm ore, the calculated pattern show s the intensity for the (422) reflection com parable to those o f the (151) and (440) reflections, like it is observed in the m easured pattern. The theoretical pattern for the inverse form has (422) m uch w eaker than the other tw o peaks. From the above observations, we conclude that the new ly synthesised phase is a norm al spinel: i.e., the Si"^^ ions occupy octahedral sites, and Ge^^ ions preferentially occupy tetrahedral positions. The result appears initially counterintuitive, as Ge'*^ ions (0.55Â) are considerably larger than Si"*^ (0.40Â). H ow ever, ab initio calculations confirm ed this result [130].

CO

§

(3

(220)

Normal spinel

)

(422)

(331)

(620)

b)

₁

(400)

(222)

(531)

Inverse spinel

— I— I— I— I— I I I I— I—r

40 45 50

X-ray energy (keV) at 20 = 12 °

F igure 3 -7 C alcu lated X -ra y diffraction p a ttern s f o r (a) n orm al sp in e l (idealised

fr o m the observed com position (Geo.4oSio.6o)3^4) ^ith Ge ord ered on tetrah edral sites a n d S i

ord ered on octah edral sites (b) inverse sp in el SF (SiG e)'’‘N4.

T he synthesis pressure and tem perature for the new ternary nitride spinel are w ithin the range attainable by large volum e press techniques. The experim ents also indicate that the new phase is recoverable to am bient conditions. In consequence, the next paragraph w ill focus on the synthesis o f larger sam ples o f the new ternary nitride spinel in a large volum e press.

3.3.3.

Synthesis of ternary spinel nitrides using multi-anvil press

techniques

Follow ing the very interesting discovery o f a new ternary spinel nitride phase using diam ond anvil cell techniques, we decided that it w ould be useful to obtain a larger sam ple o f the new phase. Thus, w e m oved from the diam ond anvil cell to the large volum e press techniques.

3.3.3.1. E xperim en tal m ethods

W e perform ed the experim ents at A rizona State U niversity using the B ig B lue press previously described and a 3-8 assem bly. W e originally only confined the sam ple w ithin the rhenium furnace w ith a small rhenium disk above and below. In a second set o f experim ents, we further enclosed the sam ple into a boron nitride capsule. The experim ental conditions w ere 20 G Pa (4950 psi o f oil pressure in the press) and tem peratures ranging from 1500 °C to above 2300 °C. In m ost cases, w e m easured the tem perature using a type C therm ocouple. H ow ever, in som e cases the therm ocouple failed upon pressurisation. T herefore, w e had to use the pow er curve obtained during the successful runs in order to estim ate the tem perature.

T he sam ple used during the synthesis experim ents w ere ground m ixtures o f a -S i]N 4 + a - ^ - G e3N 4. W e m ixed and ground the starting sam ples w ith tw o W C cubes.

3.3.3.2. R esu lts a n d discussion

In the first run, w e used a 2:1 m ixture o f Si3N 4 and G e3N 4, and heated the sam ple to 1500 °C for 2 hours. The X -ray diffraction pattern o f the quench sam ple show s very little reaction betw een silicon nitride and germ anium nitride. The resulting pattern show s the X -ray diffraction pattern o f y-Si3N 4 + y-Ge3N 4. Therefore, we decided to increase the tem perature in order to increase the kinetics. W e perform ed the follow ing experim ents at 20 G P a and 2000 °C.

W e loaded four m ixtures o f silicon nitride and germ anium nitride; 1:4, 1:2, 1:1 and 2:1. Figure 3-8 presents the X -ray diffraction patterns o f the sam ples resulting from those syntheses. The extra peaks in the pattern are from an undeterm ined phase o f rhenium alloying w ith the germ anium in the sam ple (Figure 3-9).

T he patterns clearly show that there is no com plete reaction betw een the silicon nitride and germ anium nitride even though we heated the sam ple for tw o hours. The only sam ple displaying a single spinel pattern is that o f the sam ple synthesised from a m ixture o f 4 G e3N 4 + 1 Si3N 4. The lattice param eter and R ietveld refinem ent o f the pattern give a com position (Geo.9Sio.i)3N 4 w ith all the Si atom s in octahedral sites. The difference betw een the starting com position and the product com position is m ost likely due to the large errors in the starting com position as we

only sample a very small amount o f the weighted sample. Although this result appears in contradiction with the DAC experim ents, the errors in com positions m easurem ent in the DAC experim ents can easily encounter for the difference. The lattice param eter o f the spinel considered to be "G egN /' in the DAC synthesis is not exactly that o f germanium nitride therefore it could have a com position including a small amount o f silicon. Therefore, the results are in fact consistent to this extent.

d C3

JIU

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