Figure 3 -1 7 Raman shift o f Ge - S i nitride spinels as a fu n ctio n o f composition. Com position o f the spinel m easured using electron m icroprobe on various pa rts o f the samples.
Figure 3-17 shows a summary o f the results on the Raman spectroscopy study o f the ternary spinel nitride with a diagram o f the Raman shift as a function o f com position. The peak shift change regularly as a function o f composition. Therefore, we can use Raman spectroscopy in a first approxim ation in order to determ ine the com position o f the ternary spinel nitride in the system SigN^ - GegN^.
3 .3 .5 .
Summary
From the data collected using the diamond anvil cell and the multi anvil press, we have evidence o f the synthesis o f the first ternary spinel nitride with the com position GeSi2N4. The relative intensity o f the peaks in the X-ray diffraction pattern allowed us to determ ine that Ge atom s are in the tetrahedral sites and Si atoms are in the octahedral sites. This result is consistent with that determined by Dong et al. [130]. They explain the surprising site preferences by looking at the nitrogen environm ent. The tetrahedral site formed by the three octahedrally coordinated metal atoms and one tetrahedral ly coordinated metal atom around the nitrogen atom
are m uch m ore sym m etrical w ith ^^Ge and ^ S i than in the inverse spinel case. T herefore, the norm al spinel is m ore stable than the inverse spinel at the com position G eSi2N 4.
The experim ental results from diam ond anvil cell and m ulti anvil synthesis bring som e input on the - Ge]N4 spinel phase diagram . The phase diagram at high tem perature show s som e solid solution betw een Ge2SiN4 and G e]N 4 as we could synthesise the sam ple w ith the com position G e2.7Sio.3N 4. Furtherm ore, the results also show ed that there w as no solid solution betw een G e2SiN4 and G eSi2N 4. The only com positions synthesised betw een those com positions w ere the results o f synthesis w here the sam ple did not reach equilibrium . H ow ever, the presence o f rhenium reacting w ith the sam ple as a sink for Ge w as an inconvenience during the experim ents and could lead to partially inaccurate conclusions in the m ulti-anvil syntheses.
Finally, we perform ed a R am an spectroscopic study on both the end m em bers and the interm ediate phases in the system y-Si3N 4 - y-Ge3N 4. The results show that creation o f a significant am ount o f nitrogen vacancies in the spinel structure at tem perature above 2000 K. F urtherm ore, there is a regular decrease in phonon frequencies from silicon nitride to germ anium nitride. Therefore, in a first approxim ation, it is possible to determ ine the com position o f the ternary spinel nitrides.
Chapter 4.
Pressure Induced Phase Transition in P-Gc
3N
44.1.
Introduction
The recent discovery o f new silicon and germ anium nitride phases synthesised at high pressure and high tem perature lead to a renew ed interest in the study o f their polym orphism [41, 47, 49] (see chapter 3). and G e3N 4 show identical polym orphism at low pressure (see chapter 1).
The a - and (3-forms o f Si3N 4 and G e3N 4 contain tetrahedrally-coordinated Si and Ge. The N atom s are at the centre o f a triangle form ed by three Si or G e atom s in the (3-phase. In the a - phase the N atom s are slightly out o f the triangles. The high-density spinel structures contain Si and G e in both octahedral and tetrahedral coordination to nitrogen, and the N atom s are coordinated to four Si or Ge. y-Ge3N 4 is the only solid state com pound to date that contains Ge in octahedral coordination to nitrogen.
In this chapter, w e investigate the polym orphism that occurs in p-G c3N 4 during m etastable com pression at am bient tem perature, using a com bination o f energy-dispersive (ED ) and high- resolution angle-dispersive (A D ) X -ray diffraction o f pow dered m aterials. W e carried out the experim ents at the N SL S beam line X -17C , and at D aresbury SRS beam line 9.1.
W e initiated this study follow ing the experim ents that led to the synthesis o f y-Ge3N 4 [49]. In the ED synchrotron run, we found that w e could follow the diffraction peaks from m etastably com pressed (3-G e3N 4 to P = 40 GPa, although the character o f the pattern changed above P = 20 G P a (figure 4-1) [49]. Here, w e analyse the variation in unit cell param eters obtained from these ED synchrotron runs, along w ith A D diffraction data. The V (P) plots reveal a large density decrease (5-7% ) occurring at P ^ 20-24 GPa, consistent w ith the occurrence o f a phase transition in m etastably com pressed p-G e3N 4. Ab initio calculations and R am an spectroscopy also confirm the occurrence o f that phase transition [136], [51].
40.1 GPa 2 a 27.3 GPa
I
3
19.3 GPa 9.3 GPa 0.03 GPa 10 15 20 25 30 35 40 45 50X-ray energy (keV) at 2 0 = 12 °
F igure 4-1 E n ergy dispersive X -ra y diffraction o f ^ G e^ 4 a t high pressure.F ollow ing our initial experim ental observations, J. D ong et al. [51] perform ed ab initio density functional calculations within the local density approxim ation (LD A ) to exam ine the polym orphism that m ight occur w ithin p-G e3N 4 com pressed at low T. They concluded that m etastable phase transitions are expected in the pressure range P = 20-30 GPa. The phase transition involves N atom displacem ents aw ay from special sym m etry positions w ithin the Pbs/m structure. D ong et al. first analysed the data in term s o f displacem ent patterns considering all unit cells in phase; i.e., T point {q = 0) instabilities. They predicted the occurrence o f tw o second order phase transitions, at 20 and 28 G Pa, that reduced the sym m etry first from Pôg/m to P 6 , and then to P3 (figure 4-2). T he E (V ) curve show ed inflection points at these pressures. M cM illan et al. recently studied the m etastable transitions in P-G e]N 4 by R am an scattering spectroscopy [136]. The data confirm that a phase transition occurs at P = 20 G Pa. T he num ber o f peaks observed at pressures above the transition greatly exceeds that expected for the P 6 structure, but they correspond generally to those predicted for the P3 structure. They concluded that the P 63/m -P 3 transition occurred directly in a first-order m anner, bypassing the predicted interm ediate P 6 phase.
In the theoretical study, D ong et al. already recognised that the PÔ3/m -P 3 transition could occur directly via a first order process, at a predicted transition pressure Ptr ~ 23 G Pa, because the calculated energy barrier for the first order (3-6 transition w as sm all, on the order o f 0.01 eV /atom (10.6 kJ/m ol) (figure 4-2). The transition pressure corresponds to the com m on tangent
betw een the m inim a on the calculated E (V ) curve. M cM illan et al. have term ed the new "post- phenacite" phase 0-G e3N 4, to distinguish it from previously know n a - , p- and y-polym orphs [136]. -7.25