High-pressure metastable phase transitions in β-Ge3N4 studied by Raman spectroscopy

We studied polymorphic transitions occurring in beta-Ge3N4 metastably compressed to high pressures (P = 40 GPa) at room temperature. Previous studies under high-pressure-high-temperature conditions have shown that this phase transforms into a newly discovered spinel-structured material (gamma-Ge3N4) above 10 GPa and 1200degreesC. However, ab initio theoretical studies have indicated that the phenacite-structured low-pressure beta-Ge3N4 polymorph should also undergo a series of metastable transformations upon compression at low temperatures. Here we studied these transformations by in situ Raman spectroscopy in a diamond anvil cell. The phase transitions were first predicted to involve a sequence of second-order phase changes driven by soft phonon modes at the Brillouin zone centre (the Gamma point: q = 0) resulting in displacive motions of the N atoms, causing the symmetry to descend from P6(3)/m through P (6) over bar to P3. The two transitions were predicted to occur at P = 20 and 28 GPa. However, it was also noted that a direct first-order transition between P6(3)/m and P3 phases might occur at P = 23 GPa if order-disorder processes or phonon condensations away from the Brillouin zone centre were considered. Here we present experimental evidence for a phase transition occurring at P = 20 GPa, within beta-Ge3N4 that has been metastably compressed at ambient temperature. From the number of Raman modes observed, this transition most likely corresponds to the direct P6(3)/m-P3 transformation, and it is therefore first order in character, but with a small activation energy barrier as ascertained by little or no hysteresis observed upon decompression. However, when all the minor features appearing in the Raman spectrum are accounted for, the number of observed resonances is greater than the number of zone centre modes expected even for the P3 structure. This indicates that the true unit cell is larger than expected from zone-centre mode softening, and that order-disorder processes or phonon instabilities at q not equal 0 must have occurred during the transition. Copyright (C) 2003 John Wiley Sons, Ltd.