Premium
High‐pressure metastable phase transitions in β‐Ge 3 N 4 studied by Raman spectroscopy
Author(s) -
McMillan Paul F.,
Deb Sudip K.,
Dong JianJung
Publication year - 2003
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1007
Subject(s) - raman spectroscopy , brillouin zone , diamond anvil cell , metastability , phase transition , phonon , crystallography , materials science , monoclinic crystal system , phase (matter) , spectroscopy , chemistry , analytical chemistry (journal) , condensed matter physics , crystal structure , high pressure , thermodynamics , optics , physics , chromatography , quantum mechanics , organic chemistry
We studied polymorphic transitions occurring in β‐Ge 3 N 4 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 (γ‐Ge 3 N 4 ) above 10 GPa and 1200°C. However, ab initio theoretical studies have indicated that the phenacite‐structured low‐pressure β‐Ge 3 N 4 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 Γ point: q = 0) resulting in displacive motions of the N atoms, causing the symmetry to descend from P 6 3 / m through P 6 to P 3. 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 P 6 3 / m and P 3 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 β‐Ge 3 N 4 that has been metastably compressed at ambient temperature. From the number of Raman modes observed, this transition most likely corresponds to the direct P 6 3 / m – P 3 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 P 3 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 ≠0 must have occurred during the transition. Copyright © 2003 John Wiley & Sons, Ltd.