Phase relations in silicon and germanium nitrides up to 98 GPa and 2400°C

Phase relations in silicon and germanium nitrides (Si 3 N 4 and Ge 3 N 4 ) were investigated using a Kawai‐type multianvil apparatus and a laser‐heated diamond anvil cell combined with a synchrotron radiation. The pressure‐induced phase transition from the β to γ (cubic spinel‐type structure) phase was observed in both compositions. We observed the coexistence of the β and γ phases in Si 3 N 4 at 12.4 GPa and 1800°C, while the appearance of single phase γ‐Ge 3 N 4 was observed at pressures above 10 GPa. Our observations under higher pressures revealed that γ‐Si 3 N 4 and γ‐Ge 3 N 4 have wide stability fields and no postspinel transition was observed up to 98 GPa and 2400°C in both compositions. Using the room‐temperature compression curves of these materials, the bulk moduli ( K 0 ) and their pressure derivatives ( K ′ 0 ) were determined: K 0  = 317 (16) GPa and K ′ 0  = 6.0 (8) for γ‐Si 3 N 4 ; K 0  = 254 (13) GPa and K ′ 0  = 6.0 (7) for γ‐Ge 3 N 4 .