In situ study of the high pressure high-temperature stability field of TaN and of the compressibilities of θ-TaN and TaON

The high pressure high-temperature stability of tantalum mononitrides was investigated at 8-23.5GPa and 1400-3500K by in situ powder synchrotron X-ray diffraction using the laser-heated diamond anvil cell and E-TaN as starting material. The transformation from E-TaN to ?-TaN was observed and ?-TaN was confirmed as the stable high-(p, T) tantalum mononitride. At temperatures above 3000K at 16-23.5GPa, the formation of a novel quenchable high pressure high-temperature phase was observed. Under nitrogen excess E-TaN reacted with nitrogen to the high-(p, T) phase -Ta2N3 as the main stable phase at 8-13GPa and about 2000K. This behaviour is similar to the reaction of pure Ta with nitrogen at high-(p, T) conditions. In addition, we have determined the bulk moduli of ?-TaN (B=360(15)GPa), which is less compressible than E-TaN and -Ta2N3, and of TaON (B=267(8)GPa).