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

Abstract

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