High pressure polymorphism of β-TaON

Abstract

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch–Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value K_o = 328 ± 4 GPa with K^′_o = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta–(O,N) and Ta–Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO₂ phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus K_o ∼ 370 GPa, close to the theoretically predicted value.