High pressure polymorphism of β-TaON.

Area of Science:

• Materials Science
• High-Pressure Physics
• Solid-State Chemistry

Background:

• Tantalum oxynitride (TaON) is a material with potential applications under extreme conditions.
• Understanding its behavior under high pressure is crucial for materials science and geophysics.
• Previous studies on TaON's high-pressure properties are limited.

Purpose of the Study:

• To investigate the high-pressure behavior of TaON up to 70 GPa.
• To determine the structural phase transitions and compressibility of TaON.
• To characterize the material's properties using advanced spectroscopic and diffraction techniques.

Main Methods:

• High-pressure experiments utilizing diamond anvil cells.
• Synchrotron X-ray diffraction for structural analysis.
• Raman scattering and X-ray absorption spectroscopy (XAS) for probing vibrational and electronic properties.
• Birch-Murnaghan equation of state fitting for compressibility calculations.

Main Results:

• The initial baddeleyite structure (β-TaON) exhibits a bulk modulus (Ko) of 328 ± 4 GPa.
• A pressure-induced phase transition initiates around 33 GPa.
• The high-pressure phase adopts an orthorhombic cotunnite-type structure, with a high bulk modulus of approximately 370 GPa.
• Extended X-ray absorption fine structure (EXAFS) analysis revealed changes in Ta-O/N and Ta-Ta bond distances.

Conclusions:

• TaON undergoes a significant structural transformation to a cotunnite-type phase at high pressures.
• The material demonstrates remarkable stiffness, with the high-pressure phase possessing a bulk modulus close to theoretical predictions.
• The observed sluggish transformation kinetics, similar to ZrO2, suggest complex phase transition mechanisms.