High‐pressure structural stability, equation of state, and thermal expansion behavior of cubic HfO2.

The structural stability, equation of state, and thermal expansion behavior of nanocrystalline cubic HfO2, an ultra‐high‐temperature ceramic, have been investigated using X‐ray diffraction at extreme conditions of pressures and temperatures. High‐pressure studies show that the cubic structure is stable up to 26.2 GPa, while the high‐temperature studies show the stability of the cubic structure up to 600°C. The Rietveld structure refinement of the high‐pressure data reveals the progressive transition of secondary monoclinic phase to the cubic phase at higher pressures. The phase progression is accompanied by incompressibility along the b axis and a large compressibility along the c axis of the monoclinic structure. The second‐order Birch‐Murnaghan equation of state fit to the unit cell volume data yielded a bulk modulus of 242(16) GPa for the cubic structure. A linear thermal expansion value of αa(c) = 8.80(15) × 10−6°C−1 and a volume thermal expansion value of αv = 26.5(4) × 10−6°C−1 have been determined from the in situ high‐temperature X‐ray diffraction studies. The results are discussed by comparing with the high‐pressure and high‐temperature behavior of isostructural ZrO2. To the best of our knowledge, this is the first experimental report on the structural stability of cubic HfO2 at high pressures. [ABSTRACT FROM AUTHOR]