Pre‐Transitional Behavior in Tetragonal to Cubic Phase Transition in HfO₂ Revealed by High Temperature Diffraction Experiments

Using the container‐less levitation technique, high‐temperature X‐ray diffraction experiments have been performed on HfO 2 powders to confirm the temperature dependences of the lattice constants, thermal expansion character, and phase relations. A two‐dimensional imaging plate detector is used for short‐time observations; diffraction data in a wide area are projected in one dimension for Rietveld analysis, as large‐grain growth occurred at high temperatures. The lattice constants, thermal expansions, and c / a ratios for the tetragonal (space group P 4 2 /nmc ) and cubic (space group Fm 3 m ) HfO 2 phases are measured at high temperature. The transition temperature of HfO 2 from the tetragonal phase to the cubic phase is specified to be between 2500 and 2550 °C. The pre‐transitional behavior is clearly observed around 2200 °C. As no clear lattice volume change is observed at the transition point from the tetragonal phase to the cubic phase, a phase boundary between the tetragonal phase and the cubic phase has been proposed which does not exhibit a negative slope. It has been estimated that the stable region of the cubic phase is narrow in the higher‐pressure region.