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 HfO2 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 P42/nmc) and cubic (space group Fm3m) HfO2 phases are measured at high temperature. The transition temperature of HfO2 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. High‐temperature X‐ray diffraction experiments on HfO2 powders are performed using a container‐less levitation technique up to the melting points. Below the temperature of phase transition from tetragonal to cubic, pre‐transitional behavior is observed as a decrease of the c/a ratio. Based on lattice volume changes, the phase boundary between the tetragonal phase and cubic phase exhibits a positive slope in the pressure–temperature phase diagram.