Phase Relations and Volume Changes of Hafnia under High Pressure and High Temperature

Using multi‐anvil high‐pressure devices and synchrotron radiation, X‐ray in situ observations of HfO2 under high pressure and high temperature have been performed to investigate its phase relations and compression behavior. An orthorhombic phase (orthoI) is stable from 4 to 14.5 GPa below 1250°–1400°C and transforms to a tetragonal phase, which is one of the high‐temperature forms of HfO2, above these temperatures. Another orthorhombic phase (orthoII) with a cotunnite‐type structure appears above 14.5 GPa. OrthoII is stable up to 1800°C at 21 GPa. OrthoII is quenchable to ambient conditions. The orthoI‐to‐orthoII transition is accompanied by ∼8 vol% decrease. The bulk moduli of orthoI and orthoII at room temperature are 220 and 312 GPa, respectively. This low compressibility of orthoII indicates that it is a potential candidate for very hard materials.