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 HfO 2 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 HfO 2 , 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.