Structural phase purification of bulk HfO2:Y through pressure cycling

Abstract

We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO2:7%Y from the mixed monoclinic (P21/c) + antipolar orthorhombic (Pbca) phase to pure antipolar orthorhombic (Pbca) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the Pbca metastable state at 300 K. Compression also drives polar orthorhombic (Pca21) hafnia into the tetragonal (P42/nmc) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO2