Pressure-Driven Polar Orthorhombic to Tetragonal Phase Transition in Hafnia at Room Temperature.

Oxides are legendary for their complex energy landscapes, sensitivity to external stimuli, and property control through chemical substitution. Of these, the binary oxide HfO2 is one of the most fascinating because of the extraordinary number competing phases and opportunities to stabilize unique and useful properties. In this work, we combined synchrotron-based infrared absorbance and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of polar orthorhombic hafnia (chemical formula HfO2:xY, where x = 12%) under pressure. Compression drives this system to the tetragonal form above 22 GPa-quite different from the more conventional phase diagram derived from pressurization of monoclinic HfO2 where the tetragonal phase resides at elevated temperatures. In addition to evidence for a complex energy landscape, we unveil a wide coexistence region, order-of-magnitude differences in phonon lifetimes, and an A 1g symmetry phonon in the tetragonal phase with a negative mode Grüneisen parameter that drives the system toward the cubic phase. Similar pressure pathways may connect other metastable phases in this family of materials.