Structural Evolution and Metal–Insulator–Metal Transitions in Hafnium Oxides: Implication for Memristive Devices

Hafnium oxides have been widely studied for their application in resistive random-access memory, which is a prominent emerging technology for next-generation nonvolatile memory systems. We performed a comprehensive investigation into the stoichiometry-dependent structural evolution and the remarkable electronic properties of HfOx at ambient pressure. This study employed calculations based on density functional theory augmented by particle-swarm optimization and the ab initio random structure searching methodology. Through this approach, we identified novel phases of HfO, Hf2O3, HfO2, and Hf2O5, all of which were determined to be thermodynamically, dynamically, and mechanically stable. Analysis of the electronic structures, charge density variations, and charge transfer revealed that all identified phases primarily exhibit ionic bonding characteristics. Additionally, an examination of the lattice vibrational spectra offers detailed insights into the lattice dynamics and thermodynamic properties of the P21/c-HfO2. In particular, our theoretical predictions indicate that HfOx undergoes metal–insulator–metal transitions with increasing oxygen content, a characteristic that could be integral to its potential use as a fundamental component in resistive random-access memory devices.