Vibrational Modes in High-Configurational-Entropy Rocksalt Oxides

Single-phase, cation-disordered rocksalt structure oxides such as Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O can be synthesized by annealing equimolar mixtures of five binary oxides to 1000°C and then quenching to room temperature. The resulting compounds have high configurational entropy, which may play a crucial role in stabilizing the rocksalt structure. In systems with translational symmetry, vibrational modes can be understood using a ‘phonon gas’ model. In disordered systems, this may have to be replaced by a threefold division into propagons, diffusons and locons. Initial ab initio structural relaxation of a 64-atom Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O supercell using Quantum ESPRESSO found a triclinic unit cell which disagrees with measured xray diffraction data. Additionally, ab initio calculations on 216-atom supercells resulted in unphysical, negative modes in the phonon density-of-states. These difficulties with density functional theory motivate a ‘toy model’ based on classical lattice dynamics (CLD). In this work, the CLD model is studied in Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O using the General Utility Lattice Program and compared with infrared and Raman spectra measurements. We propose that high-entropy oxides are a rich playground to study and probe phonon localization by calculating inverse participation ratios.