Localization of Vibrational Modes in High-Configurational-Entropy Rocksalt Oxides


X-ray diffraction shows that, if quenched to 295 K from high enough temperature, highly disordered Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (HEO) forms a single-phase rocksalt structure. Various probes indicate that the the material exhibits a mixture of long range order (oxygen sublattice) and disorder (random cation sublattice). In addition, long range antiferromagnetic order below TN ≈ 120 K has been demonstrated using neutron scattering despite the fact that only 60% of the ions have moments. In this work, we study the vibrational modes of HEO experimentally using Infrared and Raman spectroscopy and theoretically using the General Utility Lattice Program. New empirical interatomic potentials (EIPs) are developed for the parent binary oxides MgO, CoO, CuO, NiO, and ZnO by fitting to experimental phonon frequencies, dielectric constants, and lattice parameters. The simulated vibrational density-of-states of the parent binary oxides are in agreement with inelastic neutron scattering data and Density Functional Theory calculations and improve upon existing potentials. The EIPs are utilized in HEO by neglecting cation-cation interactions. The simulated infrared and Raman spectra of HEO are in agreement with experimental data. In 3D materials, disorder can produce mode localization as in amorphous Si, where lattice dynamical calculations show that some of the phonon eigenvectors, corresponding to modes above a threshold frequency, are characterized by exponential decay. Furthermore, the onset of phonon localization for mode frequencies above a high-frequency mobility edge in the Vibrational Density of states is accompanied by a participation ratio (PR) < 0.1. We consider the application of these concepts to HEO. The main result is that a smaller percentage of modes are localized in HEO than in a cluster of α-Si with a similar number of atoms. Additionally, we show that the number of localized modes increases if the ordered oxygen sublattice is disturbed by the random substitution of sulfur atoms.

The seminar will be chaired by Stanislav Kamba, Department of Dielectrics.