Far Infrared Reflectance Spectroscopy of High Entropy Rocksalt Structure Oxides

A high entropy oxide is formed by a random distribution of metal atoms amongst ordered oxygen atoms in a crystal lattice. Since in high entropy systems one is dealing with a supercell instead of a unit cell, one cannot simply find the number of optical modes by knowing the number of atoms in the unit cell. In this experimental work the far infrared reflectance of a rocksalt structure high entropy oxide (NiCoMgZnCu)O for both single phase prepared by air quenching the mixture of five binary oxides from 1100^oC and a multiphase sample were measured between 50 cm^-1 to 1000 cm^-1 and temperatures between 4 and 300K in order to study the phonon vibrational modes and the effect of the antiferromagnetic transition on the modes. The far infrared reflectance showed one optical mode with a sideband which resembles the FIR reflectance of diatomic rocksalt compounds. The multiphase spectrum was different and it behaves qualitatively as the combination of FIR reflectance of all constituents.