An infrared study of antiferromagnetic medium and high entropy rocksalt structure oxides

The infrared reflectance of a set of antiferromagnetic entropy-stabilized rocksalt structure oxides is measured in order to study the effect of a random array of cations, spin-phonon coupling, and anharmonicity on phonon mode behavior. Samples were prepared by conventional solid state reaction from quaternary or quinary equimolar mixtures of CoO, ZnO, CuO, NiO, and MgO. They were characterized by magnetometry, x-ray diffraction, and reflectance spectroscopy. It is found that the Neel temperature (T N) depends on the concentration of magnetic ions. The far-infrared reflectance spectra consist of one strong reststrahlen band and a weaker low frequency band, which is unexpected in a rocksalt structure. The resonance frequency of the main transverse optical ( omega T O) mode varies between 355 < omega T O < 386 cm - 1 and is controlled by both the average reduced mass of a cation-anion pair and the average effective charge. Unlike the parent compounds CoO and NiO, no splitting of omega T O due to spin-phonon coupling appeared in the reflectance spectra below T N. This is possibly due to the high scattering rate of the associated transverse optical mode, which is connected to static disorder.