Structural, Magnetic and Vibrational Studies of Entropy Stabilized Oxides

Abstract

The structural, magnetic and vibrational properties of high entropy oxide (HEO) and medium entropy oxides (MEOs) made from mixtures of five and four metal oxides were studied in this work. The HEO and MEOs obtained from solid state reactions in quinary and quaternary equimolar mixtures of oxides including ZnO, CuO, CoO, MgO and NiO which are called 5-HEO, 4-MEO(-Cu), 4-MEO(-Mg), 4-MEO(-Co), 4-MEO(-Ni) and 4-MEO(-Zn) in this thesis. The samples were sintered at high temperatures followed by air quenching. The x-ray diffraction measurements results show the formation of single rocksalt phase in all of them except 4MEO(-Ni). The magnetization measurements in zero field cooled (ZFC) and field cooled (FC) regimes under H=5000 Oe have provided strong evidence for long range magnetic ordering in 5-HEO and 4-MEOs. Two samples including 4-MEO(-Co) and 4-MEO(-Ni) exhibit high frustration factor which is a major component for spin glass phase. The AC susceptibility measurements for 4-MEO(-Co) and 4-MEO(-Ni) with the highest frustration factors have carried out in order to investigate the presence of spin glass. The segregation of Cu from the rocksalt phase in 5-HEO by resintering at intermediate temperature (700oC) was confirmed by x-ray diffraction measurements, SEM-EDS elemental mapping and magnetization measurements. Infrared refectance spectroscopy measurements (100-15000 cm-1) for 5-HEO and 4-MEOs with single rocksalt structures have revealed one strong mode and a weaker mode at lower frequency in the far infrared spectra. The Lorentz model and Kramers-Kronig (KK) analysis were implemented for the samples and the results were used in effective charge calculations which reveal that 4-MEO(-Cu) has the greatest ionic character whereas 4-MEO(-Mg) showed the least ionic character. The Raman spectroscopy which complements the infrared spectroscopy was carried out for 5-HEO and 4-MEOs. The results have indicated first and second order Raman modes in addition to a two-magnon mode at around 1600 cm-1 which was confirmed by studying the temperature dependence. While 4-MEO(-Co) also shows a peak near 1600cm-1 it is likely of different origin. In 4-MEO(-Cu) the two magnon scattering peak was not observed and it rather causes a high background which increases with decreasing temperature.