Thermodynamic studies on entropy stabilized oxide (Co,Cu,Mg,Ni,Zn)O

Abstract

Thermodynamic properties of the rock-salt entropy stabilized oxide (Co,Cu,Mg,Ni,Zn)O have been determined and the relevant functions suitable for evaluation of thermodynamic stability in particular temperature ranges associated with structural changes during heating have been measured. The energetic parameters obtained via several experimental methods under both the equilibrium and dynamic conditions (drop calorimetry, solid state electromotive force measurements, DSC/TG), together with the thermal expansion and electrical conductivity data, have been analyzed, and the relationship of the thermodynamic stability with changes in the defect structure has been revealed. Our results are consistent with a phase transition to a high entropy state around 1150 K and the large negative values of the free energy, observed in the 1173 to 1223 K temperature domain, highlight a more stable state in the sample with the rock-salt structure. The partial molar enthalpy and entropy values of oxygen dissolution in the crystalline phase increase with increasing temperature suggesting the decrease of the binding energy of oxygen and the decrease of order in the oxygen sublattice of the structure, respectively. Due to the significant increase in partial molar entropy in the temperature range of 1173–1273 K, it is likely that oxygen vacancies are not concentrated at specific oxygen sites but they are randomly distributed, supporting the previous findings related to an entropy-driven phase transformation. The experimental methods outlined in this report can be used to distinguish between the phase transitions associated with the formation of an entropy stabilized phase and highlight the correlative effects of temperature and defect structure on the thermodynamic behavior of the (Co,Cu,Mg,Ni,Zn)O compound.