Experimental thermodynamic study of the “high-entropy” oxide (MgCoNiCuZn)0.2O: entropic stabilization confirmed, but is it all that matters?

Several experimental thermochemical techniques—adiabatic calorimetry, differential scanning calorimetry, drop calorimetry, and drop solution calorimetry—were employed to obtain the heat capacity dependences and the enthalpy of formation of the single-phase complex-composition (“high-entropy”) oxide (MgCoNiCuZn)0.2O with the rock-salt structure. The enthalpy of formation from the binary oxides MO (M is either Mg, Co, Ni, Cu, or Zn) was found to be (6.92 ± 0.65) kJ mol-1, which is quite different from the previously reported estimates. The heat capacity data for (MgCoNiCuZn)0.2O between 0 K and 1420 K were successfully described by the sum of the Debye and Einstein terms. The resulting function was used to derive the standard Gibbs function, G^° (T), that is valid up to at least 1420 K – almost in the whole T range where (MgCoNiCuZn)0.2O is near oxygen-stoichiometric in air, as the thermogravimetric analysis demonstrated that in air (MgCoNiCuZn)0.2O exhibits oxygen exchange above ≈1450 K. Based on the obtained G^° (T) we confirmed the primary role of configurational entropy in stabilizing the (MgCoNiCuZn)0.2O phase relative to the constituent binary oxides. However, the nonconfigurational entropy change was found to constitute more than 15% of the positive entropy of formation from the binary oxides, which is a noticeable contribution. The thermodynamic description of (MgCoNiCuZn)0.2O allowed us to demonstrate that solid solution equilibria must be taken into account when the real-life stability of complex-composition oxides is assessed, highlighting the need for more experimental thermodynamic data and pointing out the dangers of using computation-based thermodynamic quantity estimates when it is not known how accurate they are with respect to the actual experimental values.