On the entropic contribution to the redox energetics of SrFeO3−δ

Abstract

The entropic contribution to the redox energetics of the grossly disordered high-temperature phase SrFeO_3−δ is derived from the heat capacities of SrFeO_2.500, SrFeO_2.725 and SrFeO_2.833 determined by adiabatic calorimetry. All three samples are structurally and magnetically ordered at low temperatures and the calorimetric data allow deconvolution of the total heat capacity and entropy into approximate contributions of vibrational, magnetic, electronic and configurational origin. Several thermal effects that are easily accounted for in terms of known magnetic and structural transitions are observed and discussed. The configurational entropy of the disordered high-temperature phase indicates substantial short-range-order on the oxygen sub-lattice even at high-temperatures. The standard entropy at 298 K as well as the total non-configurational entropy of SrFeO_3−δ at high-temperatures varies approximately linearly with composition. The entropies of the following oxidation reactions

are deduced. The entropy of oxidation is within the uncertainty of measurement, independent of composition. On the other hand, the entropy of oxidation varies significantly with temperature. In general, the results substantiate the importance of the vibrational characteristics of reactant and product. While it is often assumed that the entropy of oxidation is close to − 120 J K⁻¹ (mol O₂)⁻¹, the present results indicate that the entropic contribution to the redox energetics must be expected to vary to a large degree from one system to another.