Heat capacity of SrFeO3–; = 0.50, 0.25 and 0.15 – configurational entropy of structural entities in grossly non-stoichiometric oxides

The heat capacity of SrFeO3–; = 0.50, 0.25 and 0.15 has been determined for 10 < T/K < 800 K by adiabatic calorimetry. The total heat capacity has been analysed and the entropy of the magnetic and structural order–disorder transitions derived. The excess heat capacity of SrFeO2.50 relative to the mixture of the binary constituent oxides is large even far below TN= 685 K due both to vibrational and magnetic effects. Lattice energy simulations show that the maximum in the excess heat capacity observed at around 60 K is due to a change in the vibrational density of state with origin in changes in the shortest Sr–O bond length on formation of the ternary oxide from the binary ones. While the entropy of the magnetic order–disorder transitions appear to be close to the ideal spin-only values, the entropy of the structural order–disorder transitions are much smaller than expected assuming random distribution of the relevant species on the different sub-lattices. A statistical analysis of the effect of enthalpically preferred structural entities (square pyramids or tetrahedra) on the configurational entropy is presented. A significantly reduced configurational entropy in qualitative agreement with the experiments is obtained.