Entropy contribution to the stability of double perovskite Sr2FexMo2−xO6

In contrast to the substitution of exotic elements such as Mn, Cu for Fe or W for Mo in Sr2FeMoO6, the degree of ordering of Fe and Mo in Sr2FexMo2−xO6 shows a maximum at xC=0.95 and decreases as x deviates from xC. Saturation magnetization depends upon composition in a similar way. It is shown that the entropy due to antisite defects or substitutional disorder can stabilize the double perovskite structure. It is proposed that the stability of the double perovskite structure of Sr2FexMo2−xO6 is essentially determined by the combined contribution to free energy of configuration entropy and differences in ionic charge and in ionic radius. A plausible charge distribution of Fe and Mo cations in Sr2FexMo2−xO6 is proposed to elucidate the observed composition-dependence of magnetization.