Entropy contribution to the stability of double perovskite Sr_2Fe_xMo_(2-x)O_6

摘要

In contrast to the substitution of exotic elements such as Mn, Cu for Fe or W for Mo in Sr_2FeMoO_6, the degree of ordering of Fe and Mo in Sr_2Fe_xMo_(2-x)O_6 shows a maximum at x_C = 0.95 and decreases as x deviates from x_C, 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 Sr_2Fe_xMo_(2-x)O_6 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 Sr_2Fe_xMo_(2-x)O_6 is proposed to elucidate the observed composition-dependence of magnetization.