Effect of band-filling and structural distortions on the Curie temperature of Fe-Mo double perovkites

By means of high resolution neutron powder diffraction at low temperature we have characterized the structural details of $\mathrm{L}{\mathrm{a}}_{\mathrm{x}}\mathrm{S}{\mathrm{r}}_{2-\mathrm{x}}\mathrm{F}\mathrm{e}\mathrm{M}\mathrm{o}{\mathrm{O}}_6$ ($0\le \mathrm{x}\le 0.5$) and $\mathrm{C}{\mathrm{a}}_{\mathrm{x}}\mathrm{S}{\mathrm{r}}_{2-\mathrm{x}}\mathrm{F}\mathrm{e}\mathrm{M}\mathrm{o}{\mathrm{O}}_6$ ($0\leq {\rm x}\leq 0.6$) series of compounds. This study reveals a similar variation of the mean bond-angle \FeOMo in both series. In contrast, the mean bond-distance \FeMoO
increases with La but not with Ca substitution. Both series also present a different evolution of the Curie temperature ($T_C$), which raises in the La series and slightly decreases in the Ca one. We thus conclude that the enhancement of $T_C$ in the La series is due to the electron filling of the conduction band and a concomitant rising of the density of states at the Fermi level.