Spin-orbital ordering and mesoscopic phase separation in the double perovskite ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$

Neutron powder diffraction measurements on ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ reveal that this double perovskite orders ferrimagnetically and shows anomalous lattice parameter behavior below $T_C=521\mathrm{}\mathrm{K}.$ Below $\sim 300\mathrm{K}$ and $\sim 160\mathrm{K}$ we observe that the high-$T$ monoclinic crystal structure separates into two and three monoclinic phases, respectively. A magnetic field suppresses the additional phases at low T in favor of the highest-$T$ phase. These manifestations of the orbital degree of freedom of Re $5d$ electrons indicate that these electrons are strongly correlated and the title compound is a Mott insulator, with competing spin-orbitally ordered states.