Structural and magnetic properties of the colossal magnetoresistance perovskiteLa0.85Ca0.15MnO3

A single phase sample of ${\mathrm{La}}_{0.85}{\mathrm{Ca}}_{0.15}{\mathrm{MnO}}_3$ with uniform cation distribution and complete oxygen stoichiometry was synthesized by freeze-drying technique and studied by a variety of methods. The sample is ferromagnetic below $T_C=170\mathrm{}\mathrm{K}$ with ${\mu }_{\mathrm{Mn}}=2.9\mathrm{}\left(1\right)\mathrm{}{\mu }_B$ at 10 K. Precise examination of the crystal structure by a combination of electron diffraction, high-resolution electron microscopy, and neutron powder diffraction revealed a monoclinic distortion of the ${\mathrm{GdFeO}}_3-$type structure $[\mathrm{S}.\mathrm{G}.\mathrm{}{P2\mathrm{}}_1/c,$ $a=7.74476\mathrm{}\left(6\right)\mathrm{}\AA ,$ $b=5.50398\mathrm{}\left(4\right)\mathrm{}\AA ,$ $c=5.47351\mathrm{}\left(4\right)\mathrm{}\AA ,$ $\beta {=90.091\mathrm{}\left(2\right)\mathrm{}}^0]$ with nonequivalent ${\mathrm{MnO}}_2$ layers alternating along the a axis. The crystal structure is characterized by a specific pattern of Mn-O distances implying an unconventional orbital ordering type. A possible relation to charge ordering is described within the bond valence sum framework.