Structure and magnetic order in ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ $(0<x<~0.33)$

The nuclear and magnetic structures of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ have been studied by neutron powder diffraction methods for the compositions $x=0.06,$ 0.15, 0.175, 0.25, and 0.33. At low concentrations of Ca $\mathrm{}(x=0.06\mathrm{})\mathrm{}$ the oxidized sample contains cation vacancies and the structure is ferromagnetic, while in reduced samples all atomic sites are fully occupied and the structure is antiferromagnetic. This result explains the discrepancies found in published phase diagrams. In the samples with compositions $x>~0.15,$ the structural distortions associated with the magnetic and electronic transitions increase with increasing Ca content. These distortions consist mainly of an increase in the tilting of the ${\mathrm{MnO}}_6$ octahedra and a consequent sharp decrease of the Mn-Mn separations as the system becomes metallic and ferromagnetic. These readjustments of the structure may be responsible for the metallic character of the bonds at the transition from insulating to metallic behavior. The magnetic-field dependence of the lattice parameters, studied in sample $x=0.33\mathrm{}$ at two fixed temperatures (270 and 240 K), demonstrates that there is strong coupling between the magnetization and the structural properties in this system.