STRUCTURE AND MAGNETIC ORDER IN LA1-XCAXMNO3 (0-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-0.33)

The nuclear and magnetic structures of La1 - xCaxMnO3 have been studie d 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 (x = 0.06) th e oxidized sample contains cation vacancies and the structure is ferro magnetic, while in reduced samples all atomic sites are fully occupied and the structure is antiferromagnetic. This result explains the disc repancies found in published phase diagrams. In the samples with compo sitions x greater than or equal to 0.15, the structural distortions as sociated with the magnetic and electronic transitions increase with in creasing Ca content. These distortions consist mainly of an increase i n the tilting of the MnO6 octahedra and a consequent sharp decrease of the Mn-Mn separations as the system becomes metallic and ferromagneti c. These readjustments of the structure may be responsible for the met allic character of the bonds at the transition from insulating to meta llic behavior. The magnetic-field dependence of the lattice parameters , studied in sample x= 0.33 at two fixed temperatures (270 and 240 K), demonstrates that there is strong coupling between the magnetization and the structural properties in this system. [S0163-1829(98)06329-2].