Neutron diffraction studies of structural and magnetic properties of niobium doped cobaltites

Neutron powder diffraction studies of the structural and magnetic properties of the La_1-xSr_xNb_yCo _1-yO₃ solid solutions (x = 0.2, 0.5; y = 0, 0.075, 0.1) have been performed. Substitution of Co²⁺ by Nb⁵⁺ prevents the formation of Co⁴⁺ and leads to stabilization of the Co ³⁺ ions in the high-or intermediate-spin state. This is accompanied by the significant change of the Co-O bond length as well as Co-O-Co bond angle in the CoO₆ octahedron. The obtained data are in agreement with the negative sign of the magnetic exchange interactions Co³⁺-O-Co ³⁺ in a relatively wide range of the Co-O-Co bond angles. Diamagnetic dilution by the niobium ions prevents long-range magnetic ordering and strongly increases magnetoresistance at low temperature.