Metal-insulator transitions in anion-excess LaMnO_3+δ controlled by the Mn⁴⁺ content

Abstract

LaMnO_3+δ samples with Mn⁴⁺ content up to 50% have been prepared by different methods. The structure of LaMnO_3+δ changes from orthorhombic to cubic (via rhombohedral) with increase in the Mn⁴⁺ content. LaMnO_3+δ samples containing greater than 20% Mn⁴⁺ are ferromagnetic and show resistivity maxima at a temperature T_t which is close to the ferromagnetic Curie temperature. The resistivity maximum is due to the occurrence of a metal-insulator transition. In samples heated to the same temperature, the value of T_t increases with % Mn⁴⁺. For a given sample, T_t increases with the temperature of heat treatment due to the increase in particle size. The onset of ferromagnetism in LaMnO_3+δ accompanied by an insulator-metal transition is similar to that found in La_1−xCa_xMnO₃ and La_1−xSr_xCoO₃.