Metal-Insulator Transitions in Anion-Excess LaMnO3+δ Controlled by the Mn4+ Content

LaMnO3+δ samples with Mn4+ content up to 50% have been prepared by different methods. The structure of LaMnO3+δ changes from orthorhombic to cubic (via rhombohedral) with increase in the Mn4+ content. LaMnO3+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 LaMnO3+δ accompanied by an insulator-metal transition is similar to that found in La1-x Ca x MnO3 and La1-x Sr x CoO3.