Combustion synthesis, characterization and metal–insulator transition studies of nanocrystalline La 1− xCa xMnO 3 (0.0 ≤ x ≤ 0.5)

Abstract

Nanocrystalline CMR materials La 1− x Ca x MnO 3 (0.0 ≤ x ≤ 0.5) are prepared by a low temperature, self-propagating combustion synthesis. As-formed and heat treated calcium doped lanthanum manganites are crystalline, showing cubic symmetry. However, as-formed cubic LaMnO 3 on calcination at 900 °C for 6 h, transformed into rhombohedral phase. The microstructure and morphology of the compounds show that the particles are nearly spherical in shape and are agglomerated. Differential thermal analysis (DTA) curve reveals a broad exothermic peak around 900 °C, while the thermogravimetric (TG) curve exhibits a gradual weight loss from 650 °C onwards, corresponding to reduction of Mn 4+ to Mn 3+. The surface area of manganites increases from 12.5 to 24 m 2 g −1 with increase in calcium content. Both undoped and doped lanthanum manganites show two IR active vibrational modes at 400 and 600 cm −1, corresponding to bending ( ν b) and stretching modes ( ν s) of the Mn–O–Mn bond, respectively. The metal–insulator transition temperatures ( T M–I) are found to lie in the range 117–235 K for combustion derived samples, which are low compared to those prepared by ceramic route.