Combustion synthesis, characterization and metal-insulator transition studies of nanocrystalline La1-xCaxMnO3 (0.0≤x≤0.5)

Nanocrystalline CMR materials La1-xCaxMnO3 (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 LaMnO3 on calcination at 900 degrees 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 degrees C, while the thermogravimetric (TG) curve exhibits a gradual weight loss from 650 degrees C onwards, corresponding to reduction of Mn4+ to Mn3+. 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 (nu(b)) and stretching modes (nu(s)) of the Mn-O-Mn bond, respectively. The metal-insulator transition temperatures (TM-1) are found to lie in the range 117-235 K for combustion derived samples, which are low compared to those prepared by ceramic route. (c) 2006 Elsevier B.V. All rights reserved.