Combustion synthesis, characterization and metal–insulator transition studies of nanocrystalline $La_{1-x}Ca_xMnO_3 (0.0 \leq x \leq 0.5)$

Nanocrystalline CMR materials $La_{1-x}Ca_xMnO_3 (0.0 \leq x \leq 0.5)$ 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 $(V_b)$ and stretching modes $(V_s)$ 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