Effect of the sintering temperature on the properties of nanocrystalline Ca1-xSmxMnO3 (0 <= x <= 0.4) powders

Nanocrystalline Ca1-xSmxMnO3 (0 <= x <= 0.4) manganites were prepared by a soft chemical method (Pechini method) followed by auto-combustion and sintering in air at 1073 or 1473 K. Single-phase powders with general composition Ca1-xSmxMnO3 were obtained after 18 h annealing. The particle and grain sizes of the substituted Sm-manganites did not exhibit variation with samarium content, but increase with increasing the sintering temperature. All manganites show two active IR vibrational modes near 400 and 600 cm(-1) characteristic of the BO6 octahedron vibrations. For the samples sintered at T-s = 1473 K, the partial substitution of calcium by samarium in the CaMnO3 phase induces a marked decrease in the electrical resistivity, in the temperature range of 300-900 K, and at the same time a metal-to-insulator transition occurs; for T-s = 1073 K all the samples present semiconductor behaviour. With the increase of the annealing temperature the grain size increases and a metal-semiconductor transition appears. The results can be ascribed to the Mn4+/Mn3+ ratio and particle grain size. The effects of particle size on the electrical properties can be attributed to the domain status, changes in the Mn-O-Mn bond angle and Mn-O bond length. (C) 2011 Elsevier B.V. All rights reserved.