Low-temperature synthesis of nanocrystalline Ca1-xHoxMnO3-δ (0≤x≤0.3) powders

Nanocrystalline Ca1-xHoxMnO3-delta (0 <= x <= 0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 degrees C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm(-1). The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 degrees C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO3. The results can be well attributed to the Mn4+/Mn3+ ratio and to the particle grain size. (C) 2008 Elsevier Ltd. All rights reserved.