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

Nanocrystalline Ca1−xHoxMnO3−δ (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 °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 °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.