Lattice crossover and mixed valency in the LaCo1−xRhxO3 solid solution

The full LaCo1−xRhxO3 solid solution was investigated utilizing structural, electrical transport, magnetic, and thermal conductivity characterization. Strong evidence for at least some conversion of Rh3+/Co3+ to Rh4+/Co2+ is found in both structural and electrical transport data. The crystal structure is that of a rhombohedrally distorted perovskite over the range 0.0≤x≤0.1. The common orthorhombic distortion of the perovskite structure is found over the range 0.2≤x≤1.0. A crossover of all three orthorhombic cell edges occurs at x=0.5 giving the appearance of a cubic structure, which actually remains orthorhombic. The octahedra in the orthorhombic structure must be distorted for x values less than 0.5, and the observed distortion suggests orbital ordering for Co2+. Electrical resistivity measurements as a function of temperature show semiconducting-like regions for all compositions. There is a steady increase in electrical resistivity as the Rh content increases. Large positive thermopower values are generally obtained above 475 K. With increasing Rh substitution there is a decrease in thermal conductivity, which slowly rises with increasing temperature due to increased electrical conductivity. The electronic part of the thermal conductivity is suppressed significantly upon Rh substitution. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo0.5Rh0.5O3 at 775 K, and is expected to reach 0.15 at 1000 K.

Graphical abstractA complete solid solution is formed between LaCoO3 and LaRhO3. The effect of Rh substitution for Co on the lattice crossover, mixed valency and physical properties is described.Figure optionsDownload full-size imageDownload as PowerPoint slide