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

Jun Li, Andrew E. Smith, Kyei Sing Kwong, Cynthia Powell, Arthur W. Sleight, M. A. Subramanian

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Abstract

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.

Original languageEnglish
Pages (from-to)1388-1393
Number of pages6
JournalJournal of Solid State Chemistry
Volume183
Issue number6
DOIs
Publication statusPublished - Jun 2010
Externally publishedYes

Keywords

  • Cobalt oxides
  • Magnetism
  • Perovskites
  • Rhodium oxides
  • Solid solutions
  • Thermoelectrics

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