Electrical properties of lanthanum orthochromite

The determination of the electrical conductivity and Seebeck voltage, from 20 to 1450 deg. C, in the perovskite LaCrO₃ bas been used to study the influence of the sintering atmosphere (air, hydrogen, vacuum) and/or stoichiometry and cation substitutions (bi, tri and tetravalent cations) on the electrical properties and to identify the conduction mechanisms. The conduction with a low activation energy (0.16 to 0,22 eV) occurring most frequently is identified as a small polaron conduction. The polarons Cr³⁺ + O arise from the high temperature volatilization of chromium in oxygenated atmospheres and/or the surface oxidation of the chromite. Only in dense and vacuum sintered materials has the intrinsic semi-conduction been observed, with an activation energy corresponding to E_g = 2.2 eV. The 'metallic conduction' has not been reached through different cation substitutions. Substituting Al³⁺ for Cr³⁺ leads to a small lowering of the conductivity and to a lower volatilization rate of chromium. The only practical way of enhancing the electrical conductivity of LaCrO₃ is shown to be the creation of Cr⁴⁺ in the material. (author)