Abstract

The electric conductivity of monazite-type LaCrO₄ and zircon-type Nd_1 − xCa_xCrO₄ (x = 0–0.2) were characterized by measurements of dc electric conductivity and the Seebeck coefficient in the 300–600 K range. The LaCrO₄ and NdCrO₄ were found to be n-type semiconductors, while commonly known monazite- or zircon-type oxides are insulators. In mixed valence compounds of Cr^V and Cr^VI, Nd_1 − xCa_xCrO₄ (x = 0.1, 0.2), hole hopping conduction arising from the mixed valency was also observed. The Nd_1 − xCa_xCrO₄ (x = 0.1, 0.2) compounds obeyed Curie–Weiss behavior above the Nèel temperature, and the observed magnetic moments for the Cr^V ions were in good agreement with the theoretical values. Experimental results and the calculated spin densities by the UHF method indicated that most of the unpaired electrons from Cr^V were localized on Cr atoms. The ab initio MO calculations for CrO₄³⁻ clusters in NdCrO₄ revealed that the SOMO is the degenerate dπ* state and that the LUMO is the pσ* state (O 2p origin): the SOMO forms the degenerate states at the top of the valence band and the LUMO forms a wide conduction band. For NdCrO₄ and LaCrO₄ the electronic conduction mechanism as semiconductors was explained by the band model. For Nd_1 − xCa_xCrO₄ (x = 0.1, 0.2) electronic conduction was described by the band model combined with hopping conduction of holes in degenerate dπ* states. It is concluded that the electronic conductivity of these compounds is caused by an intermixing of the ligand-to-metal charge-transfer (LMCT) state into the ionic configuration.