The electric conductivity of monazite-type LaCrO4
and zircon-type Nd1 − xCaxCrO4
(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 CrV and CrVI, Nd1 − xCaxCrO4
(x = 0.1,
0.2), hole hopping conduction arising from the mixed valency was also
observed. The Nd1 − xCaxCrO4
(x = 0.1,
0.2) compounds obeyed Curie–Weiss behavior above the Nèel
temperature, and the observed magnetic moments for the CrV 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 CrV 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 Nd1 − xCaxCrO4
(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.
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