Electrical Transport in Rare-Earth Oxides

Sesquioxides and nonstoichiometric oxides of rare-earths (Ln) exhibit electrical conductivities in the range 10⁻⁹−10⁻¹ Ω⁻¹ cm⁻¹. The sesquioxides exhibit mixed conduction with some contribution from ionic conductivity and major contribution from electronic conductivity. Seebeck coefficient data as well as the oxygen partial pressure dependence of conductivity indicate that LnO_x compounds are mixed valence semiconductors where oxides with 1.50 ≤ x ≤ 1.75 are p-type semiconductors and oxides with 1.75 ≤ x ≤ 2.00 are n-type semiconductors. The conductivity of LnO_x (Ln = Pr or Tb) goes through a maximum at x ≈ 1.75; Seebeck coefficients are sensibly constant with temperature and approach zero value at x ≈ 1.75. Employing the conductivities and Seebeck coefficients, transport parameters have been calculated. The mechanism of conduction in these oxides can be understood in terms of the hopping model and the small polaron theory. Fully ionised cation vacancies seem to be the predominant defects contributing to the defect structure in rare-earth sesquioxides.