Electrical transport in light rare-earth vanadates

This paper reports electrical transport studies of rare-earth vanadates (RVO₄ with R = Ce, Pr, Nd, Sm, Eu and Gd), prepared by solid state reaction and characterized by X-ray diffraction studies. TGA study (300 to 1200 K) shows no weight loss; possible phase transitions in the range 1075 to 1300 K have been indicated by DTA studies. All these vanadates are typical semiconducting compounds with room temperature electrical conductivity (σ) lying between 10⁻⁴ and 10⁻²Ω⁻¹m⁻¹. Measurements of σ and the Seebeck coefficient (S) are reported in the temperature interval 400 to 1200 K. Two linear regions 400 to T ₁K and T ₁ to T ₂K have been obtained from the log σ against T ⁻¹ as well as the S against T ⁻¹ plots followed by a peak around T ₃ and minima around T ₄K. T ₄>T ₃>T ₂>T ₁, are different for different vanadates. It has been concluded that in the interval 400 to T ₁K, conduction is of the extrinsic hopping type with Ce⁴⁺ in CeVO₄, Pr⁴⁺ in PrVO₄ and V⁴⁺ in Nd to Gd vanadates as dominant defect centres. In the temperature interval T ₁, to T ₂K, the conduction has been shown to be of the intrinsic band type in all vanadates with polarons of intermediate coupling strength as the dominant charge carriers. Above T ₂ all vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller. T ₄ is close enough to the temperature corresponding to the DTA peak to be termed the phase transition temperature.