Specific features of ion transport in nonstoichiometric Sr1−xRxF2+x phases (R=La, Y) with the fluorite-type structure
Abstract Temperature and concentration dependence of ionic conductivity (δ) was studied for the crystals of nonstoichiometric Sr 1− x R x F 2+ x phases (R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu, Y;0 x ⩽0.40) having the fluorite structure ( Fm 3 space group). The temperature dependence of σ of the crystals in the region 300–800 K obeys the Arrhenius equation. In order to explain the ion transport features observed (absolute σ and ΔH σ values, their concentration dependence) in the anion excess solid solutions, a model of “defect regions” is proposed. Within the framework of the model, the conductivity of a solid solution is determined by the ratio of the conductivities of the “defect region” and the matrix. It is shown that in relation to electrical characteristics, the whole set of nonstoichiometric phases can be divided into three groups, R=La, Ce, Pr, Nd: R=Gd, Tb, Dy, Ho; R=Er, Lu, Y. This division correlates well with a number of other crystallophysical and crystallochemical characteristics. The average concentration of charge carriers (F − ) and their average mobility in the Sr 0.8 R 0.2 F 2.2 crystals were calculated: n =2.2×10 27 m −3 , μ 500 K =2.8×10 −11 m 2 / s/V for solid solutions with R=La, Ce, Pr, Nd and n =6.8×10 26 m −3 , μ 500 K =3×10 −14 m 2 / sV for solid solutions with R=Er, Lu, Y.