Defect structure and ionic conductivity of as-grown R1–ySryF3–y (R = Ce, Pr, or Nd) crystals with high SrF2 content

Abstract

The structure of defect nonstoichiometric phases of Ce_0.88Sr_0.12F_2.88, Pr_0.85Sr_0.15F_2.85, and Nd0.85Sr0.15F2.85 single crystals has been investigated by X-ray diffraction at room temperature. Crystals R_1–ySr_yF_3–y (R = Ce, Pr, or Nd) belong to the tysonite structural type (LaF₃), which has two forms (α and β). The morphotropic transition from the trigonal β form of the crystals to the hexagonal α form is confirmed, which was previously found for the first time when analyzing the nonstoichiometric phase of La_1–ySr_yF_3–y with a SrF2 content above 10 mol % (y ≥ 0.1). Temperature dependences of fluorine-ion conductivity are obtained. It is established that the conductivity of the α form of Ce_0.88Sr_0.12F_2.88 and R_0.85Sr_0.15F_2.85 crystals (R = Pr or Nd) is lower than that for the β form of R_0.95Sr_0.05F_2.95 (R = La–Nd) by 1–2 orders of magnitude. The sublattice of fluorine atoms in the α-form crystals is characterized by an elevated (in comparison with the β form) content of vacancies and more uniform structural and dynamic properties, which leads to a decrease in the mean diffusion mobility of fluorine ions and an increase in the ion-transport activation enthalpy.