Crystal structure, electrical, and thermal properties of Ca_0.5Th_0.5VO₄

Abstract

Ca_0.5Th_0.5VO₄ was prepared by a solid-state reaction of component oxides and characterized by powder x-ray diffraction (XRD) at ambient and higher temperatures and impedance spectroscopy. Crystal structure was refined by Rietveld refinements from powder XRD data. At room temperature, Ca_0.5Th_0.5VO₄ has a zircon-type tetragonal (I4₁/amd) lattice with unit cell parameters: a = 7.2650(1) and c = 6.4460(1) Å. Despite the large charge difference, Ca²⁺ and Th⁴⁺ are statistically distributed over a single site. The crystal structure of Ca_0.5Th_0.5VO₄ is built from the (Ca/Th)O₈ (bisdisphenoid) and VO₄ tetrahedra. The in situ high-temperature XRD studies on Ca_0.5Th_0.5VO₄ revealed anisotropic thermal expansion behavior with coefficients of thermal expansion α_c = 10.96 × 10⁻⁶/°C and α_a = 5.32 × 10⁻⁶/°C. The impedance measurements carried out in the temperature range from ambient to 800 °C indicate semiconducting behavior with appreciable ionic conductivity above 400 °C. The activation energy obtained from the temperature-dependent AC conductivity data is ∼1.37 eV. In wider range of frequencies and temperatures, the relative permittivity of approximately 50 to 60 is observed for Ca_0.5Th_0.5VO₄.