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Ca0.5Th0.5VO4 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, Ca0.5Th0.5VO4 has a zircon-type ˚. tetragonal (I41/amd) lattice with unit cell parameters: a = 7.2650(1) and c = 6.4460(1) A 2+ 4+ Despite the large charge difference, Ca and Th are statistically distributed over a single site. The crystal structure of Ca0.5Th0.5VO4 is built from the (Ca/Th)O8 (bisdisphenoid) and VO4 tetrahedra. The in situ high-temperature XRD studies on Ca0.5Th0.5VO4 revealed anisotropic thermal expansion behavior with coefficients of thermal expansion ac = 10.96
106/ C and aa = 5.32
106/ 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 Ca0.5Th0.5VO4. I. INTRODUCTION

The ABO4-type compounds have been of interest because of their luminescent scintillators and laser host applications.1 ABO4-type compounds crystallize in scheelite, monazite, zircon, and wolframite, etc. type structures, depending on the ionic radii and charges of A and B cations. With rare earth as one of the trivalent ions, monazite and zircon are the only known structure types for B = P and V cations.2 Vanadates of all rare-earth ions, with the exception of lanthanum, crystallize in zircon-type structures.2 However, the electrical properties of these materials are relatively less explored due to their highly insulating nature. Zircon-type compounds have been proposed as promising gate materials in field-effect transistors due to their high-dielectric constants and appreciable stability in wider range of frequency and temperature.3 The spinlattice interaction in rare-earth vanadates and phosphates is reflected as discontinuity in magnetic, specific heats and thermal and electrical conductivity.4 Antiferroelectric transitions associated with the magnetic transition have been reported due to spin-lattice correlation in these phosphates or vanadates.5 More recently, the analogous phosphates (RPO4) with rare-earth ions have been reported to show high-quality factor (Qf > 60 K GHz) in microwave region.6 All rare-earth vanadates exhibit semiconducting behavior with band gap ranging between 1.2 and 1.7 eV.7 Studies on the transport properties of CeVO4 and EuVO4 a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0449 J. Mater. Res., Vol. 24, No. 12, Dec 2009

http://journals.cambridge.org

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indicate appreciably higher electrical conductivity comparable with fluorite-based ionic conductors.8–10 The emf measurements on the zircon-type CeVO4 indicate the