Thermal and X-Ray Diffraction Studies of Doped and Undoped Single Crystal and Polycrystalline BaTiO 3
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INTRODUCTION A ferroelectric is normally a spontaneously polarized material with its polarization reversible under an external electric field. Barium titanate is a perovskite ferroelectric with three possible orientations for the polar axis [1]. The large electro-optic effect displayed by single crystals of BaTiO3 enables it to be used in photonic switching devices [2]. The storage of holograms using the photo refractive effect of undoped, and niobium- and iron-doped BaTiO 3 crystals has also been demonstrated [2-3]. Resolutions as high as 0.25 pm have been reported. The holograms of BaTiO3 were comparable with those obtained for commonly used iron doped LiNbO 3 or (K,Ta) NbO 3 crystals [4]. The hologram characteristics were found to vary greatly with crystal quality and dopants. Barium strontium titanite is another interesting ferroelectric material for uncooled IR detectors [5]. This material typically operates near the phase transition temperature, with dc bias applied [6]. Therefore, it would be interesting to make a careful study of the variations of the Curie temperature (Tc) in BaTiO 3 single crystals with effects of doping with foreign ions. Although the effect of doping on the crystallographic transition temperatures of BaTiO3 has been studied [7], little work has been reported for single crystal materials. For optoelectronic applications, single crystal materials are highly desirable. In this paper we report the effect of doping with Nb, Sr and Fe on the three crystallographic transitions of BaTiO 3 using differential calorimetric techniques and x-ray diffraction.
EXPERIMENTAL Solid State Synthesis of Ceramic Powders BaTiO 3 was prepared by treating a stiochiometric mixture of BaCO 3 and TiO 2 with concentrated nitic acid. The acid was added dropwise until a paste formed, and then the mixture 543 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
was fumed by gentle heating in a drying oven at 150°C for approximately 12 hours. The sample was allowed to cool, then ground in an agate mortar, transferred to an alumina crucible and heated in air at 600°C for 12 hours. The resulting product was allowed to cool, ground then reheated at 1200'C for a total of 72 hours. During the final heating period the sample was cooled and reground three times. BaTi1-xMxO3 (M = Nb, Fe) and Bal-xSrxTiO3 were prepared according to the procedure described for barium titanate. Nb 20 5 , Fe20 3 and SrCO 3 were used as dopants in these syntheses. X-ray Analysis Powder diffraction patterns of the products were obtained with a Philips diffractometer using a monochromated high intensity CuKcz radiation ( X = 1.5405,A). Patterns were taken from 12" < 20 < 70* and cell parameters were determined by a least squares refinement of the reflections. Differential Scanning Calorimetry (DSC) Transition temperatures associated with the three phase transitions were measured on a Perkin-Elmer DSC-4 TADS system following the procedure described in reference [6]. The temperature was controlled by a Perkin Elmer CCA7 cooling access
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