Abstract
Polycrystalline ceramic samples of pure and germanium (Ge4+) doped fresnoite of general formula Ba2TiOSi2−xGexO7 (x=0.0 and 0.2) have been prepared by solid state reaction technique. The formation of the single phase compound was confirmed by X-ray diffraction and the structural parameters were refined by the Rietveld refinement technique. The dc conductivity of both the materials has been measured as a function of temperature from room temperature to 753 K and activation energy was calculated using the relation σ = σoexp(-Ea/kt). The activation energy 4.74 eV obtained for the pure compound is very high in comparison with 1.47 eV of Ge4+-substituted compound. The frequency and temperature dependent dielectric behavior of both the compounds have been studied. The real and imaginary parts of the dielectric constant increase with the increase of temperature.
Highlights
Fresnoite, a rare barium titanium silicate mineral, Ba2TiOSi2O7, was discovered during the geological investigation of the sanbornite deposits at Fresno County, California [1]
The detailed Rietveld refinement of Xray diffraction data for both the compounds was reported in our earlier paper [12]
The resistivities of these samples are so high that they are almost temperature independent from room temperature (RT) to 643 K and the results are plotted for an elevated temperature range from 643 to 753 K
Summary
A rare barium titanium silicate mineral, Ba2TiOSi2O7, was discovered during the geological investigation of the sanbornite deposits at Fresno County, California [1]. The mineral shows a noncentrosymmetric tetragonal crystal structure with space group P4bm [2]. The material shows good ferroelectric, piezoelectric, pyroelectric, optical, and other physical properties useful for different practical devices [3–8]. The single crystal data of fresnoite shows a broad peak in dielectric permittivity at ∼433 K but the same peak is absent in dielectric loss curve. It is surprising that the ceramic samples fail to reveal the dielectric anomaly shown in case of single crystal at 433 K [3]. The single crystal X-ray data taken at 297 K and 573 K revealed no change in symmetry except small changes in atomic position, the largest of which were less than 0.02 A [8]. The ceramic sample showed good hysteresis loop at room temperature but the shape of the loop remained unchanged when temperature increased to 875 K. It is surprising that the high temperature X-ray, DTA, and specific heat measurements gave no indication of phase transition
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