High dielectric SrGeO3 perovskite prepared by high pressure synthesis
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Arizona State University, Tempe AZ 85287-1604, USA Abstract High pressure synthesis of SrGeO3 perovskite and its properties are reported. The metastable material recovered to room pressure shows a high dielectric constant (>100), due to a "stretched" crystal lattice. Introduction There has been much interest in the recent literature concerning the transformation of silicate materials (MgSiO3, CaSiO 3 ) to the perovskite structure under the high pressure conditions of the Earth's lower mantle 1 . The MgSiO3 perovskite may be retained metastably on decompression, but CaSiO3 perovskite transforms to glass on return to ambient pressure. This observation indicates a fundamental lattice instability which may be related to the relative size of Si4+ and the alkaline earth cation in this family of perovskites. Recent lattice dynamics studies indicate that these instabilities involve a soft polar transverse optic mode, in which the atomic displacements are the same as those associated with the paraelectric-ferroelectric phase change in transition metal perovskites 2 . This observation suggests a new method for preparing high dielectric perovskite materials, via recovery of metastable phases synthesized at high pressure. The high pressure phase should be sufficiently stable to be retained on decompression, but would have a lattice sufficiently "expanded" to give anomalous dielectric behaviour 3-5. In this study we report our results on the structure and properties of the highpressure perovskite phase of SrGeO3. Results Exploratory work showed that SrGeO3 with the ambient-pressure pseudowollastonite structure transforms to a garnet phase above lGPa and to a perovskite above 5 GPa at temperatures of 1073-1273 K6 . The X-ray powder pattern of SrGeO3 perovskite, synthesized at pressure of 6 GPa and temperature of 1273 K, recovered at ambient pressure 501 Mat. Res. Soc. Symp. Proc. Vol. 398 0 1996 Materials Research Society
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Raman shift ( cm'") Figure 1. Raman spectra of SrGe03 pervoskite as a function of low (a) and high (b) temperature. 502
corresponds to the pattern of the ideal cubic phase 6 . However, Raman spectroscopy reveals a first-order spectrum, indicating a small distortion of the crystal lattice (Fig. 1). To distinguish between first- and second-order features in the Raman spectra, we took measurements at temperatures down to 77 K. The intensities of the observed Raman bands at 95, 225, 509, 708, and 1016 cm- 1 change as expected with Bose-Einstein statistics for the first order Raman effect 7 . We observe no changes in the spectra on cooling that would suggest a low-temperature phase transition. Nor can we see the soft mode behavior. The perovskite phase of SrGeO3 is very sensitive to heating from laser, and it immediately transforms into a mixture of amorphous and other phases w
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