Optical-vibration properties and pressure-induced phase transition in (In,Sc)2Ge2O7 pyrogermanates

Abstract

Thortveitite-type (T-type) scandium and indium pyrogermanates were obtained through a solid-state reaction route. The crystal structures, morphologies, and chemical compositions of these compounds were investigated by X-ray diffraction and transmission electron microscopy techniques. The optical-vibration properties were evaluated by Raman scattering and infrared spectroscopy. Reflectivity far-infrared data were obtained for sintered Sc2Ge2O7 samples, which exhibited good surfaces after polishing, allowing the determination of the infrared optical functions, the complete polar phonon characteristics, the intrinsic dielectric constant and the unloaded quality factor. Because of the poor surface reflectivity of In2Ge2O7 samples, only mid-infrared absorbance data could be collected by using loose powders in an attenuated total reflectance accessory. By using polarized Raman spectroscopy on both ceramic materials, at room temperature and ambient pressure, all the 15 active Raman modes predicted by group theory for the monoclinic C2/m T-type structure were identified and assigned. Further, the phase stabilities of In2Ge2O7 and Sc2Ge2O7 were studied by high-pressure Raman spectroscopy. In the limit of pressures studied here (up to 10 GPa), a reversible structural phase transition (SPT) for In2Ge2O7 and Sc2Ge2O7 was induced by pressures of 4.2 GPa and 2.4 GPa, respectively. These would correspond to the reported C2/m (#12) ↔ P21/c (#14) SPT. Representative Raman spectra of the high-pressure phase for both materials were fitted and 24 first-order Raman modes were depicted for each material, a number greater than compatible with a proposed distorted and oxygen-deficient fluorite-like structure, but explained by an analysis of the gtroup-subgroup constraints of this SPT.