Crystal Growth, composition, structure, and Raman spectroscopy of novel Ga,Ge-rich topaz

Abstract

Single crystals of Ga-, Ge-, and Ga,Ge-rich (up to 41.42 wt% GeO2 and 33.95 wt% Ga2O3 or 0.98 apfu (atom per formula unit) Ge and 0.90 apfu Ga) and undoped topazes were grown on natural topaz seeds as a newly overgrown layer up to 5 mm thick. The thermogradient hydrothermal method was used at a temperature range of 600 − 650 °C and a pressure of 100 MPa under the condition of reverse temperature solubility factor of silica and alumina in the fluoride solution. The chemical composition and crystal structure of the grown topazes, as well as the distribution of gallium and germanium in the overgrown layer, were investigated by electron microprobe analysis (EMPA), single crystal X-ray diffraction (SCXRD) and Raman spectroscopy. The overgrown layers of Ge-, and Ga,Ge-rich topazes have a zonal distribution of elements, two zones corresponding to topaz Al2SiO4(F,OH)2 and krieselite (Al,Ga)2GeO4(F,OH)2 compositions are distinguished. The structural and spectroscopic studies show linear dependences of unit cell parameters and Raman shift on germanium and gallium contents and confirm the existence of a complete series of topaz-krieselite solid solution.