Single crystals of phenakite-like Be₂(Si_1−xGe_x)O₄ solid solution: novel experimental data on hydrothermal crystal growth, X-ray diffraction and Raman spectroscopy study

Abstract

The pioneering hydrothermal synthesis of the compound Be₂[(Si_1−xGe_x)O₄] with a phenakite structure (the size of individual crystals up to 1 mm) was carried out in acidic alkaline-containing fluoride solutions at a temperature of 625 °C and a pressure of ~ 150 MPa. Uniform (x = 0, 0.80 and 1) and zonal (x = 0.04 − 0.025) Be₂[(Si_1−xGe_x)O₄] crystals synthesized in the Li-containing mineralizer were obtained. The possibility of formation of intermediate compounds under hydrothermal conditions remains an open question. In the Na-containing mineralizer, only Be₂SiO₄ crystallizes due to the formation of insoluble sodium germanates. The fading of formation of Be₂[(Si_1−xGe_x)O₄] was determined with the use of technique of temperature-induced zoning and can be explained by the fact that newly formed crystals screen the surface of the initial BeO. The instantaneous growth rates of the prismatic faces of Ge-substituted phenakite crystals, decreasing from 18 microns/day to 2, were determined using the technique of temperature-induced zonality. The crystal structures of Be₂[(Si_1−xGe_x)O₄] samples with x = 0, 0.80 and 1 were refined by direct X-ray diffraction methods, and the linear dependence of the unit cell parameters and bond lengths on the germanium content has been quantitively described. First Raman spectroscopy study of Be₂[(Si_1−xGe_x)O₄] on zonal crystals indicated the linear shift of vibration bands in Raman spectra to a lower frequencies with an increase in germanium concentration (x up to 0.25). A new Raman band of Ge–O stretching vibrations at ~ 1115 cm⁻¹, which is not common for natural and synthetic germanium-free phenakites, was observed.