ABSTRACT
Spontaneous crystals of krieselite (Ge analogue of topaz) with the chemical formula Al2[(Ge0.75Si0.25)O4](F1.63OH0.37) were synthesized using a thermo-gradient hydrothermal method at a temperature of 600/650 °C and pressure of 100 MPa. The unit cell parameters are: a = 8.9732(8) Å, b = 8.4823(7) Å, c = 4.7379(5) Å, V = 360.62(6) Å3, space group Pnma. The F-/OH- content of the samples was refined by FTIR spectroscopy method. Raman spectroscopy showed the main differences between the spectra of krieselite and topaz at the ambient conditions. The assignment of observed and calculated Ag bands (cm-1) for non-polarized Raman spectra was carried out. Using in situ Raman spectroscopy at high pressures, the dependence of the shift in the position of the main bands of the krieselite Raman spectrum on the pressure was established, and the corresponding paths of pressure induced distortion of crystal structure was assumed. According to the data of Raman spectroscopy, it was revealed that krieselite does not undergo the phase transitions up to 30 GPa. The probable way of crystal structure distortion within the space group Pnma was proposed based on simulation of high-pressure Raman spectra.
ABSTRACT
We present complex spectroscopic data of the synthetic brunogeierite (Fe2+)2Ge4+O4 with space group Fd3¯m of spinel structure: a = 8.3783 (6) Å, V = 588.12 (13) Å3. Brunogeierite crystals up to 200 µm in size were crystallized by the interaction of a boric acid solution on a metal iron wire in the presence of germanium oxide (GeO2) at 600/650 °C and 100 MPa. Mössbauer spectrum of synthetic brunogeierite consists of the symmetric doublet with the parameters IS = 1.104(1) mm/s and QS = 2.845(1) mm/s, that corresponds to the octahedral position of iron ions ([VI]Fe2+). The Raman spectra of Fe2GeO4 crystal consist of an intense main band at 756 cm-1 and four less intense bands at â¼644, 472, 302, and â¼205 cm-1 at ambient conditions. All five bands inherent in the spectrum of cubic spinel are present and gradual change in high pressure spectra up to 30 GPa. The color of the crystal changes from brown-orange to reddish at the center at 22.7 GPa and became opaque black up to 30.2 GPa. Herewith, in the high pressure spectra, we observed the splitting of some bands and the appearance of additional bands in a wide pressure range (from 1.6 to 30 GPa). The factor group analysis with the lattice dynamics calculation of potential crystal structure distortions shows the decreasing of the structure symmetry to tetragonal or rhombohedral in this pressure range.
ABSTRACT
We present Raman spectroscopic analysis of the synthetic Ga,Ge-rich tourmalines at pressures up to 30 GPa for the first time. Based on the experimental data, we obtained correlation between the Raman band positions and the cation substitutions in the tetrahedral and octahedral sites at ambient conditions. A new band of Ge-O stretching vibrations, which is not common in natural tourmalines, occurs at ~ 870 cm-1. The main Raman bands shift to the higher frequencies with increasing pressure due to decrease in the bond lengths, associated with structural deformations. The lattice dynamic calculated spectra of tourmalines with various compositions resemble those measured experimentally. Analysis of experimental and theoretical data reveals a possible phase transition at ~ 18.4 GPa in the tourmalines with up to 10 wt% Ga2O3 and 9 wt% GeO2.