Synthetic brunogeierite Fe₂GeO₄: XRD, Mössbauer and Raman high-pressure study

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) ų. 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 ∼644, 472, 302, and ∼205 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.