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

The Renewed Interest on Brunogeierite, GeFe2O4, a Rare Mineral of Germanium: A Review

“…The cubic symmetry of GFO was maintained up to 5 K, without structural distortion, as demonstrated by synchrotron X-ray powder and neutron diffraction measurements [ 25 ]. However, by increasing the pressure, it was suggested that, at about 20–22 GPa, a reduction from cubic ( Fd-3m ) to tetragonal ( I4 1 /amd ) symmetry occurred, as determined from the analysis of the band shifts of the Raman spectra and the colour change of crystals [ 26 ]. The same authors suggested, on the base of the presence of additional bands in the Raman spectra, a partial inversion of the spinel structure, which at ambient pressure and temperature had normal structure.…”

“…As previously discussed, GeFe 2 O 4 can be found in nature in peculiar locations in the form of single crystals, which were usually employed for crystal structure determination [ 20 ]. However, single crystals were also synthesized in the laboratory and were used, for example, for high pressure Raman studies and for the determination of optical and electrical properties [ 15 , 26 ]. For electrochemical and electrocatalytic applications, GFO was used instead in the form of nano-powders, which are more easily synthesized.…”

“…In a more recent paper [ 26 ], the synthesis of GFO single crystals was carried out in thermo-gradient hydrothermal conditions in an autoclave at about 600–650 °C and 100 MPa with the use of gold-lined inserts. Nutrient materials were stoichiometric amounts of iron wire and germanium oxide, located along the insert length and at the bottom, respectively.…”

“…The Mössbauer spectroscopy, being specific on Fe ions, could be useful to determine their oxidation states and coordination, so suggesting the possible presence of impurity phases. This technique was applied for many years on ferrous spinels and on GFO [ 16 , 19 , 26 , 27 ]. A typical Mössbauer spectrum of GFO single crystal [ 26 ] consists of a symmetric doublet with an isomeric shift (IS) = 1.104(1) mm/s and a quadrupole splitting (QS) = 2.845(1) mm/s, corresponding to Fe 2+ ions in octahedral sites, confirming the normal spinel structure of GFO.…”

“…This technique was applied for many years on ferrous spinels and on GFO [ 16 , 19 , 26 , 27 ]. A typical Mössbauer spectrum of GFO single crystal [ 26 ] consists of a symmetric doublet with an isomeric shift (IS) = 1.104(1) mm/s and a quadrupole splitting (QS) = 2.845(1) mm/s, corresponding to Fe 2+ ions in octahedral sites, confirming the normal spinel structure of GFO. The reported IS values are generally similar in the different papers, while the small differences in QS values could be due to the synthesis methods and the purity of the samples.…”