Two new magnesium and magnesium-lead fluorogermanates and revision of the Mg28Ge7.5O38F10 phase

A new compound, Mg3Ge1-δ O4(1-δ)F2(1+2δ) was obtained by high temperature solid state reactions and flux method and its structure was characterized by single crystal X-ray diffraction. The structure relates to the silicate mineral norbergite, Mg3SiO4(OH)F, a member of the humite mineral group. The diffraction experiment revealed a deficiency of the Ge site in the new structure, which can be compensated by the substitution of either OH− groups or F− anions for O2− anions. The combination of the NMR, neutron scattering, and IR spectroscopy experiments suggest that the F− substitution is most likely; thus, the Mg3Ge1-δ O4(1-δ)F2(1+2δ) formula is suggested. The F− substitution is present for the side product of the Mg3Ge1-δ O4(1-δ)F2(1+2δ) synthesis, the compound known as Mg28Ge7.5O38F10. Refinement of the twined Mg28Ge7.5O38F10 crystal showed a slightly higher Ge content and the final composition was assigned as Mg28Ge8O40F8. The PbF2 flux method yielded a new compound, Mg2Pb2Ge2O7F2. The structure of this digermanate is related to the group of naturally occurring and synthetic silicates containing R 3+ cations. High-temperature powder XRD experiments showed that the Mg3Ge1-δ O4(1-δ)F2(1+2δ) phase cannot be synthesized from Mg2GeO4 and MgF2 in air due to the transformation of magnesium fluoride into MgO.