Abstract
Single crystals of magnesium fluorogermanate have been grown from a melt composition of 3.5MgO·0.5MgF 2·GeO 2 in a lead fluoride flux. The crystals are isomorphous with magnesium germanate, Mg 28Ge 10O 48; the space group is Pbam and a = 14.343(1), b = 10.196(1), and c = 5.9075(4) Å. A final unit cell composition of Mg 28Ge 7.5O 38F 10 was deduced from a detailed analysis of high quality X-ray diffraction data. A residual, R = 0.045, was achieved with a data-parameter ratio of 17 in the final anisotropic refinement. Analysis of scattering factors, thermal parameters and site occupancy factors revealed defect clusters centered around a partial germanium vacancy (38.8% occupied) at a unique tetrahedral site. Fluorine and oxygen occupancies of the contiguous anion sites are discussed in detail.
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