A new indium yttrium germanate presenting the thortveitite structure with symmetry described by the space group C2/

A new iron indium germanate has been prepared as polycrystalline powder material which crystallizes in the monoclinic system (S.G. C2=m, No. 12). The structure was characterized by X-ray powder diffraction and Rietveld refinement of the resulting diffraction pattern. The cell parameters are a ¼ 6:5124 (4) The structure contains R þ3 cations (R¼Fe, In) almost equally distributed in distorted RO 6 octahedral sites. These octahedra are joined by edge sharing forming a hexagonal arrangement on the ab planes. The RO 6 octahedra layers are held together by sheets of isolated Ge 2 O 7 diorthogroups constituted by a double tetrahedra sharing a common vertex. This compound has the thortveitite structure and keeps a strong relationship with the FeYGe 2 O 7 germanate, which presents two R þ3 sites with six-coordinated (R¼ Fe) and seven-coordinated (R¼Y) oxygens, corresponding to the different symmetry given by the monoclinic space group P2 1 =m (No. 11).

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In this article we present the main features and structural relationships between thortveitite-type and thortveitite-like germanates compounds. We describe in detail the crystal structures, how they are built and the difference between them. Bondvalence and polyhedra distortion analysis are made and crystal structure stability ranges are given.