In1.06Ho0.94Ge2O7: a thortveitite-type compound

Abstract

A new indium holmium digermanate, In1.06Ho0.94Ge2O7, with a thortveitite-type structure, has been prepared as a polycrystalline powder material by high-temperature solid-state reaction. This new compound crystallizes in the monoclinic system (space group C2/c, No. 15). The structure was characterized by Rietveld refinement of powder laboratory X-ray diffraction data. The In3+ and Ho3+ cations occupy the same octahedral site, forming a hexagonal arrangement on the ab plane. In their turn, the hexagonal arrangements of (In/Ho)O-6 octahedral layers are held together by sheets of isolated diortho groups comprised of double tetrahedra sharing a common vertex. In this compound, the Ge2O7 diortho groups lose the ideal Dad point symmetry and also the C-2h point symmetry present in the thortveitite diortho groups. The Ge-O-Ge angle bridging the diortho groups is 160.2 (3)degrees, compared with 180.0degrees for Si-O-Si in thortveitite (Sc2Si2O7). The characteristic mirror plane in the thortveitite space group (C2/m, No. 12) is not present in this new thortveitite-type compound and the diortho groups lose the C-2h point symmetry, reducing to C-2.