Y(0.76)Ho(0.24)FeGe(2)O(7): a new member of thortveitite-like layered compounds.

Y(0.76)Ho(0.24)FeGe(2)O(7) (yttrium holmium iron digermanate) was synthesized by solid-state reaction at 1573 K. This thortveitite-like compound presents a crystallographic group-subgroup isotranslational (klassengleiche) relation with some other pyrogermanates, such as FeInGe(2)O(7), In(1.08)Gd(0.92)Ge(2)O(7) and InYGe(2)O(7), which are configurationally isotypic with the Sc(2)Si(2)O(7) thortveitite structure first reported by Zachariasen [(1930 ▶). Z. Kristallogr.73, 1-6]. Holmium cations share with yttrium the 4f Wyckoff position at the center of a seven-coordinated pentagonal bipyramid, while Fe atoms also occupy one site with Wyckoff position 4f at the center of the octahedron. All these sites have the point symmetry C(1). Two types of Ge(2)O(7) diorthogroups with point symmetry C(1h) are present in the structure, each one of them defining a layer type which alternates with the other. These diorthogroups have their tetrahedral groups in an eclipsed conformation.

A synchrotron study of Na(2.27)Ho(7.73)(SiO(4))(6)O(0.72).

A well crystallized powder sample of sodium holmium orthosilicate oxyapatite, Na(2.27)Ho(7.73)(SiO(4))(6)O(0.72), was obtained after mechanical milling and thermal treatment at 1123 K. Crystal structure analysis was performed from the results of Rietveld refinement of the synchrotron diffraction data. As in other rare-earth orthosilicate apatites, sodium cations appear located sharing with holmium the 4f Wyckoff position at the center of a tricapped trigonal prism. In its turn, holmium almost fully occupies the 6h position at the center of a seven-coordinated penta-gonal bipyramid. A small quantity of Na atoms was found at this site. No vacancies are present in the two independent crystallographic sites available for Ho and Na atoms.

Compositionally induced phase transitions and symmetry relations between space groups in the layered system FeIn1-xYxGe2O7.

Symmetry reduction in the basic structure of thortveitite-type compound FeInGe2O7-mC22 (C12/m1, No. 12) promoted by the incorporation of yttrium in the formula FeIn1-xYxGe2O7 gives rise to a derivative structure called thortveitite-like AA'Ge2O7-mP44, with symmetry described by the space group P121/m1 (No. 11) for x = 0.50, 0.75 and 0.90. The structure remains unchanged within the structural type of thortveitite when x = 0.25. In order to establish structural models for performing a Rietveld refinement to the derivative structure, symmetry relations between space groups connecting the basic and derivative structures were used. The higher contrast to X-rays of Fe3+, In3+ and Y3+ as well as by the behaviour during the refinements of the isotropic thermal displacements, the values of interatomic distances and calculated bond-valence sums for each atom in the asymmetric unit, were helpful for elucidating the relocation of cations in the different available crystallographic sites generated by the symmetry reduction.

In1.08Gd0.92Ge2O7: a new member of the thortveitite family.

Indium gadolinium digermanium heptaoxide, In(1.08)Gd(0.92)Ge(2)O(7), with a thortveitite-type structure, has been prepared as a polycrystalline powder material by a high-temperature solid-state reaction. As in the mineral thortveitite, the crystal structure belongs to the monoclinic system, with space group C2/m (No. 12). The precise structural parameters were obtained by applying the Rietveld method of refinement to the X-ray powder diffraction data. This layered structure presents, on one side, a honeycomb-like arrangement of the unique octahedral site, which is occupied randomly by In and Gd atoms, and, on the other side, sheets of isolated Ge(2)O(7) diortho-groups made up of double tetrahedra sharing a common vertex and displaying C(2h) point symmetry. This compound showed a remarkable photoluminescence effect when it was irradiated with the X-ray beam during the X-ray diffraction measurements, and with the alpha beam during the Rutherford back-scattering spectrometry experiments employed to analyze the chemical stoichiometry.

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

A new indium holmium digermanate, In(1.06)Ho(0.94)Ge(2)O(7), 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 In(3+) and Ho(3+) 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 Ge(2)O(7) diortho groups lose the ideal D(3d) 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 180.0 degrees for Si-O-Si in thortveitite (Sc(2)Si(2)O(7)). 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).