Ivonne Rosales

The crystal structure of FeInGe2O7

Abstract 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 = 6.5124(4) Å, b = 8.5914(5) Å, c = 4.8936(3) Å, β = 102.683(2)°, V = 267.12(3) Å3 and Z = 2. The structure contains R+3 cations (R=Fe, In) almost equally distributed in distorted RO6 octahedral sites. These octahedra are joined by edge sharing forming a hexagonal arrangement on the ab planes. The RO6 octahedra layers are held together by sheets of isolated Ge2O7 diorthogroups constituted by a double tetrahedra sharing a common vertex. This compound has the thortveitite structure and keeps a strong relationship with the FeYGe2O7 germanate, which presents two R+3 sites with six-coordinated (R=Fe) and seven-coordinated (R=Y) oxygens, corresponding to the different symmetry given by the monoclinic space group P21/m (No. 11)

Grain growth and phase transformations induced by shock waves on alpha-GeO2 powder

An impact experiment on a mixture of water and microcrystalline alpha-GeO2 powder was performed with a single-stage gas gun. The recovered sample contained micrometer-scale crystals of different sizes and morphologies that correspond to 88% of alpha-GeO2, 6.0% of monoclinic phase (P21/c, space group No. 14), 4.9% of orthorhombic phase (Pnnm, space group No. 58) and 1.1% of rutile-type phase.

A synchrotron study of Na2.27Ho7.73(SiO4)6O0.72

A well crystallized powder sample of sodium holmium orthosilicate oxyapatite, Na2.27Ho7.73(SiO4)6O0.72, 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 pentagonal 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.

Raman analysis of an impacted α-GeO2–H2O mixture

Through a Raman analysis, we detected polymorphism at high pressure on mixtures of α-GeO2 microcrystalline powder and water under impact experiments with a single-stage gas gun. The Raman measurements taken from recovered samples show two vibrational modes associated with water-related species. After the impact, the size of the α-GeO2 crystallites was approximately 10 times higher showing molten zones and a lot of porous faces. Raman examination showed some unknown peaks possibly associated with other GeO2 polymorphs detected by X-ray diffraction experiments and perhaps stabilized in the porous of the α-GeO2 crystallites.

Phase transitions induced by shock compression on a gypsum mineral: X-ray and micro-Raman analysis

As a part of systematic researches of phase transitions induced by shock compression in phosphates, silicates, germanates and sulfates, in this article we report preliminary results obtained from shock recovery experiments on powders of a gypsum mineral. The shock experiment was performed in a light gas gun until a pressure close to 14 GPa reached. The experimental techniques employed to analyze the shock effects on recovered samples were: Scanning Electron Microscopy (SEM), X-ray Powder Diffraction (XRD) and Micro-Raman Spectroscopy (MRS). The SEM observations show a high plasticity in the impacted sample composed mainly by gypsum and bassanite quantified by Rietveld analysis of the XRD. The results indicate the partial dehydration of gypsum as a result of impact. The MRS analysis suggests the presence of micro-mixtures of gypsum, bassanite and anhydrite heterogeneously distributed throughout the recovered sample.

Y0.76Ho0.24FeGe2O7: a new member of thortveitite-like layered compounds

Y0.76Ho0.24FeGe2O7 (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 FeInGe2O7, In1.08Gd0.92Ge2O7 and InYGe2O7, which are configurationally isotypic with the Sc2Si2O7 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 Ge2O7 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.

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.

Symmetry Relations Between Space Groups in Layered Germanate Structures: Modeling Crystal Structures

Structural models for the new layered germanates ScInGe2O7 and ScFeGe2O7 were analyzed within the framework of symmetry relations between space groups. These compounds were supposed to be hettotypes of the thortveitite mineral, (Sc,Y)2Si2O7, which was considered as the aristotype. Thortveitite crystallizes in the monoclinic system, and the symmetry is described by the space group type C2/m. Other monoclinic hettotypes for the thortveitite are FeInGe2O7 (PDF 01-070-8447, ICSD 94487), space group C2/m (No. 12); TbInGe2O7 (PDF 01-072-6515, ICSD 96360), space group C2/c (No. 15); and FeYGe2O7 (PDF 01-072-6099, ICSD 95935), space group P21/m (No. 11). All these space groups are related by symmetry. By the use of these relations, we proposed starting models for the crystal structures of ScInGe2O7 and ScFeGe2O7. For ScInGe2O7 this was found to be isostructural to FeInGe2O7 reported by our laboratory [1]. The structural data for this compound were obtained by conventional Rietveld refinement of the powder diffraction data of X-rays, using the GSAS program and EXPGUI [2, 3] interface. For ScFeGe2O7 the symmetry related structural model was found in the triclinic system by symmetry reduction from the space group C2/m (unique axis b) to the triclinic space group P1 (figure 1). Rietveld refinement was performed reaching to the following results: lattice parameters a = 5.3434 (8), b = 5.3145 (8), c = 4.8732 (7 ) Å, α = 99 468 (2), β = 97 257 (2), γ = 104 609 (2) °, V = 130.03 (5) A3, Z = 1; WRp = 0.047, Rp = 0.04 and reduced χ2 of 2.176 for 64 variables. This study was sponsored by CONACyT project CB-2011/167624.

Rietveld refinement of the mixed boracite Fe1.59Zn1.41B7O13Br

The structural characterization of the new iron–zinc heptaborate bromide with composition Fe1.59Zn1.41B7O13Br, prepared by chemical transport is reported. A rigid-body model with constrained generalized coordinates was defined in order to hold the positions of the B atoms at reasonable interatomic distances that typically would reach unacceptable values because of the weak scattering power of boron. There are three independent sites for the B atoms of which two are tetrahedrally coordinated. The bond-valence sum around the third B atom, located on a threefold rotation axis, was calculated considering two cases of coordination of boron with oxygens: trigonal-planar and tetrahedral. The contribution of the fourth O atom to the bond-valence sum was found to be only 0.06 v.u., indicating the presence of a very weak bond in the right position to have a distorted tetrahedral coordination in favour of the trigonal-planar coordination for the third B atom. X-ray fluorescence (XRF) was used to determinate the Fe/Zn ratio.

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).