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.

Y 0.76 Ho 0.24 FeGe 2 O 7 (yttrium holmium iron digermanate) was synthesized by solid-state reaction at 1573 K. This thortveititelike 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][2][3][4][5][6]. Holmium cations share with yttrium the 4f Wyckoff position at the center of a sevencoordinated 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.

Comment
The crystal structure of the original thortveitite was first reported by Zachariasen (1930) and has the formula Sc 2 Si 2 O 7 .
In this silicate, the substitution of silicon has given rise to germanates, phosphates, arsenates and vanadates which present layered structures.
The ionic substitution from Si to Ge and Sc to trivalent metals and rare earths in thortveitite, give often germanates with thortveitite-like crystal structure RMGe 2 O 7 , where R and M represent cations of rare earths, transition metals, divalent or trivalent elements in octahedral coordination. The frameworks of these phases are built up from corner-sharing octahedra along ab planes forming a hexagonal disposition on layers interspersed with layers of Ge 2 O 7 groups in staggered conformation (in Fig. 1a the octahedra appear in dark cyan, while the Ge 2 O 7 group in yellow color).
Some ionic substitutions give rise to seven-coordinated cations occupying the half of octahedral sites in the thortveitite structure. In such case, the generalized formula can be written as MRX 2 O 7 where X 2 O 7 is the same diorthogroup mentioned before presenting almost the same features as in thortveitite. The octahedral sites split in two new sites: half for cation M and other half for cation R, such as the cases for R = Y, Tb-Yb (Cascales et al., 1998a(Cascales et al., ,b, 2002. M remains with octahedral coordination while R changes its coordination to seven. In the present work we present the crystal structure of the new compound Y 1-x Ho x FeGe 2 O 7 with x = 0.24. Flattened chains of RO 7 polyhedra (in yellow) are linked in the c direction through pairs of MO 6 octahedra with which they share edges forming layers running parallel to the bc crystal plane.
The most important feature in the structure previously described is the presence of Ge-O-Ge angles in the Ge 2 O 7 group different from 180° giving rise to seven-coordinated cations in a half of the octahedral sites in the idealized thortveitite structure. The results of the Rietveld refinement established the presence of two crystalline phases for the method of synthesis GeO 2 (CERAC 99.999%) according to the method reported by Cascales et al. (1998b). This mixture was first powdered using an agate mortar; and then was heated in air in a tube furnace at 1573 K for 5 d with intermediate regrinding. At the end of the reaction, some vitreous phase impregnated and segregated at the bottom of the crucible was attributed to the presence of amorphous GeO 2 . Small amount of Fe 2 O 3 was also detected as trace phase. The characterization of the bulk material by conventional X-ray powder diffraction data indicated two phases well crystallized. One of them showed reflections that were explained matching the isostructural phase YFeGe 2 O 7 (PDF 01-072-6099) and the other one was identified as Y 2 Ge 2 O 7 (PDF 38-288).

Refinement
The structural model for YFeGe 2 O 7 (ICSD 95935) was taken for start the Rietveld refinement of Y 1-x Ho x FeGe 2 O 7 with x = 1/5, while for the secondary phase, the data used for Y 2 Ge 2 O 7 (ICSD 240989) was those reported by Redhammer et al. (2007). The Rietveld refinement was made using the Fullprof program (Rodríguez-Carvajal, 2006). A pseudo-Voigt function modified by Thompson et al. (1987) was chosen to generate the peak shape of the diffraction reflections. The following parameters were refined: zero point and scale factors, cell parameters, half-width profile parameters, overall temperature factors, preferred orientation, atomic coordinates, and asymmetries. For the Y 2 Ge 2 O 7 phase no preferred orientation was considered, and the atomic coordinates were fixed to their starting values and an overall temperature factor was considered.
The background was refined first by mean of a linear interpolation between 55 background points with adjustable heights.
At the end of the refinement, the values for all of these heights of the background were fixed. The final Rietveld refinement of conventional diffraction pattern is shown in Figure 21.   Geometric parameters (Å, °)