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Acta Crystallogr Sect E Struct Rep Online. 2009 July 1; 65(Pt 7): i49–i50.
Published online 2009 June 6. doi:  10.1107/S1600536809020467
PMCID: PMC2969248

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

Abstract

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 [triangle]). 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.

Related literature

The method of preparation was based on work published by Cascales et al. (1998b [triangle]). For related structures, see: Zachariasen (1930 [triangle]); Cascales et al. (1998a [triangle],b [triangle], 2002 [triangle]); Bucio et al. (2001 [triangle]); Redhammer et al. (2007 [triangle]).

Experimental

Crystal data

  • Y0.76Ho0.24FeGe2O7
  • M r = 420.17
  • Monoclinic, An external file that holds a picture, illustration, etc.
Object name is e-65-00i49-efi1.jpg
  • a = 9.6496 (2) Å
  • b = 8.5073 (2) Å
  • c = 6.6712 (2) Å
  • β = 100.621 (1)°
  • V = 538.27 (2) Å3
  • Z = 4
  • Cu Kα radiation
  • T = 295 K
  • Specimen shape: flat sheet
  • 20 × 20 × 0.2 mm
  • Specimen prepared at 1573 K
  • Particle morphology: spherical, brown

Data collection

  • Bruker Advance D8 diffractometer
  • Specimen mounting: packed powder sample container
  • Specimen mounted in reflection mode
  • Scan method: step
  • min = 10, 2θmax = 80.0°
  • Increment in 2θ = 0.02°

Refinement

  • R p = 0.07
  • R wp = 0.09
  • R exp = 0.06
  • R B = 0.03
  • S = 1.53
  • Wavelength of incident radiation: 1.54175 Å
  • Profile function: pseudo-Voigt modified by Thompson et al. (1987 [triangle])
  • 843 reflections
  • 60 parameters

Data collection: DIFFRAC/AT (Siemens, 1993 [triangle]); cell refinement: DICVOL91 (Boultif & Louër, 1991 [triangle]); data reduction: FULLPROF (Rodríguez-Carvajal, 2006 [triangle]); program(s) used to solve structure: coordinates taken from an isotypic compound (Cascales et al., 2002 [triangle]) show [/query]>; program(s) used to refine structure: FULLPROF ; molecular graphics: ATOMS (Dowty, 2000 [triangle]); software used to prepare material for publication: ATOMS.

Table 1
Selected geometric parameters (Å, °)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809020467/br2109sup1.cif

Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536809020467/br2109Isup2.rtv

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors acknowledge the collaboration of Adolfo Cordero and Edilberto Hernández for performing the X-ray diffraction measurements at the Instituto de Física, Universidad Nacional Autónoma de México (IFUNAM), and Angel Osornio for his technical support. IR acknowledges the fellowship of the Consejo Nacional de Ciencia y Tecnología (CONACyT) for a postdoctoral position, and projects CONACyT SEP-2007-81700, CONACyT SEP-2007-80380 and DGAPA-PAPIIT IN109308.

supplementary crystallographic information

Comment

The crystal structure of the original thortveitite was first reported by Zachariasen (1930) and has the formula Sc2Si2O7. 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 RMGe2O7, 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 Ge2O7 groups in staggered conformation (in Fig. 1a the octahedra appear in dark cyan, while the Ge2O7 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 MRX2O7 where X2O7 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,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 Y1-xHoxFeGe2O7 with x = 0.24.

Fig. 1b show the crystal structure of Y0.76Ho0.24FeGe2O7 in which the bridging O atoms at the middle of the Ge2O7 diorthogroup are displaced up (u) or down (d) along a direction normal to the cb plane. RO7 polyhedra are connected alternately by either a vertex or an edge into chains along the b axis, Fig. 1b (medium slate blue colored polyhedra). In the same direction only isolated pairs of associated MO6 octahedra exist, as can be seen in the same figure (light gray octahedra). Flattened chains of RO7 polyhedra (in yellow) are linked in the c direction through pairs of MO6 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 Ge2O7 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 used. The quantitative analysis gave 91.2 (8)% for Y0.76Ho0.24FeGe2O7 and 8.8 (3)% for Y2Ge2O7. With these results, the chemical reaction compatible with the quantitative analysis is:

4Y2O3 + Ho2O3 + 5Fe2O3 + 30GeO2→ 8.43Y0.76Ho0.24FeGe2O7 + 0.79Y2Ge2O7 + 11.56 GeO2 + 0.78Fe2O3.

Experimental

The reactive mixture was prepared from Y2O3 (Aldrich.99.99%), Ho2O3 (Aldrich.99.9%), Fe2O3 (Aldrich.99.99%) and GeO2 (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 GeO2. Small amount of Fe2O3 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 YFeGe2O7 (PDF 01-072-6099) and the other one was identified as Y2Ge2O7 (PDF 38-288).

Refinement

The structural model for YFeGe2O7 (ICSD 95935) was taken for start the Rietveld refinement of Y1-xHoxFeGe2O7 with x = 1/5, while for the secondary phase, the data used for Y2Ge2O7 (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 Y2Ge2O7 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.

Figures

Fig. 1.
(a) View of a layer in the thortveitite structure (ab projection). Diorthogroups X2O7 are represented as yellow tetrahedra and MO6 octahedra in dark cyan. (b) View of thortveitite-like structure of Y0.76Ho0.24FeGe2O7 (ac projection). Diorthogroups Ge2O7 ...
Fig. 2.
Rietveld refinement for Y0.76Ho0.24FeGe2O7 X-ray diffraction data. Observed (crosses), calculated (solid line) and difference (bottom trace) plots are represented; vertical marks correspond to the allowed Bragg reflections for Y0.76Ho0.24FeGe2O7 (top) ...

Crystal data

Y0.76Ho0.24FeGe2O7Z = 4
Mr = 420.17F(000) = 768.0
Monoclinic, P21/mDx = 5.186 Mg m3
Hall symbol: -P 2ybCu Kα radiation, λ = 1.54175 Å
a = 9.6496 (2) ÅT = 295 K
b = 8.5073 (2) Åbrown
c = 6.6712 (2) Åflat sheet, 20 × 20 mm
β = 100.621 (1)°Specimen preparation: Prepared at 1573 K
V = 538.27 (2) Å3

Data collection

Bruker Advance D8 diffractometerData collection mode: reflection
Radiation source: sealed X-ray tube, Cu KαScan method: step
graphitemin = 10°, 2θmax = 80.00°, 2θstep = 0.02°
Specimen mounting: packed powder sample container

Refinement

Least-squares matrix: full with fixed elements per cycle3501 data points
Rp = 0.07Profile function: pseudo-Voigt modified by Thompson et al. (1987)
Rwp = 0.0960 parameters
Rexp = 0.06Weighting scheme based on measured s.u.'s
RBragg = 0.03(Δ/σ)max = 0.02
R(F2) = 0.03Background function: linear interpolation between a set background points with refinable heights
χ2 = 2.341

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

xyzUiso*/UeqOcc. (<1)
Y0.7521 (11)0.5406 (3)0.7509 (10)0.0120 (5)0.76000
Ho0.7521 (11)0.5406 (3)0.7509 (10)0.0120 (5)0.24000
Fe0.7467 (15)0.4473 (5)0.2648 (16)0.0120 (5)
Ge10.5263 (14)0.750000.0459 (17)0.0120 (5)
Ge20.5471 (13)0.250000.4897 (14)0.0120 (5)
Ge30.9469 (13)0.250000.0252 (15)0.0120 (5)
Ge40.0310 (13)0.250000.5427 (18)0.0120 (5)
O10.627 (3)0.427 (3)0.499 (5)0.0120 (5)
O20.874 (4)0.250000.389 (4)0.0120 (5)
O30.966 (3)0.250000.766 (5)0.0120 (5)
O40.590 (3)0.250000.792 (4)0.0120 (5)
O50.845 (3)0.070 (3)0.028 (4)0.0120 (5)
O60.130 (6)0.250000.119 (6)0.0120 (5)
O70.150 (3)0.083 (3)0.502 (3)0.0120 (5)
O80.375 (5)0.250000.334 (5)0.0120 (5)
O90.606 (4)0.250000.186 (5)0.0120 (5)
O100.640 (3)0.584 (3)0.070 (3)0.0120 (5)

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
???????

Geometric parameters (Å, °)

Fe—O12.11 (4)Ge1—O101.78 (3)
Fe—O22.15 (2)Ge1—O10vi1.78 (3)
Fe—O5i1.99 (3)Ge2—O11.69 (3)
Fe—O7ii2.04 (2)Ge2—O1i1.69 (3)
Fe—O92.16 (2)Ge2—O41.98 (3)
Fe—O101.90 (2)Ge2—O81.78 (4)
Ho—O12.11 (3)Ge3—O3vii1.77 (4)
Ho—O42.96 (2)Ge3—O51.82 (3)
Ho—O5iii2.12 (3)Ge3—O5i1.82 (3)
Ho—O6ii2.20 (3)Ge3—O6viii1.76 (6)
Ho—O7ii2.11 (3)Ge4—O2ix1.67 (3)
Ho—O8ii2.18 (3)Ge4—O3ix1.73 (4)
Ho—O10iv2.59 (3)Ge4—O71.87 (3)
Ge1—O4ii1.70 (3)Ge4—O7i1.87 (3)
Ge1—O9v1.82 (3)
O1—Ho—O457.2 (1)O1—Ho—O8ii87.3 (2)
O1—Ho—O5iii125 (2)O1—Ho—O10iv117.1 (2)
O1—Ho—O6ii149 (2)Ge1x—O4—Ge2127.7 (16)
O1—Ho—O7ii73.6 (2)Ge3iv—O3—Ge4viii165 (2)

Symmetry codes: (i) x, −y+1/2, z; (ii) −x+1, y+1/2, −z+1; (iii) x, −y+1/2, z+1; (iv) x, y, z+1; (v) −x+1, y+1/2, −z; (vi) x, −y+3/2, z; (vii) x, y, z−1; (viii) x+1, y, z; (ix) x−1, y, z; (x) −x+1, y−1/2, −z+1.

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BR2109).

References

  • Boultif, A. & Louër, D. (1991). J. Appl. Cryst.24, 987–993.
  • Bucio, L., Ruvalcaba-Sil, J. L., Rosales, I., García-Robledo, J. & Orozco, E. (2001). Z. Kristallogr 216, 438–441.
  • Cascales, C., Bucio, L., Gutiérrez-Puebla, E., Rasines, I. & Fernández-Díaz, M. T. (1998a). Phys. Rev. B, 57, 5240–5249.
  • Cascales, C., Bucio, L., Gutiérrez-Puebla, E., Rasines, I. & Fernández-Díaz, M. T. (1998b). J. Alloys Compd.275–277, 629–632.
  • Cascales, C., Fernández-Díaz, M. T., Monge, M. A. & Bucio, L. (2002). Chem. Mater.14, 1995–2003.
  • Dowty, E. (2000). ATOMS for Windows Shape Software, Kingsport, Tennessee, USA.
  • Redhammer, G. J., Roth, G. & Amthauer, G. (2007). Acta Cryst. C63, i93–i95. [PubMed]
  • Rodríguez-Carvajal, J. (2006). FULLPROF. http://www.ill.eu/sites/fullprof/php/reference.html
  • Siemens (1993). DIFFRAC/AT Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin, USA.
  • Thompson, P., Cox, D. E. & Hastings, J. B. (1987). J. Appl. Cryst.20, 79–83.
  • Zachariasen, W. H. (1930). Z. Kristallogr.73, 1–6.

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