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Indium gadolinium digermanium heptaoxide, In1.08Gd0.92Ge2­O7, 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 Ge2O7 diortho-groups made up of double tetrahedra sharing a common vertex and displaying C2h 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 α beam during the Rutherford back-scattering spectrometry experiments employed to analyze the chemical stoichiometry.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102013343/br1375sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108270102013343/br1375Isup2.rtv
Contains datablock I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270102013343/br1375Isup3.hkl
Contains datablock I

Comment top

Since Zachariasen (1930) published his work on the Sc2Si2O7 thortveitite structure, many compounds belonging to this structure type have been synthesized and their complete and precise structural data reported. In 1962, as a result of a reinvestigation of the crystal structure of thortveitite, Cruickshank et al. (1962) considered the possibility of exchanging Sc3+ with other cations, such as Y3+ and Fe3+, to observe the effects that this might have on the crystal symmetry, specifically on the point symmetry shown by the Ge2O7 diorthogroup and the presence or otherwise of a mirror plane, being the difference between C2 and C2/m as possible space groups for thortveitite.

In subsequent years, many compounds were synthesized which presented the thortveitite structure or variations of it, namely Sm2Si2O7 (Smolin et al., 1970), Y2Si2O7 (Batalieva & Pyatenko, 1971), Pr2Si2O7 (Felshe, 1971), In2Si2O7 (Reid et al., 1977; Gaewdang et al., 1994) and Gd2Si2O7 (Smolin & Shepelev, 1967). In the same way, the Si atom has been exchanged for Ge, giving rise to the pyrogermanate compounds Er2Ge2O7 (Smolin, 1970), Gd2Ge2O7 (Smolin et al., 1971), Eu2Ge2O7 (Chigarov et al., 1983) and so on. Many of these germanium-based compounds also kept the thortveitite structure, but some others changed their crystal symmetry.

Recently, the crystal structure of iron indium digermanate, FeInGe2O7, has been reported (Bucio et al., 2001), also having the typical thortveitite layered structure. This compound belongs to a new kind of monoclinic germanates, the general stoichiometric formula of which is MRGe2O7, where M and R represent trivalent metals (such as Al, Ga or Fe) and rare earth ions, respectively. Among these compounds can be found crystal symmetries described by the space groups C2/m (No. 12), as in thortveitite, C2 (No. 5), as in the gittinsite type, represented by InTbGe2O7 (Juarez-Arellano et al., 2002), P21/c (No. 14), as in FeRGe2O7 (R is La, Pr, Nd or Gd; Bucio et al., 1996), and P21/m (No. 11), as in FeRGe2O7 (R is Y or Tb—Yb; Cascales et al., 1998).

This kind of compound has been of great interest in laser crystal physics. For instance, the incorporation of R3+ activators into single-centred hosts up to full substitution of all cations gives the possibility of obtaining so called self-activated crystals. Other recently reported layered compounds with interesting optical responses are GdMnGe2O7 (Taviot-Gueho et al., 1999) and MnEuGe2O7 (Juarez-Arellano et al., 2001), these being the only cases in which an orthorhombic symmetry (space group A222, No. 21) can be found.

In recent years, compounds with rare earth cations, specially Gd3+, have been developed and employed as scintillators for applications in a variety of fields, such as medical imaging, high-energy physics, and space-borne gamma-ray astronomy (Moses et al., 1997; Yasunobu et al., 2001). The present work is devoted to the synthesis and crystal-structure characterization of a new gadolinium-based compound having the formula In1.08Gd0.92Ge2O7. This allowed us to study and understand the optical and other physical properties of this layered compound.

The title structure is built up of layers of InO6 and GdO6 octahedra in a nearly hexagonal arrangement, which are kept apart by Ge2O7 pyrogermanate groups presenting C2 h point group symmetry. The average (In,Gd)—O distance of 2.252 Å, evaluated over the unique octahedral site occupied by both In and Gd atoms, is very close to the sum of the ionic radii; rIn3+/Gd3+ + rO2- = 2.269 Å, where rIn3+/Gd3+ is the average value of the ionic radii for In3+ and Gd3+. The values used for these calculations were 0.80, 0.938 and 1.40 Å for rIn3+, rGd3+ and rO2-, respectively, according to the values reported by Shannon (1976).

The Ge—O distances in In1.08Gd0.92Ge2O7 are in the range 1.695–1.703 Å (average 1.698 Å). Therefore, the GeO4 tetrahedra are approximately regular, more than those described for the In2Ge2O7 compound reported by Gaewdang et al. (1994). In this last case, the average In—O distance for the InO6 octahedra is 2.177 Å, which is in agreement with the sum of the ionic radii of In and O, and lower than the corresponding value of 2.252 Å for the InGdGe2O7 compound reported here. All these findings are consistent with the addition of Gd.

Experimental top

InGdGe2O7 was prepared as a polycrystalline powder material by solid-state reaction from a stoichiometric mixture of analytical grade Gd2O3, GeO2 and In2O3. The sample was ground and heated in air at 1423 K for 5 d with intermediate regrindings. The standard X-ray powder diffraction analysis indicated that the final sample was well crystallized and appeared completely free of secondary crystalline phases. The elemental composition was determined by Rutherford backscattering spectrometry (RBS), following the procedure previously reported by Bucio et al. (2001). The stoichiometric values for In, Gd, Ge and O elements were 1.0, 1.0, 2.6 and 7.9% (±7%), respectively. The amounts of Ge and O are slightly bigger because of the presence of amorphous GeO2.

Refinement top

The powder diffraction pattern was indexed using the TREOR program (Werner, 1985). Following the criteria employed by Cruickshank et al. (1962), we considered that the title new InGdGe2O7 compound crystallizes in the thortveitite structure, the symmetry of which is described by the space group C2/m (No. 12). The Rietveld method was used to refine the crystal structure, using initial parameters from the In2Ge2O7 thortveitite-type structure (ICDD card No. 26–768, structural data from Gaewdang et al., 1994). The following parameters were refined: zero point, scale factor, six background polynomial coefficients, unit-cell parameters, half-width and asymmetry parameters for the peak shape, atomic coordinates and isotropic atomic displacement parameters. A total of 31 independent parameters were refined for the InGdGe2O7 germanate.

Computing details top

Data collection: DIFFRAC/AT (Siemens, 1993); cell refinement: DICVOL91 (Boultif & Louer, 1991); data reduction: Please provide missing information; program(s) used to solve structure: Please provide missing information; program(s) used to refine structure: FULLPROF (Wiles & Young, 1981); molecular graphics: ATOMS (Dowty, 1994); software used to prepare material for publication: ATOMS.

Figures top
[Figure 1] Fig. 1. The observed (+++) and calculated (solid line) X-ray powder diffraction profiles for InGdGe2O7 at room temperature. The difference profile is given at the bottom. Vertical marks correspond to the positions of the allowed Bragg reflections.
[Figure 2] Fig. 2. The projection of the InGdGe2O7 structure on the ab plane, showing the honeycomb-like arrangement of InO6 (or GdO6) octahedra (grey). The Ge2O7 diorthogroups (white) link to the hexagonal rings of octahedra at three points.
[Figure 3] Fig. 3. The RO6 octahedral layers, where R represents In or Gd atoms, are held together alternately along the c axis by sheets of isolated Ge2O7 diorthogroups. The shading convention is the same as in Fig. 2.
Indium gadolinium digermanium heptaoxide top
Crystal data top
In1.08Gd0.92Ge2O7V = 292.83 (3) Å3
Mr = 525.90Z = 2
Monoclinic, C2/mDx = 5.964 Mg m3
Hall symbol: -C 2yCu Kα1, Cu Kα2 radiation, λ = 1.54056, 1.544 Å
a = 6.8713 (4) ÅT = 295 K
b = 8.8805 (5) Åwhite
c = 4.8976 (3) Åflat sheet, 20 × 20 mm
β = 101.525 (2)°Specimen preparation: Prepared at 1423 K
Data collection top
Siemens D5000
diffractometer
Data collection mode: reflection
Radiation source: sealed X-ray tube, Cu KαScan method: step
Graphite monochromator2θmin = 14°, 2θmax = 90°, 2θstep = 0.02°
Specimen mounting: packed powder sample container
Refinement top
Least-squares matrix: full with fixed elements per cycle3801 data points
Rp = 0.118Profile function: pseudo-Voigt modified according to Thompson et al. (1987)
Rwp = ?31 parameters
Rexp = 0.07Weighting scheme based on measured s.u.'s
RBragg = 0.026
R(F2) = 0.02Background function: polynomial
χ2 = 2.822
Crystal data top
In1.08Gd0.92Ge2O7β = 101.525 (2)°
Mr = 525.90V = 292.83 (3) Å3
Monoclinic, C2/mZ = 2
a = 6.8713 (4) ÅCu Kα1, Cu Kα2 radiation, λ = 1.54056, 1.544 Å
b = 8.8805 (5) ÅT = 295 K
c = 4.8976 (3) Åflat sheet, 20 × 20 mm
Data collection top
Siemens D5000
diffractometer
Scan method: step
Specimen mounting: packed powder sample container2θmin = 14°, 2θmax = 90°, 2θstep = 0.02°
Data collection mode: reflection
Refinement top
Rp = 0.118R(F2) = 0.02
Rwp = ?χ2 = 2.822
Rexp = 0.073801 data points
RBragg = 0.02631 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
In0.000000.3033 (1)0.000000.0057 (6)0.54 (2)
Gd0.000000.3033 (1)0.000000.0057 (6)0.46 (2)
Ge0.2284 (3)0.000000.4143 (4)0.013 (1)
O10.000000.000000.500000.014 (2)
O20.388 (1)0.000000.727 (2)0.014 (2)
O30.2394 (9)0.1596 (7)0.228 (1)0.014 (2)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
???????
Geometric parameters (Å, º) top
Gd—O2i2.239 (6)Ge—O11.703 (2)
Gd—O32.202 (6)Ge—O21.698 (9)
Gd—O3ii2.316 (6)Ge—O31.695 (6)
O2i—Gd—O2iii77.5 (5)O3—Gd—O3v114.1 (4)
O2i—Gd—O3152.5 (5)O3ii—Gd—O3v163.7 (5)
O2i—Gd—O3iv91.0 (3)O1—Ge—O2103.9 (6)
O2i—Gd—O3ii78.9 (4)O1—Ge—O3106.0 (4)
O2i—Gd—O3v88.4 (4)O2—Ge—O3113.3 (2)
O3—Gd—O3iv109.1 (4)O3—Ge—O3vi113.5 (6)
O3—Gd—O3ii75.9 (3)
Symmetry codes: (i) x1/2, y+1/2, z1; (ii) x+1/2, y+1/2, z; (iii) x+1/2, y+1/2, z+1; (iv) x, y, z; (v) x1/2, y+1/2, z; (vi) x, y, z.

Experimental details

Crystal data
Chemical formulaIn1.08Gd0.92Ge2O7
Mr525.90
Crystal system, space groupMonoclinic, C2/m
Temperature (K)295
a, b, c (Å)6.8713 (4), 8.8805 (5), 4.8976 (3)
β (°) 101.525 (2)
V3)292.83 (3)
Z2
Radiation typeCu Kα1, Cu Kα2, λ = 1.54056, 1.544 Å
Specimen shape, size (mm)Flat sheet, 20 × 20
Data collection
DiffractometerSiemens D5000
diffractometer
Specimen mountingPacked powder sample container
Data collection modeReflection
Scan methodStep
2θ values (°)2θmin = 14 2θmax = 90 2θstep = 0.02
Refinement
R factors and goodness of fitRp = 0.118, Rwp = ?, Rexp = 0.07, RBragg = 0.026, R(F2) = 0.02, χ2 = 2.822
No. of data points3801
No. of parameters31
No. of restraints?

Computer programs: DIFFRAC/AT (Siemens, 1993), DICVOL91 (Boultif & Louer, 1991), Please provide missing information, FULLPROF (Wiles & Young, 1981), ATOMS (Dowty, 1994), ATOMS.

Selected geometric parameters (Å, º) top
Gd—O2i2.239 (6)Ge—O11.703 (2)
Gd—O32.202 (6)Ge—O21.698 (9)
Gd—O3ii2.316 (6)Ge—O31.695 (6)
O1—Ge—O2103.9 (6)O2—Ge—O3113.3 (2)
O1—Ge—O3106.0 (4)O3—Ge—O3iii113.5 (6)
Symmetry codes: (i) x1/2, y+1/2, z1; (ii) x+1/2, y+1/2, z; (iii) x, y, z.
 

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