Acta Cryst. (2009). E65, i60 [ doi:10.1107/S1600536809026579 ]
Y2GeO5 (yttrium germanium pentaoxide) was synthesized by solid-state reaction at 1443 K. The arrangement, which has monoclinic symmetry, is isostructural with Dy2GeO5 and presents two independent sites for the Y atoms. Around these atoms there are distorted six-coordinated YO6 octahedra and seven-coordinated YO7 pentagonal bipyramids. The YO7 polyhedra are linked together, sharing their edges along a surface parallel to ab, forming a sheet. Each of these parallel sheets is interconnected by means of GeO4 tetrahedra, sharing an edge (or vertex) on one side and a vertex (or edge) on the other adjacent side. Parallel sheets of YO7 polyhedra are also interconnected by undulating chains of YO6 octahedra along the c axis. These octahedra are joined together, sharing a common edge, to form the chain and share edges with the YO7 polyhedra of the sheets.
The reactive mixture was prepared from Y2O3 (Aldrich.99.99%) and GeO2 (CERAC 99.999%) according to the stoichiometric proportions desired. The mixture was first powdered using an agate mortar; and then was heated in air in a tube furnace at 1373 K for 5 days with intermediate regrindings. A second thermal treatment at 1443 K for two days was applied. The characterization of the bulk material by conventional X-ray powder diffraction data indicated the presence of a well crystallized phase showing reflections that match with the isostructural phase DyGeO5 (PDF 01–078–0478). Very small amount of a secondary phase Y2Ge2O7 (PDF 38–288) was identified.
The starting structural parameters for perform a Rietveld refinement of the Y2GeO5 phase were taken from the isostructural data reported for Dy2GeO5 (ICSD 61373) by Brixner et al. (1985). For modeling the second phase Y2Ge2O7 (ICSD 240989), the data were those reported by Redhammer et al. (2007). The following parameters were refined: zero point and scale factors, cell parameters, half-width profile parameters, overall temperature factors, atomic coordinates, and asymmetries. For the Y2Ge2O7 phase the atomic coordinates were fixed to their starting values. The final Rietveld refinement of conventional diffraction pattern is shown in Fig. 2.
Data collection: DIFFRAC/AT (Siemens, 1993); cell refinement: DICVOL91 (Boultif & Lou1"er 1991); data reduction: FULLPROF (Rodríguez-Carvajal, 2006); program(s) used to solve structure: coordinates were taken from isotypic compound; program(s) used to refine structure: FULLPROF (Rodríguez-Carvajal, 2006); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: ATOMS (Dowty, 2000).
![[Figure 1]](./br2110fig1thm.gif)
| Fig. 1. (a) View of a YO7 layer in the Y2GeO5 structure (ab projection). YO7 polyhedra are represented in medium slate blue while GeO4 tetrahedra are represented in yellow. (b) View of the Y2GeO5 structure (ac projection). The layers formed by YO7 polyhedra are linked together by GeO4 tetrahedra. (c) Chains of YO6 octahedra (in gray) undulating along the c axis sharing common edges and linked with other chains by mean of GeO4 tetrahedra. (d) Unit cell for Y2GeO5 structure. |
![[Figure 2]](./br2110fig2thm.gif)
| Fig. 2. Rietveld refinement for 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 Y2GeO5 (top) and Y2Ge2O7 (bottom) as secondary phase. |
| Y2Ge1O5 | Z = 8 |
| Mr = 330.43 | F000 = 1200 |
| Monoclinic, I2/a | Dx = 4.868 Mg m−3 |
| Hall symbol: -I 2ya | Cu Kα radiation, λ = 1.540560 Å |
| a = 10.4706 (2) Å | T = 300 K |
| b = 6.8292 (1) Å | Specimen form: flat sheet; particle morphology spherical; white |
| c = 12.8795 (2) Å | 20 × 20 × 0.2 mm |
| β = 101.750 (3)º | Specimen preparation: temperature 1443 K |
| V = 901.66 (3) Å3 |
| Bruker Advance D8 diffractometer | Scan method: step |
| Monochromator: graphite | T = 300 K |
| Specimen mounting: packed powder sample container | 2θmin = 8.00, 2θmax = 80.02º |
| Specimen mounted in reflection mode | Increment in 2θ = 0.02º |
| Least-squares matrix: full with fixed elements per cycle | Profile function: pseudo-Voigt modified by Thompson et al. (1987) |
| Rp = 0.053 | 105 parameters |
| Rwp = 0.069 | Weighting scheme based on measured s.u.'s ? |
| Rexp = 0.024 | (Δ/σ)max = 0.02 |
| S = 2.90 | Extinction coefficient: ? |
| Y2Ge1O5 | β = 101.750 (3)º |
| Mr = 330.43 | V = 901.66 (3) Å3 |
| Monoclinic, I2/a | Z = 8 |
| a = 10.4706 (2) Å | Cu Kα radiation, λ = 1.540560 Å |
| b = 6.8292 (1) Å | T = 300 K |
| c = 12.8795 (2) Å | 20 × 20 × 0.2 mm |
| Bruker Advance D8 diffractometer | Scan method: step |
| Specimen mounting: packed powder sample container | 2θmin = 8.00, 2θmax = 80.02º |
| Specimen mounted in reflection mode | Increment in 2θ = 0.02º |
| Rp = 0.053 | Excluded region(s): ? |
| Rwp = 0.069 | Profile function: pseudo-Voigt modified by Thompson et al. (1987) |
| Rexp = 0.024 | 105 parameters |
| RB = ? | ? restraints |
| S = 2.90 | Preferred orientation correction: ? |
| x | y | z | Uiso*/Ueq | ||
| Y1 | 0.3011 (2) | 0.6277 (2) | 0.6380 (1) | 0.0096 (7) | |
| Y2 | 0.0708 (2) | 0.2567 (3) | 0.5355 (1) | 0.0090 (7) | |
| Ge1 | 0.6236 (2) | 0.5933 (3) | 0.8155 (2) | 0.0121 (9) | |
| O1 | 0.1210 (9) | 0.604 (1) | 0.5178 (8) | 0.009 (2) | |
| O2 | 0.2950 (9) | 0.298 (1) | 0.6172 (7) | 0.009 (2) | |
| O3 | 0.5212 (9) | 0.654 (1) | 0.6971 (8) | 0.009 (2) | |
| O4 | 0.551 (1) | −0.006 (1) | 0.4155 (8) | 0.009 (2) | |
| O5 | 0.2412 (8) | 0.572 (1) | 0.7926 (8) | 0.009 (2) |
| Y1—O1 | 2.189 (8) | Y2—O2i | 2.655 (10) |
| Y1—O1i | 2.321 (10) | Y2—O3iv | 2.327 (10) |
| Y1—O2 | 2.270 (8) | Y2—O4i | 2.358 (9) |
| Y1—O3 | 2.283 (8) | Y2—O4v | 2.287 (9) |
| Y1—O5 | 2.238 (10) | Ge1—O2vi | 1.767 (8) |
| Y1—O5ii | 2.316 (8) | Ge1—O3 | 1.727 (8) |
| Y2—O1 | 2.447 (8) | Ge1—O4vii | 1.732 (10) |
| Y2—O1iii | 2.203 (8) | Ge1—O5viii | 1.739 (9) |
| Y2—O2 | 2.386 (8) | ||
| Y1—O1—Y1ix | 53.6 (2) | Y1—O2—Y2xiii | 118.8 (3) |
| Y1—O1—Y2x | 128.7 (3) | Y2xii—O2—Y2xiii | 61.19 (6) |
| Y1ix—O1—Y2x | 78.64 (6) | Y1—O3—Y2xiv | 110.7 (3) |
| Y1ix—O1—Y2xi | 90.44 (7) | Y2xv—O4—Y2xvi | 124.20 (7) |
| Y2x—O1—Y2xi | 157.05 (6) | Y1—O5—Y1xvii | 89.8 (2) |
| Y1—O2—Y2xii | 97.4 (2) |
| Symmetry codes: (i) −x+1/2, y, −z+1; (ii) −x+1/2, −y+3/2, −z+3/2; (iii) −x, −y+1, −z+1; (iv) x−1/2, −y+1, z; (v) x−1/2, −y, z; (vi) −x+1, y+1/2, −z+3/2; (vii) x, −y+1/2, z+1/2; (viii) x+1/2, −y+1, z; (ix) −x+3/2, y, −z+1; (x) −x, −y, −z; (xi) x+1/2, −y+1, z+1; (xii) −x+1/2, y, −z; (xiii) −x+1, −y, −z+1; (xiv) −x+3/2, y+1, −z; (xv) x+1, y, z+1; (xvi) −x+3/2, y, −z; (xvii) −x+3/2, y+2, −z+2. |
The authors acknowledge the collaboration of Manuel Aguilar Franco for performing the conventional X-ray diffraction measurements, and projects CONACyT SEP-2007–81700.
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Field emission display (FED) constitutes the next generation of information display devices. Its advantages include portable size with low power consumption, broad viewing angle, and wide operating-temperature range among others (Zhao et al., 2003). New multicomponent oxide phosphor, Mn-activated Y2O3—GeO2, is promising as the thin-film emitting layer for thin-film electroluminescent (TFEL) devices (Minami et al., 2001). The oxide phosphor for use in those electroluminescent devices is formed from yttrium oxide and a transition metal as an activator, or from Y—Ge—O oxide and one metallic element to form M:Y2GeO5 where M is a metal (Minami et al., 2002; Minami et al., 2004). Other reported use for Y2GeO5 consists in piezoelectric ceramics in the form of films which include a complex oxide material having an oxygen octahedral structure and a paraelectric material having a catalytic effect for the complex oxide material in a mixed state. Paraelectric material could be a layered compound having an oxygen tetrahedral structure which includes one compound with the form MSiOx (M=metal) and Y2GeO5 (Natori et al., 2004). Fig. 1a show a fragment of the crystal structure of Y2GeO5 along the ab plane in which YO7 polyhedra share common edges forming a mesh. These YO7 polyhedra are represented as medium slate blue. Over the mesh, there are isolated GeO4 tetrahedra, which are represented in yellow in Fig. 1a. Each one of these parallel sheets are interconnected by means of GeO4 tetrahedra, sharing an edge (or vertix) in one side and a vertix (or edge) in the other adjacent side respectively as can be seen in Fig. 1b. Undulating chains of YO6 octahedra along the c axis are represented in gray in Fig. 1c in which the YO7 polyhedra were not represented in order lo clarify this feature of the arrangement. The chains of YO6 octahedra also interconnect the parallel sheets of YO7 polyhedra, as can be see in the unit cell of Y2GeO5 represented in Fig. 1d. Bond valence calculations were made using the recommended bond-valence parameters for oxides published by Brese & O'Keeffe (1991). Bond valence sum (BVS) around six-coordinated Y1, seven-coordinated Y2, and Ge give the values of 3.03, 2.76 and 4.08 respectively, being the first and the last closer to the values of +3 and +4 expected for the yttrium and germanium atoms respectively. The second value of 2.76 was first interpreted as stretched bonds around Y2 exist, but this suggestion was withdrawn because there is no compressed cation in the unit cell capable to balance the supposed stretched bonds around Y2, as it is established in the Brown's bond valence model (Brown, 1981) for evaluating the existence of stresses in the crystal. In fact, calculating the so called Global Instability Index, which is obtained as the root mean square of the bond-valence sum deviation for all the N atoms present in the asymmetric unit (Brown, 1992) a value of 0.06 was obtained suggesting no strain. This is a remarkably low value for a Rietveld refinement (for a well refined and unstrained structure this is less than 0.1). Then, the low value of the bond valence sum around Y2 is well within normal limits for a Rietveld refinment where larger deviations are typically found.