Rietveld refinement of Y₂GeO₅

Abstract

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.

Related literature

For the isotypic structure of Dy₂GeO₅, see: Brixner et al. (1985 ▶). Different synthesis methods have been reported for this compound, including preparation by conventional r.f. magnetron sputtering (Minami et al., 2003 ▶), solid-state reactions at high temperatures (Zhao et al., 2003 ▶), MOCVD and LSMCD (Natori et al., 2004 ▶). For bond-valence parameters, see: Brese & O’Keeffe (1991 ▶), and for the bond-valence model, see: Brown (1981 ▶, 1992 ▶). For oxide phosphors, see: Minami et al. (2001 ▶, 2002 ▶, 2004 ▶). Data used to model the second phase present in the reaction product, Y₂Ge₂O₇, were taken from Redhammer et al. (2007 ▶). For related literature on technological applications, see: Fei et al. (2003 ▶).

Experimental

Crystal data

• Y₂GeO₅

• M _r = 330.43

• Monoclinic,

• a = 10.4706 (2) Å

• b = 6.8292 (1) Å

• c = 12.8795 (2) Å

• β = 101.750 (3)°

• V = 901.66 (3) ų

• Z = 8

• Cu Kα radiation

• T = 300 K

• Specimen shape: flat sheet

• 20 × 20 × 0.2 mm

• Specimen prepared at 1443 K

• Particle morphology: spherical, white

Data collection

• Bruker Advance D8 diffractometer

• Specimen mounting: packed powder sample container

• Specimen mounted in reflection mode

• Scan method: step

• 2θ_min = 8.0, 2θ_max = 80.0°

• Increment in 2θ = 0.02°

Refinement

• R _p = 0.053

• R _wp = 0.069

• R _exp = 0.024

• S = 2.90

• Wavelength of incident radiation: 1.540560 Å

• Profile function: pseudo-Voigt modified by Thompson et al. (1987 ▶)

• 582 reflections

• 105 parameters

Data collection: DIFFRAC/AT (Siemens, 1993 ▶); cell refinement: DICVOL91 (Boultif & Louëer 1991 ▶); data reduction: FULLPROF (Rodríguez-Carvajal, 2006 ▶); method used to solve structure: coordinates taken from an isotypic compound (Brixner et al., 1985 ▶); program(s) used to refine structure: FULLPROF; molecular graphics: ATOMS (Dowty, 2000 ▶); software used to prepare material for publication: FULLPROF.

Supplementary Material

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

supplementary crystallographic information

Comment

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 MSiO_x (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.

Experimental

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.

Refinement

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.

Figures

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.

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.

Crystal data

Y2GeO5	Z = 8
Mr = 330.43	F(000) = 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	Particle morphology: spherical
c = 12.8795 (2) Å	white
β = 101.750 (3)°	flat sheet, 20 × 20 mm
V = 901.66 (3) Å3	Specimen preparation: Prepared at 1443 K

Data collection

Bruker Advance D8 diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, Cu Kα	Scan method: step
graphite	2θmin = 7.98°, 2θmax = 80.00°, 2θstep = 0.02°
Specimen mounting: packed powder sample container

Refinement

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
χ2 = 8.410	Background function: 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.
3704 data points

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

	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)

Geometric parameters (Å, °)

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.