Rubidium gadolinium bis­(tungstate)

The structure of rubidium gadolinium bis­(tungstate), RbGd(WO₄)₂, has been determined. The crystal is built up from corner- and edge-sharing WO₆ octa­hedral and GdO₈ polyhedral groups, giving rise to a Gd-WO₄ polyhedral backbone surrounding structural cavities filled with Rb⁺ cations. The Gd and Rb atoms lie on twofold axes.

S1. Comment

Monoclinic double tungstate single crystals, with the formula KRE(WO₄)₂ (KGW), RE being the rare earths, are currently receiving attention as hosts for self-induced frequency shifting (Kaminskii et al., 1998). The low-temperature phase of potassium gadolinium tungstate, KGd(WO₄)₂, is a promising laser material and has been extensively studied (Kushawaha et al., 1993). In this paper, we present the synthesis and structural determination of the new rubidium gadolinium bis(tungstate), RbGd (WO₄)₂ (RGW), refined from single-crystal data. The structure is a three-dimensional network of WO₆ octahedra and GdO₈ square-antiprism polyhedra sharing edges and corners. Fig. 1 shows the framework of the RGW crystal projected along the [101] direction.

The WO₆ ocatahedra are distorted, with each coordinating W cation displaced by 0.36 Å from the mean of its six O vertices. This displacement `dipole' is orientated along a pseudo threefold axis of the octahedra, with inversion-related octahedral pairs joined by a common O1···O1ⁱ edge [2.405 Å; symmetry code: (i) −x, −y + 1, −z]. The inversion symmetry centre at the shared edge midpoint is strongly suggestive of antiferroelectric properties. The W—O bonds span the range 1.768 (9)–2.325 (1) Å, with a mean distance of 1.959 (7) Å. Pairs of distorted octahedral units form a double chain along the crystallographic c axis and are connected by sharing common O3 vertices. The shortest W1···W1ⁱ distance is 3.265 (0) Å, across the shared edge of the edge-sharing octahedra. Between the corner-sharing octahedra, the distance between W atoms is significantly larger, at 3.80 (3) Å [W1···W1ⁱⁱ; symmetry code: (ii) x, −y + 1, z + 1/2]. These values are almost in the same range as those found in KGd(WO₄)₂ and KYb(WO₄)₂ (Pujol et al., 2001)

The Gd sites are eight-coordinated, with four pairs of Gd—O distances ranging from 2.310 (1) to 2.707 (9) Å, and arranged as a square antiprism. These polyhedra form a single chain along the [101] direction by alternately sharing O2—O4ⁱⁱⁱ edges [3.039 Å; symmetry code: (iii) −x + 1/2, y − 1/2, −z + 1/2] and O2^iv···O2^v edges [3.028 Å; symmetry codes: (iv) x − 1/2, −y + 1/2, z − 1/2; (v) −x, y,-z + 1/2]. The Gd···Gd1^vi distance within a chain is 4.049 Å [symmetry code: (vi) −x + 1/2, −y + 1/2, −z + 1], while between chains the Gd1···Gd1ⁱ distance is 6.7 Å. The shared edge between WO₆ octahedra, O1···O1ⁱ [2.405 Å], is the shortest in the structure and, likewise, the second-shortest edge, O1^vii···O2 [2.519 Å; symmetry code: (vii) Please provide symmetry code] is one that is shared between the tungstate and gadolinium polyhedra.

The Rb⁺ cation is 12-coordinated by O atoms, forming a distorted icosahedron. These polyhedra form a bidimensional layer consisting of chains, which share edges in the [101] and [110] directions. These chains fill the holes in the framework of gadolinium and tungstate polyhedra. All the polyhedra contributing to the monoclinic RGW structure are strongly connected by shared edges and vertices.

S2. Experimental

GdRb(WO₄)₂ single crystals were grown by the top-seeded solution growth (TSSG) slow-cooling technique, using Rb₂W₂O₇ as solvent. The solution composition of 20mol% of solute and 80% of solvent was chosen. The solutions used in the crystal growth experiments, weighing about 200 g, were prepared in a cylindrical Pt crucible, 60 mm in diameter, by melting and decomposing the appropriate quantities of Gd₂O₃, Rb₂O₃ and WO₃. Homogenization of the solutions was achieved by maintaining them at about 323 K above the expected saturation temperature for 24 h. After homogenization of the solution, a Pt disc, diameter 11 mm, rotating at 30 r.p.m. was placed in it. The solution temperature was then decreased at a rate of 0.1 K h⁻¹ until crystals nucleated on the disc, and then at a rate of 1–1.5 K h⁻¹. Crystals of RGW were removed after the sample had reached room temperature.

S3. Refinement

A numerical or analytical absorption correction should be used for the intensity data, because the product of the linear absorption coefficient (µ) and the median crystal dimension is greater than 3.0, but there are no regular faces of the crystal for that approach. Therefore, a ψ-scan absorption correction was attempted. After the correction, the structure was solved easily. The positions of three heavy atoms were obtained correctly from the Patterson map and four O atoms from Fourier syntheses. All atoms were refined with anisotropic displacement parameters. The value of the maximum residual electron density in the final difference Fourier map is less than 0.1 of the heaviest atomic number and the value of the minimum is larger than −0.1 of that. The peak and hole are 0.79 and 0.74 Å, respectively, from the W atom. The result of the structure analysis proved that the ψ-scan absorption correction can work well for this data.

Computing details

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL (Bruker, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: Please provide missing details.

Figures

Fig. 1. A projection of the RGW structure along the [101] direction. Displacement ellipsoids are drawn at the 50% probability level.

Rubidium gadolinium bis(tungstate)

Crystal data

RbGd(WO4)2	F(000) = 1252
Mr = 738.42	Dx = 7.550 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 10.6953 (12) Å	Cell parameters from 33 reflections
b = 10.5017 (11) Å	θ = 5.0–15.0°
c = 7.6064 (11) Å	µ = 52.87 mm^−1
β = 130.504 (7)°	T = 293 K
V = 649.61 (14) Å^3	Prism, colourless
Z = 4	0.20 × 0.18 × 0.13 mm

Data collection

Bruker P4 diffractometer	848 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.085
Graphite monochromator	θmax = 30.0°, θmin = 3.2°
/w scans	h = −1→15
Absorption correction: ψ scan (XSCANS; Siemens, 1996)	k = −14→1
Tmin = 0.013, Tmax = 0.045	l = −10→8
1197 measured reflections	3 standard reflections every 97 reflections
951 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.050	w = 1/[σ^2(Fo^2) + (0.089P)^2 + 31.6366P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.136	(Δ/σ)max < 0.001
S = 1.11	Δρmax = 5.38 e Å^−3
951 reflections	Δρmin = −6.38 e Å^−3
57 parameters	Extinction correction: SHELXTL (Bruker, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0070 (5)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
W1	0.19389 (6)	0.49876 (4)	0.23272 (9)	0.0093 (3)
Gd1	0.0000	0.22800 (8)	0.2500	0.0102 (3)
Rb1	0.0000	0.8015 (2)	0.2500	0.0207 (4)
O1	0.0228 (11)	0.6071 (9)	−0.0297 (17)	0.0131 (17)
O2	0.2728 (12)	0.3423 (8)	0.3698 (16)	0.0127 (16)
O3	0.1869 (13)	0.5719 (10)	0.4367 (18)	0.0154 (18)
O4	0.3688 (13)	0.5767 (9)	0.3062 (17)	0.0152 (18)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
W1	0.0088 (4)	0.0102 (4)	0.0111 (4)	−0.00011 (13)	0.0074 (3)	0.00057 (13)
Gd1	0.0114 (4)	0.0099 (5)	0.0122 (5)	0.000	0.0090 (4)	0.000
Rb1	0.0179 (9)	0.0247 (10)	0.0221 (10)	0.000	0.0141 (8)	0.000
O1	0.009 (4)	0.016 (4)	0.011 (4)	−0.002 (3)	0.004 (4)	−0.001 (3)
O2	0.012 (4)	0.008 (4)	0.011 (4)	−0.002 (3)	0.004 (3)	−0.002 (3)
O3	0.015 (4)	0.019 (4)	0.019 (4)	0.001 (3)	0.014 (4)	−0.002 (4)
O4	0.020 (5)	0.010 (4)	0.019 (5)	−0.005 (3)	0.014 (4)	0.000 (3)

Geometric parameters (Å, º)

W1—O4	1.768 (9)	Rb1—O3^x	2.853 (11)
W1—O3	1.776 (10)	Rb1—O3	2.853 (10)
W1—O2	1.832 (9)	Rb1—O3^iii	2.880 (10)
W1—O1	1.970 (10)	Rb1—O3^xii	2.880 (10)
W1—O1^i	2.084 (9)	Rb1—O4^iii	2.975 (10)
W1—O3^ii	2.325 (10)	Rb1—O4^xii	2.975 (10)
W1—Gd1	3.5717 (9)	Rb1—O1	3.070 (10)
W1—Rb1^iii	3.7238 (15)	Rb1—O1^x	3.070 (10)
W1—Rb1^iv	3.8037 (15)	Rb1—O2^xiii	3.130 (10)
W1—Rb1	3.844 (2)	Rb1—O2^xiv	3.130 (10)
W1—Rb1^i	4.2231 (19)	Rb1—O4^xiii	3.359 (10)
Gd1—O1^v	2.310 (10)	Rb1—O4^xiv	3.359 (10)
Gd1—O1^i	2.310 (10)	O1—W1^i	2.084 (9)
Gd1—O4^vi	2.335 (9)	O1—Gd1^i	2.310 (10)
Gd1—O4^vii	2.335 (9)	O2—Gd1^ix	2.384 (9)
Gd1—O2^viii	2.384 (9)	O2—Rb1^iv	3.130 (10)
Gd1—O2^ix	2.384 (9)	O3—W1^v	2.325 (10)
Gd1—O2	2.707 (9)	O3—Rb1^iii	2.880 (10)
Gd1—O2^x	2.707 (9)	O4—Gd1^xv	2.335 (9)
Gd1—W1^x	3.5717 (9)	O4—Rb1^iii	2.975 (10)
Gd1—Rb1^i	3.8158 (6)	O4—Rb1^iv	3.359 (10)
Gd1—Rb1^xi	3.8158 (6)
O4—W1—O3	99.2 (5)	O2—Gd1—Rb1^i	71.0 (2)
O4—W1—O2	103.5 (4)	O2^x—Gd1—Rb1^i	113.5 (2)
O3—W1—O2	99.0 (5)	W1—Gd1—Rb1^i	69.64 (3)
O4—W1—O1	98.6 (4)	W1^x—Gd1—Rb1^i	118.50 (4)
O3—W1—O1	95.6 (4)	O1^v—Gd1—Rb1^xi	53.6 (3)
O2—W1—O1	151.1 (4)	O1^i—Gd1—Rb1^xi	135.7 (3)
O4—W1—O1^i	157.0 (4)	O4^vi—Gd1—Rb1^xi	51.2 (2)
O3—W1—O1^i	102.8 (4)	O4^vii—Gd1—Rb1^xi	121.1 (2)
O2—W1—O1^i	79.8 (4)	O2^viii—Gd1—Rb1^xi	121.6 (2)
O1—W1—O1^i	72.7 (4)	O2^ix—Gd1—Rb1^xi	55.0 (2)
O4—W1—O3^ii	79.4 (4)	O2—Gd1—Rb1^xi	113.5 (2)
O3—W1—O3^ii	172.3 (3)	O2^x—Gd1—Rb1^xi	71.0 (2)
O2—W1—O3^ii	88.7 (4)	W1—Gd1—Rb1^xi	118.50 (4)
O1—W1—O3^ii	77.2 (4)	W1^x—Gd1—Rb1^xi	69.64 (3)
O1^i—W1—O3^ii	78.0 (4)	Rb1^i—Gd1—Rb1^xi	170.69 (8)
O4—W1—Gd1	151.4 (3)	O3^x—Rb1—O3	64.6 (4)
O3—W1—Gd1	87.6 (3)	O3^x—Rb1—O3^iii	149.8 (3)
O2—W1—Gd1	47.8 (3)	O3—Rb1—O3^iii	85.2 (3)
O1—W1—Gd1	108.5 (3)	O3^x—Rb1—O3^xii	85.2 (3)
O1^i—W1—Gd1	37.8 (3)	O3—Rb1—O3^xii	149.8 (3)
O3^ii—W1—Gd1	97.3 (2)	O3^iii—Rb1—O3^xii	125.0 (4)
O4—W1—Rb1^iii	51.8 (3)	O3^x—Rb1—O4^iii	126.2 (3)
O3—W1—Rb1^iii	48.7 (3)	O3—Rb1—O4^iii	97.9 (3)
O2—W1—Rb1^iii	98.0 (3)	O3^iii—Rb1—O4^iii	54.9 (3)
O1—W1—Rb1^iii	110.2 (3)	O3^xii—Rb1—O4^iii	100.3 (3)
O1^i—W1—Rb1^iii	151.0 (3)	O3^x—Rb1—O4^xii	97.9 (3)
O3^ii—W1—Rb1^iii	131.0 (2)	O3—Rb1—O4^xii	126.2 (3)
Gd1—W1—Rb1^iii	122.86 (3)	O3^iii—Rb1—O4^xii	100.3 (3)
O4—W1—Rb1^iv	62.0 (3)	O3^xii—Rb1—O4^xii	54.9 (3)
O3—W1—Rb1^iv	136.8 (3)	O4^iii—Rb1—O4^xii	129.1 (4)
O2—W1—Rb1^iv	54.8 (3)	O3^x—Rb1—O1	55.8 (3)
O1—W1—Rb1^iv	124.2 (3)	O3—Rb1—O1	55.8 (3)
O1^i—W1—Rb1^iv	104.7 (3)	O3^iii—Rb1—O1	107.9 (3)
O3^ii—W1—Rb1^iv	49.1 (3)	O3^xii—Rb1—O1	107.8 (3)
Gd1—W1—Rb1^iv	94.15 (4)	O4^iii—Rb1—O1	151.8 (3)
Rb1^iii—W1—Rb1^iv	97.12 (4)	O4^xii—Rb1—O1	72.0 (3)
O4—W1—Rb1	95.2 (3)	O3^x—Rb1—O1^x	55.8 (3)
O3—W1—Rb1	44.2 (3)	O3—Rb1—O1^x	55.8 (3)
O2—W1—Rb1	141.4 (3)	O3^iii—Rb1—O1^x	107.8 (3)
O1—W1—Rb1	52.4 (3)	O3^xii—Rb1—O1^x	107.9 (3)
O1^i—W1—Rb1	95.8 (3)	O4^iii—Rb1—O1^x	72.0 (3)
O3^ii—W1—Rb1	128.2 (3)	O4^xii—Rb1—O1^x	151.8 (3)
Gd1—W1—Rb1	108.55 (3)	O1—Rb1—O1^x	96.6 (4)
Rb1^iii—W1—Rb1	67.767 (19)	O3^x—Rb1—O2^xiii	113.3 (3)
Rb1^iv—W1—Rb1	157.00 (6)	O3—Rb1—O2^xiii	80.5 (3)
O4—W1—Rb1^i	116.2 (3)	O3^iii—Rb1—O2^xiii	58.2 (3)
O3—W1—Rb1^i	143.8 (3)	O3^xii—Rb1—O2^xiii	113.7 (3)
O2—W1—Rb1^i	67.0 (3)	O4^iii—Rb1—O2^xiii	112.9 (3)
O1—W1—Rb1^i	86.7 (3)	O4^xii—Rb1—O2^xiii	59.6 (3)
O1^i—W1—Rb1^i	43.6 (3)	O1—Rb1—O2^xiii	57.5 (2)
O3^ii—W1—Rb1^i	39.7 (2)	O1^x—Rb1—O2^xiii	136.0 (2)
Gd1—W1—Rb1^i	57.900 (19)	O3^x—Rb1—O2^xiv	80.5 (3)
Rb1^iii—W1—Rb1^i	159.64 (4)	O3—Rb1—O2^xiv	113.3 (3)
Rb1^iv—W1—Rb1^i	63.190 (17)	O3^iii—Rb1—O2^xiv	113.7 (2)
Rb1—W1—Rb1^i	132.54 (3)	O3^xii—Rb1—O2^xiv	58.2 (3)
O1^v—Gd1—O1^i	82.9 (5)	O4^iii—Rb1—O2^xiv	59.6 (3)
O1^v—Gd1—O4^vi	99.8 (3)	O4^xii—Rb1—O2^xiv	112.9 (3)
O1^i—Gd1—O4^vi	148.2 (3)	O1—Rb1—O2^xiv	136.0 (2)
O1^v—Gd1—O4^vii	148.2 (3)	O1^x—Rb1—O2^xiv	57.5 (2)
O1^i—Gd1—O4^vii	99.8 (3)	O2^xiii—Rb1—O2^xiv	164.3 (3)
O4^vi—Gd1—O4^vii	94.3 (5)	O3^x—Rb1—O4^xiii	151.6 (3)
O1^v—Gd1—O2^viii	131.0 (3)	O3—Rb1—O4^xiii	125.1 (3)
O1^i—Gd1—O2^viii	78.9 (3)	O3^iii—Rb1—O4^xiii	49.5 (3)
O4^vi—Gd1—O2^viii	75.6 (3)	O3^xii—Rb1—O4^xiii	81.7 (3)
O4^vii—Gd1—O2^viii	80.1 (3)	O4^iii—Rb1—O4^xiii	81.16 (16)
O1^v—Gd1—O2^ix	78.9 (3)	O4^xii—Rb1—O4^xiii	54.1 (3)
O1^i—Gd1—O2^ix	131.0 (3)	O1—Rb1—O4^xiii	104.9 (2)
O4^vi—Gd1—O2^ix	80.1 (3)	O1^x—Rb1—O4^xiii	152.6 (3)
O4^vii—Gd1—O2^ix	75.6 (3)	O2^xiii—Rb1—O4^xiii	51.5 (2)
O2^viii—Gd1—O2^ix	143.9 (4)	O2^xiv—Rb1—O4^xiii	112.7 (2)
O1^v—Gd1—O2	80.8 (3)	O3^x—Rb1—O4^xiv	125.1 (3)
O1^i—Gd1—O2	59.6 (3)	O3—Rb1—O4^xiv	151.6 (3)
O4^vi—Gd1—O2	152.1 (3)	O3^iii—Rb1—O4^xiv	81.7 (3)
O4^vii—Gd1—O2	73.7 (3)	O3^xii—Rb1—O4^xiv	49.5 (3)
O2^viii—Gd1—O2	124.9 (4)	O4^iii—Rb1—O4^xiv	54.1 (3)
O2^ix—Gd1—O2	72.7 (3)	O4^xii—Rb1—O4^xiv	81.16 (16)
O1^v—Gd1—O2^x	59.6 (3)	O1—Rb1—O4^xiv	152.6 (3)
O1^i—Gd1—O2^x	80.8 (3)	O1^x—Rb1—O4^xiv	104.9 (2)
O4^vi—Gd1—O2^x	73.7 (3)	O2^xiii—Rb1—O4^xiv	112.7 (2)
O4^vii—Gd1—O2^x	152.1 (3)	O2^xiv—Rb1—O4^xiv	51.5 (2)
O2^viii—Gd1—O2^x	72.7 (3)	O4^xiii—Rb1—O4^xiv	61.3 (3)
O2^ix—Gd1—O2^x	124.9 (4)	W1—O1—W1^i	107.3 (4)
O2—Gd1—O2^x	127.4 (4)	W1—O1—Gd1^i	139.3 (5)
O1^v—Gd1—W1	68.9 (2)	W1^i—O1—Gd1^i	108.7 (4)
O1^i—Gd1—W1	33.6 (2)	W1—O1—Rb1	97.0 (4)
O4^vi—Gd1—W1	168.7 (2)	W1^i—O1—Rb1	108.5 (4)
O4^vii—Gd1—W1	95.9 (2)	Gd1^i—O1—Rb1	89.2 (3)
O2^viii—Gd1—W1	111.1 (2)	W1—O2—Gd1^ix	134.3 (5)
O2^ix—Gd1—W1	97.6 (2)	W1—O2—Gd1	102.0 (4)
O2—Gd1—W1	30.11 (19)	Gd1^ix—O2—Gd1	107.3 (3)
O2^x—Gd1—W1	99.16 (19)	W1—O2—Rb1^iv	96.6 (4)
O1^v—Gd1—W1^x	33.6 (2)	Gd1^ix—O2—Rb1^iv	86.5 (3)
O1^i—Gd1—W1^x	68.9 (2)	Gd1—O2—Rb1^iv	135.4 (3)
O4^vi—Gd1—W1^x	95.9 (2)	W1—O3—W1^v	135.6 (5)
O4^vii—Gd1—W1^x	168.7 (2)	W1—O3—Rb1	110.1 (5)
O2^viii—Gd1—W1^x	97.6 (2)	W1^v—O3—Rb1	108.9 (3)
O2^ix—Gd1—W1^x	111.1 (2)	W1—O3—Rb1^iii	103.7 (4)
O2—Gd1—W1^x	99.16 (19)	W1^v—O3—Rb1^iii	93.3 (3)
O2^x—Gd1—W1^x	30.1 (2)	Rb1—O3—Rb1^iii	94.8 (3)
W1—Gd1—W1^x	74.48 (2)	W1—O4—Gd1^xv	153.6 (6)
O1^v—Gd1—Rb1^i	135.7 (3)	W1—O4—Rb1^iii	100.4 (4)
O1^i—Gd1—Rb1^i	53.6 (3)	Gd1^xv—O4—Rb1^iii	91.1 (3)
O4^vi—Gd1—Rb1^i	121.1 (2)	W1—O4—Rb1^iv	90.3 (3)
O4^vii—Gd1—Rb1^i	51.2 (2)	Gd1^xv—O4—Rb1^iv	102.2 (3)
O2^viii—Gd1—Rb1^i	55.0 (2)	Rb1^iii—O4—Rb1^iv	125.9 (3)
O2^ix—Gd1—Rb1^i	121.6 (2)

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