Cadmium(II) metaarsenate(V), CdAs₂O₆

CdAs₂O₆ is isotypic with other metaarsenates MAs₂O₆ (M = Ca, Mn, Ni, Co, Hg, Pb) and adopts the PbSb₂O₆ structure type. The Cd and As atoms are situated on positions with site symmetry (m) and (32), respectively. They are coordinated octahedrally by O atoms with distances d(Cd-O) = 2.302 (2) Å and d(As-O) = 1.826 (1) Å.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](As-O) = 0.001 Å
• R factor = 0.018
• wR factor = 0.042
• Data-to-parameter ratio = 20.0

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Like other metaarsenates M^IIAs₂O₆ reported by Magnèli (1941) [structure refinements: M = Ca, Pb (Losilla et al., 1995); Mn, Ni, Co (Nakua & Greedan, 1995); Hg (Weil, 2000; Mormann & Jeitschko, 2000], CdAs₂O₆ crystallizes in the PbSb₂O₆ structure type (Wells, 1984), which is based on a hexagonal array of O atoms. Layers of octahedral interstices alternate along the c axis of which two-thirds are filled by As atoms and one-third by M atoms. The AsO₆ octahedra are connected by edge sharing to form honeycomb sheets with the composition [As₂O₆]^2- (Fig. 1). The M atoms are situated below and above the vacant sites of the [As₂O₆]^2- layers, which leads to isolated MO₆ octahedra with site symmetry (3.m) for the M atoms. As atoms have site symmetry (3.2) (Fig. 2). The resulting distances of d(Cd—O) = 2.302 (2) Å and d(As—O) = 1.826 (1) Å compare well with d(Cd—O) = 2.31 Å and d(As—O) = 1.82 Å calculated from the radii for six-coordinated Cd and As and three-coordinated oxygen given by Shannon (1976).

S2. Experimental

Single crystals of CdAs₂O₆ were prepared by chemical transport reaction of microcrystalline material in sealed and evacuated silica ampoules using PtCl₂ as transport agent (993 K → 953 K, 14 d). Microcrystalline CdAs₂O₆ was synthesized by solid-state reaction of the binary oxides in closed silica ampoules at 953 K for 5 d.

S3. Refinement

The crystal shape was optimized by minimizing the internal R value of ψ scan data for ten selected reflections using the program HABITUS (Herrendorf, 1993). The habit so derived was used for the numerical absorption correction.

Computing details

Data collection: STADI4 (Stoe & Cie, 1995); cell refinement: STADI4 (Stoe & Cie, 1995); data reduction: STADI4; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1995); software used to prepare material for publication: SHELX97.

Figures

	Fig. 1. Projection of the structure along [001] (left) and [100] (right). CdO6 octahedra are red and AsO6 octahedra are yellow.
	Fig. 2. The unit cell with anisotropic displacement ellipsoids at the 90% probability level; Cd atoms are red and As atoms are yellow.

Cadmium metaarsenate(V)

Crystal data

As2CdO6	Dx = 6.059 Mg m^−3
Mr = 358.24	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P31M	Cell parameters from 25 reflections
a = 4.8269 (10) Å	θ = 9.4–17.7°
c = 4.866 (1) Å	µ = 22.22 mm^−1
V = 98.18 (4) Å^3	T = 293 K
Z = 1	Prismatic, brown
F(000) = 162	0.33 × 0.31 × 0.22 mm

Data collection

Siemens–Stoe AED-2 diffractometer	240 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.046
Graphite monochromator	θmax = 40.0°, θmin = 4.2°
ω/2θ scans	h = −8→8
Absorption correction: numerical (HABITUS; Herrendorf, 1993)	k = −8→8
Tmin = 0.019, Tmax = 0.129	l = −8→8
2446 measured reflections	3 standard reflections every 120 min
240 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)] = 0.018	w = 1/[σ^2(Fo^2) + (0.0116P)^2 + 0.3303P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.042	(Δ/σ)max < 0.001
S = 1.23	Δρmax = 1.27 e Å^−3
240 reflections	Δρmin = −0.88 e Å^−3
12 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.267 (15)

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
Cd1	0.0000	0.0000	0.0000	0.00641 (13)
As1	0.3333	0.6667	0.5000	0.00374 (13)
O1	0.6243 (4)	0.0000	0.2913 (3)	0.0062 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.00700 (14)	0.00700 (14)	0.00522 (16)	0.00350 (7)	0.000	0.000
As1	0.00321 (14)	0.00321 (14)	0.00480 (16)	0.00161 (7)	0.000	0.000
O1	0.0067 (4)	0.0042 (5)	0.0068 (6)	0.0021 (3)	0.0023 (4)	0.000

Geometric parameters (Å, º)

Cd1—O1^i	2.3019 (17)	As1—O1^x	1.8256 (11)
Cd1—O1^ii	2.3019 (17)	As1—O1^xi	1.8256 (11)
Cd1—O1^iii	2.3019 (17)	As1—O1^ii	1.8256 (11)
Cd1—O1^iv	2.3019 (17)	As1—As1^xii	2.7868 (6)
Cd1—O1^v	2.3019 (17)	As1—As1^xiii	2.7868 (6)
Cd1—O1^vi	2.3019 (17)	As1—As1^viii	2.7868 (6)
As1—O1^vii	1.8256 (11)	O1—As1^viii	1.8256 (11)
As1—O1^viii	1.8256 (11)	O1—As1^xiv	1.8256 (11)
As1—O1^ix	1.8256 (11)	O1—Cd1^xv	2.3019 (17)
O1^i—Cd1—O1^ii	180.00 (10)	O1^ix—As1—O1^ii	96.45 (11)
O1^i—Cd1—O1^iii	93.94 (6)	O1^x—As1—O1^ii	92.05 (6)
O1^ii—Cd1—O1^iii	86.06 (6)	O1^xi—As1—O1^ii	80.49 (8)
O1^i—Cd1—O1^iv	93.94 (6)	O1^vii—As1—As1^xii	40.25 (4)
O1^ii—Cd1—O1^iv	86.06 (6)	O1^viii—As1—As1^xii	95.57 (5)
O1^iii—Cd1—O1^iv	86.06 (6)	O1^ix—As1—As1^xii	40.25 (4)
O1^i—Cd1—O1^v	86.06 (6)	O1^x—As1—As1^xii	131.77 (6)
O1^ii—Cd1—O1^v	93.94 (6)	O1^xi—As1—As1^xii	131.77 (6)
O1^iii—Cd1—O1^v	93.94 (6)	O1^ii—As1—As1^xii	95.57 (5)
O1^iv—Cd1—O1^v	180.00 (6)	O1^vii—As1—As1^xiii	95.57 (5)
O1^i—Cd1—O1^vi	86.06 (6)	O1^viii—As1—As1^xiii	40.25 (4)
O1^ii—Cd1—O1^vi	93.94 (6)	O1^ix—As1—As1^xiii	131.77 (6)
O1^iii—Cd1—O1^vi	180.00 (6)	O1^x—As1—As1^xiii	40.25 (4)
O1^iv—Cd1—O1^vi	93.94 (6)	O1^xi—As1—As1^xiii	95.57 (5)
O1^v—Cd1—O1^vi	86.06 (6)	O1^ii—As1—As1^xiii	131.77 (6)
O1^vii—As1—O1^viii	96.45 (11)	As1^xii—As1—As1^xiii	120.0
O1^vii—As1—O1^ix	80.49 (8)	O1^vii—As1—As1^viii	131.77 (6)
O1^viii—As1—O1^ix	92.05 (6)	O1^viii—As1—As1^viii	131.77 (6)
O1^vii—As1—O1^x	92.05 (6)	O1^ix—As1—As1^viii	95.57 (5)
O1^viii—As1—O1^x	80.49 (8)	O1^x—As1—As1^viii	95.57 (5)
O1^ix—As1—O1^x	168.86 (10)	O1^xi—As1—As1^viii	40.25 (4)
O1^vii—As1—O1^xi	168.86 (10)	O1^ii—As1—As1^viii	40.25 (4)
O1^viii—As1—O1^xi	92.05 (6)	As1^xii—As1—As1^viii	120.0
O1^ix—As1—O1^xi	92.05 (6)	As1^xiii—As1—As1^viii	120.0
O1^x—As1—O1^xi	96.45 (11)	As1^viii—O1—As1^xiv	99.51 (8)
O1^vii—As1—O1^ii	92.05 (6)	As1^viii—O1—Cd1^xv	126.97 (5)
O1^viii—As1—O1^ii	168.86 (10)	As1^xiv—O1—Cd1^xv	126.97 (5)

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