Cadmium(II) diarsenate(V), Cd₂As₂O₇

Cd₂As₂O₇ crystallizes in the thortveitite structure type and is isotypic with other M^II₂As₂O₇ diarsenates(V) (M = Mg, Mn, Co, Ni, Ca). The bridging O atom of the diarsenate group shows large thermal motion perpendicular to the As-O-As axis. No phase transition has been observed down to 193 K.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](As-O) = 0.002 Å
• R factor = 0.036
• wR factor = 0.086
• Data-to-parameter ratio = 50.8

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level A:



PLAT_213  Alert A Atom  O1     has ADP max/min Ratio ...........       5.20


 Alert Level C:



DIFMX_01  Alert C The maximum difference density is > 0.1*ZMAX*0.75
          _refine_diff_density_max given =      3.876
          Test value =      3.600

DIFMX_02  Alert C The minimum difference density is > 0.1*ZMAX*0.75
          The relevant atom site should be identified.

General Notes



ABSTM_02  The ratio of expected to reported Tmax/Tmin(RR) is > 1.10
          Tmin and Tmax reported:      0.040     0.258
          Tmin and Tmax expected:      0.042     0.331
          RR =      1.219
          Please check that your absorption correction is appropriate.


 1 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 2 Alert Level C = Please check

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S1. Comment

A first attempt to analyze the crystal structure of Cd₂As₂O₇ was performed by Neelakantan & Calvo (1970). The authors reported a close relationship between Cd₂As₂O₇, Zn₂As₂O₇ and Mg₂As₂O₇, but a reliable structure refinement was not possible for the cadmium and zinc compound due to systematic twinning of the examined crystals.

Cd₂As₂O₇ crystallizes in the thortveitite-type structure (Zachariasen, 1930) and is isotypic with other divalent M₂As₂O₇ diarsenates(V), M = Mg (Neelakantan & Calvo, 1970), Mn (Buckley et al., 1990; Aranda et al., 1991), Co (Buckley, Bramwell & Day, 1990), Ni (Buckley et al., 1990) and Ca (Pertlik, 1980). The main structural features are As₂O₇ groups with a linear (As—O—As) bridge showing large thermal motion of the bridging O atom perpendicular to the (As—O—As)- axis (Fig. 1), and edge-sharing MO₆ octahedra, which form two-dimensionally infinite honeycomb sheets extending parallel the ac plane. The As₂O₇ groups are situated below and above the vacant sites of the cationic layers (Fig. 2).

Since thortveitite-type structures are often dimorphous (having the thortveitite-type arrangement as the high-temperature modification), an examination of Cd₂As₂O₇ has also been performed at a temperature of 193 K (Weil, 2001). No phase transition was observed down to this temperature. Lattice parameters, bond lengths and angles calculated for both datasets are nearly the same. The mean distance ¯d(Cd—O) = 2.29 Å compares well with that of 2.31 Å calculated from the radii for six-coordinated Cd and three-coordinated O given by Shannon (1976). The conformation of the As₂O₇-anion is staggered with a longer distance d(As—O) to the bridging O atom as compared to the terminal atoms. The mean distance ¯d(As—O) = 1.676 Å is in good agreement with the mean ¯d(As—O) = 1.669 Å calculated for other M₂As₂O₇ diarsenates(V).

Next to the linear model a so called split-atom model was introduced which describes a statistical disorder of the bridging oxygen atom within X₂O₇– thortveitite type groups (X = Si, P, V, As; Nord, 1984; Stefanidis & Nord, 1984). Under consideration of this model (space group C2/m, half occupation of the bridging O1 atom with fractional coordinates of x = 0, y = 0.0257 (13), z = 0) a bent diarsenate anion with an As—O—As angle of 164.5 (8)° and a slightly longer distance of d(As—O1) = 1.719 (2) Å results.

S2. Experimental

Light-brown single crystals of Cd₂As₂O₇ up to 3 mm in length were prepared by chemical-transport reaction of microcrystalline material in evacuated and sealed silica tubes using PtCl₂ as the transport agent (temperature gradient 923 K → 873 K, two weeks). Microcrystalline Cd₂As₂O₇ was synthesized by solid-state reaction of the binary oxides in closed silica ampoules at 873 K for 11 d.

S3. Refinement

In the final electron density difference map the remaining maximum of 3.9 e Å^-1 and minimum of -3.5 e Å^-1 (room temperature measurement) are located at a distance of 1.30 Å from As and 0.57 Å from Cd, respectively. They may be due to insufficient absorption correction caused by incorrect distance measurement of the crystal faces relatively to the center of the crystal.

Computing details

Data collection: STADI4 (Stoe & Cie, 1995); cell refinement: STADI4; 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: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. The As2O7 group (linear model) with anisotropic displacement ellipsoids drawn at the 90% probability level.
	Fig. 2. Projection of the structure along [001] (left) and [010] (right). CdO6 octahedra are red, As2O7 groups are yellow. Unit-cell dimensions are marked with blue stars.

(I)

Crystal data

Cd2As2O7	F(000) = 436
Mr = 486.64	Dx = 5.421 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
a = 6.9446 (14) Å	Cell parameters from 24 reflections
b = 9.0365 (14) Å	θ = 8.2–26.5°
c = 4.8530 (8) Å	µ = 18.13 mm^−1
β = 101.77 (1)°	T = 293 K
V = 298.15 (9) Å^3	Translucent plate, light brown
Z = 2	0.28 × 0.16 × 0.06 mm

Data collection

AED-2 diffractometer (Siemens, Stoe)	1550 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.035
Graphite monochromator	θmax = 50.0°, θmin = 3.8°
ω/2θ scans	h = −14→14
Absorption correction: numerical numerical absorption correction on the basis of measured and indexed crystal faces	k = −19→19
Tmin = 0.040, Tmax = 0.258	l = −10→10
6242 measured reflections	3 standard reflections every 120 min
1626 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)] = 0.036	w = 1/[σ^2(Fo^2) + (0.0304P)^2 + 3.2702P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.086	(Δ/σ)max < 0.001
S = 1.23	Δρmax = 3.88 e Å^−3
1626 reflections	Δρmin = −3.45 e Å^−3
32 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.367 (11)

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
Cd	0.0000	0.30427 (3)	0.5000	0.01120 (8)
As	0.22491 (5)	0.0000	0.91086 (7)	0.00851 (8)
O1	0.0000	0.0000	0.0000	0.070 (4)
O2	0.3843 (5)	0.0000	0.2172 (5)	0.0148 (4)
O3	0.2344 (3)	0.1541 (2)	0.7247 (5)	0.0153 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd	0.00994 (10)	0.01029 (10)	0.01280 (10)	0.000	0.00099 (6)	0.000
As	0.00690 (11)	0.01061 (12)	0.00793 (12)	0.000	0.00128 (8)	0.000
O1	0.017 (3)	0.113 (12)	0.090 (9)	0.000	0.034 (4)	0.000
O2	0.0210 (11)	0.0120 (8)	0.0084 (7)	0.000	−0.0043 (7)	0.000
O3	0.0164 (7)	0.0135 (6)	0.0159 (6)	0.0041 (5)	0.0032 (5)	0.0058 (5)

Geometric parameters (Å, º)

Cd—O3^i	2.226 (2)	As—O3	1.669 (2)
Cd—O3	2.226 (2)	As—O1^v	1.7036 (4)
Cd—O2^ii	2.2819 (16)	O1—As^vii	1.7036 (4)
Cd—O2^iii	2.2819 (16)	O1—As^viii	1.7036 (4)
Cd—O3^ii	2.356 (2)	O2—As^viii	1.663 (3)
Cd—O3^iv	2.356 (2)	O2—Cd^ii	2.2819 (16)
As—O2^v	1.663 (3)	O2—Cd^ix	2.2819 (16)
As—O3^vi	1.669 (2)	O3—Cd^ii	2.356 (2)
O3^i—Cd—O3	104.89 (12)	O3^ii—Cd—O3^iv	161.63 (10)
O3^i—Cd—O2^ii	154.10 (10)	O2^v—As—O3^vi	113.05 (10)
O3—Cd—O2^ii	92.60 (9)	O2^v—As—O3	113.05 (10)
O3^i—Cd—O2^iii	92.60 (9)	O3^vi—As—O3	113.14 (16)
O3—Cd—O2^iii	154.10 (10)	O2^v—As—O1^v	104.51 (12)
O2^ii—Cd—O2^iii	78.38 (10)	O3^vi—As—O1^v	106.09 (7)
O3^i—Cd—O3^ii	115.71 (9)	O3—As—O1^v	106.09 (7)
O3—Cd—O3^ii	76.16 (8)	As^vii—O1—As^viii	180.0
O2^ii—Cd—O3^ii	86.66 (10)	As^viii—O2—Cd^ii	128.78 (6)
O2^iii—Cd—O3^ii	79.09 (10)	As^viii—O2—Cd^ix	128.78 (6)
O3^i—Cd—O3^iv	76.16 (8)	Cd^ii—O2—Cd^ix	101.62 (10)
O3—Cd—O3^iv	115.71 (9)	As—O3—Cd	131.93 (12)
O2^ii—Cd—O3^iv	79.09 (10)	As—O3—Cd^ii	120.33 (11)
O2^iii—Cd—O3^iv	86.66 (10)	Cd—O3—Cd^ii	103.84 (8)

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