Olivine-type NaCd(AsO₄)

The title compound, sodium cadmium orthoarsenate, adopts the olivine [Mg₂(SiO₄)] structure type in space group Pnma, with Na (site symmetry -1) and Cd (.m.) replacing the two Mg positions, and the AsO₄ tetra­hedron (.m.) the SiO₄ tetra­hedron. The crystal structure is made up of a nearly hexa­gonal closed-packed arrangement of O atoms stacked along [001]. The Na and Cd atoms occupy one half of the octa­hedral voids in alternate layers stacked along [100], and one eighth of the tetra­hedral voids are occupied by As atoms.

Related literature

For a review of the crystal chemistry of olivines, see: Brown (1982). For the isotypic phosphate analogue, see: Ivanov et al. (1974); Hata et al. (1979). For other phases in the system Na–Cd–P–O, see: Murashova & Chudinova (1997); Bennazha et al. (2000); Ben Amara et al. (1979). For standardization of structure data, see: Gelato & Parthé (1987).

Experimental

Crystal data

• NaCd(AsO₄)

• M_r = 274.32

• Orthorhombic, P n m a

• a = 11.1585 (17) Å

• b = 6.550 (1) Å

• c = 5.2330 (8) Å

• V = 382.47 (10) ų

• Z = 4

• Mo Kα radiation

• μ = 14.27 mm⁻¹

• T = 293 K

• 0.06 × 0.04 × 0.04 mm

Data collection

• Bruker SMART CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2005) T_min = 0.481, T_max = 0.599

• 4252 measured reflections

• 656 independent reflections

• 615 reflections with I > 2σ(I)

• R_int = 0.030

Refinement

• R[F² > 2σ(F²)] = 0.017

• wR(F²) = 0.043

• S = 1.10

• 656 reflections

• 40 parameters

• Δρ_max = 0.67 e Å⁻³

• Δρ_min = −0.75 e Å⁻³

Table 1
Selected bond lengths (Å)

Cd—O2	2.229 (3)
Cd—O1^i	2.2629 (18)
Cd—O1^ii	2.2629 (18)
Cd—O3	2.321 (2)
Cd—O1	2.4030 (18)
Cd—O1^iii	2.4030 (18)
Na—O2	2.3779 (19)
Na—O2^iv	2.3779 (19)
Na—O3^v	2.3814 (18)
Na—O3^vi	2.3814 (18)
Na—O1^v	2.5031 (18)
Na—O1^vi	2.5031 (18)
As—O3^vii	1.677 (2)
As—O2^viii	1.687 (2)
As—O1^iii	1.7030 (17)
As—O1	1.7030 (17)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) -x, -y, -z; (v) ; (vi) ; (vii) x, y, z-1; (viii) .

Data collection: SMART (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Up to now there are no reports on crystal structure determinations of phases in the system Na–Cd–As–O, whereas for the analogous system Na–Cd–P–O several phases with compositions NaCd(PO₄) (Ivanov et al., 1974; Hata et al., 1979), NaCd(PO₃)₃ (Murashova & Chudinova, 1997), Na₂Cd₃(P₂O₇)₂ (Bennazha et al., 2000) and Cd₄Na(PO₄)₃ (Ben Amara et al., 1979) have been structurally fully characterized. In this article the synthesis and crystal structure of NaCd(AsO₄) is reported.

NaCd(AsO₄) crystallizes in the olivine [Mg₂(SiO₄)] structure type. A review on the crystal chemistry of olivines and spinels has been published some time ago by Brown (1982). The olivine structure is characterized by an almost hexagonal closed-packed arrangement of oxygen atoms parallel to (001) with a stacking sequence ABAB along [001] with one half of the octahedral voids occupied by magnesium sites, and one-eighth of the tetrahedral voids by silicon sites. In the structure of NaCd(AsO₄), the two magnesium sites and the silicon site are replaced by unique sodium and cadmium sites and an arsenic site, respectively. The resulting NaO₆ octahedron has 1 symmetry, and the CdO₆ octahedron and the AsO₄ tetrahedron both have .m. symmetry.

In comparison with the isotypic structure of NaCd(PO₄) (Ivanov et al., 1974), the Cd—O and Na—O distances are virtually the same, with mean bond lengths for the CdO₆ and NaO₆ octahedra of 2.307 (71) Å and 2.398 (54) Å in the phosphorus compound, and of 2.314 (75) Å and 2.421 (64) Å, respectively, in the arsenic compound. As expected, only the distances within the PO₄ (mean P—O distance 1.541 (12) Å) and AsO₄ (1.693 (13) Å) tetrahedra differ, because of the different sizes of P(V) and As(V).

Related literature

For a review of the crystal chemistry of olivines, see: Brown (1982). For the isotypic phosphate analogue, see: Ivanov et al. (1974); Hata et al. (1979). For other phases in the system Na–Cd–P–O, see: Murashova & Chudinova (1997); Bennazha et al. (2000); Ben Amara et al. (1979). For standardization of structure data, see: Gelato & Parthé (1987).

S2. Experimental

To an aqueous 0.05 M solution (20 ml) of cadmium acetate, an aqueous solution of 0.05 M Na₂HAsO₄ (20 ml) was added. Together with the mother liquor, parts of the colourless precipitate were transferred to a 10 ml Teflon insert (filling degree one-half). The insert was then sealed and heated in a steel autoclave under autogenous pressure at 338 K for ten days. Few colourless single crystals of the title compound with a block-like form were obtained after the reaction time, and were manually separated from the reaction mixture that was not further examined.

S3. Refinement

Atomic coordinates of the isotypic NaCd(PO₄) (Ivanov et al., 1974) in space group Pnma were taken as starting parameters for refinement of NaCd(AsO₄). Besides a structure refinement of NaCd(PO₄) in Pnma, also refinements of this structure in the non-centrosymmetric space group No. 33 were performed with settings in Pn2₁a (Ivanov et al., 1974) and P2₁nb (Hata et al., 1979). However, adopting these non-centrosymmetric structure models for refinement of NaCd(AsO₄) converged with significantly higher residual parameters and with large correlation matrix elements. Therefore the structure model in space group Pnma was considered as being correct, in agreement with other olivine-type structures. Structure data were finally standardized with STRUCTURE TIDY (Gelato & Parthé, 1987).

Computing details

Data collection: SMART (Bruker, 2005); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

Fig. 1. The olivine-type crystal structure of NaCd(AsO4) in a projection along [001]. Displacement ellipsoids are drawn at the 74% probability level.

Sodium cadmium orthoarsenate

Crystal data

NaCd(AsO4)	F(000) = 496
Mr = 274.32	Dx = 4.764 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 2012 reflections
a = 11.1585 (17) Å	θ = 3.7–30.9°
b = 6.550 (1) Å	µ = 14.27 mm^−1
c = 5.2330 (8) Å	T = 293 K
V = 382.47 (10) Å^3	Block, colourless
Z = 4	0.06 × 0.04 × 0.04 mm

Data collection

Bruker SMART CCD diffractometer	656 independent reflections
Radiation source: fine-focus sealed tube	615 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
ω scans	θmax = 31.0°, θmin = 3.7°
Absorption correction: multi-scan (SADABS; Bruker, 2005)	h = −16→15
Tmin = 0.481, Tmax = 0.599	k = −8→9
4252 measured reflections	l = −7→7

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.017	Secondary atom site location: difference Fourier map
wR(F^2) = 0.043	w = 1/[σ^2(Fo^2) + (0.0225P)^2 + 0.1997P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max < 0.001
656 reflections	Δρmax = 0.67 e Å^−3
40 parameters	Δρmin = −0.75 e Å^−3

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	0.21753 (2)	0.2500	0.50818 (5)	0.00974 (8)
Na	0.0000	0.0000	0.0000	0.0140 (3)
As	0.39805 (3)	0.2500	0.06207 (6)	0.00795 (9)
O1	0.32677 (15)	0.0493 (3)	0.2024 (3)	0.0122 (3)
O2	0.0386 (2)	0.2500	0.3191 (5)	0.0127 (5)
O3	0.3944 (2)	0.2500	0.7417 (5)	0.0116 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd	0.00894 (13)	0.00954 (14)	0.01073 (13)	0.000	0.00028 (7)	0.000
Na	0.0209 (7)	0.0128 (8)	0.0084 (7)	0.0086 (6)	−0.0005 (5)	−0.0027 (5)
As	0.00785 (16)	0.00855 (18)	0.00743 (16)	0.000	0.00039 (11)	0.000
O1	0.0157 (8)	0.0086 (8)	0.0124 (8)	−0.0029 (6)	0.0025 (6)	0.0004 (6)
O2	0.0088 (10)	0.0188 (14)	0.0106 (11)	0.000	0.0015 (8)	0.000
O3	0.0119 (11)	0.0158 (13)	0.0069 (10)	0.000	0.0003 (8)	0.000

Geometric parameters (Å, º)

Cd—O2	2.229 (3)	Na—O3^v	2.3814 (18)
Cd—O1^i	2.2629 (18)	Na—O3^vi	2.3814 (18)
Cd—O1^ii	2.2629 (18)	Na—O1^v	2.5031 (18)
Cd—O3	2.321 (2)	Na—O1^vi	2.5031 (18)
Cd—O1	2.4030 (18)	As—O3^vii	1.677 (2)
Cd—O1^iii	2.4030 (18)	As—O2^viii	1.687 (2)
Na—O2	2.3779 (19)	As—O1^iii	1.7030 (17)
Na—O2^iv	2.3779 (19)	As—O1	1.7030 (17)
O2—Cd—O1^i	90.21 (5)	O3^v—Na—O1^vi	98.07 (7)
O2—Cd—O1^ii	90.21 (5)	O3^vi—Na—O1^vi	81.93 (7)
O1^i—Cd—O1^ii	120.07 (9)	O1^v—Na—O1^vi	180.00 (5)
O2—Cd—O3	174.59 (9)	O3^vii—As—O2^viii	113.04 (12)
O1^i—Cd—O3	87.09 (5)	O3^vii—As—O1^iii	114.82 (8)
O1^ii—Cd—O3	87.09 (5)	O2^viii—As—O1^iii	105.97 (8)
O2—Cd—O1	99.14 (7)	O3^vii—As—O1	114.82 (8)
O1^i—Cd—O1	152.11 (7)	O2^viii—As—O1	105.97 (8)
O1^ii—Cd—O1	86.33 (4)	O1^iii—As—O1	101.06 (12)
O3—Cd—O1	85.37 (7)	As—O1—Cd^vi	125.24 (9)
O2—Cd—O1^iii	99.14 (7)	As—O1—Cd	95.84 (8)
O1^i—Cd—O1^iii	86.33 (4)	Cd^vi—O1—Cd	131.47 (7)
O1^ii—Cd—O1^iii	152.11 (7)	As—O1—Na^ii	90.41 (7)
O3—Cd—O1^iii	85.37 (7)	Cd^vi—O1—Na^ii	109.66 (7)
O1—Cd—O1^iii	66.33 (8)	Cd—O1—Na^ii	92.76 (6)
O2—Na—O2^iv	180.00 (9)	As^v—O2—Cd	132.00 (14)
O2—Na—O3^v	89.37 (6)	As^v—O2—Na	95.20 (9)
O2^iv—Na—O3^v	90.63 (6)	Cd—O2—Na	118.29 (8)
O2—Na—O3^vi	90.63 (6)	As^v—O2—Na^ix	95.20 (9)
O2^iv—Na—O3^vi	89.37 (6)	Cd—O2—Na^ix	118.29 (8)
O3^v—Na—O3^vi	180.00 (10)	Na—O2—Na^ix	87.04 (9)
O2—Na—O1^v	67.30 (7)	As^x—O3—Cd	123.17 (13)
O2^iv—Na—O1^v	112.70 (7)	As^x—O3—Na^ii	121.26 (9)
O3^v—Na—O1^v	81.93 (7)	Cd—O3—Na^ii	98.11 (8)
O3^vi—Na—O1^v	98.07 (7)	As^x—O3—Na^xi	121.26 (9)
O2—Na—O1^vi	112.70 (7)	Cd—O3—Na^xi	98.11 (8)
O2^iv—Na—O1^vi	67.30 (7)	Na^ii—O3—Na^xi	86.89 (8)

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