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Acta Cryst. (2013). E69, i75    [ doi:10.1107/S1600536813027256 ]

Olivine-type NaCd(AsO4)

M. Weil

CCDC reference: 964597

Abstract top

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.

Comment top

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 top

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 top

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.

Refinement top

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 top

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 top
[Figure 1] 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 top
Crystal data top
NaCd(AsO4)F(000) = 496
Mr = 274.32Dx = 4.764 Mg m3
Orthorhombic, PnmaMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2nCell parameters from 2012 reflections
a = 11.1585 (17) Åθ = 3.7–30.9°
b = 6.550 (1) ŵ = 14.27 mm1
c = 5.2330 (8) ÅT = 293 K
V = 382.47 (10) Å3Block, colourless
Z = 40.06 × 0.04 × 0.04 mm
Data collection top
656 independent reflections
Radiation source: fine-focus sealed tube615 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.030
ω scansθmax = 31.0°, θmin = 3.7°
Absorption correction: multi-scan
(SADABS; Bruker, 2005)
h = 1615
Tmin = 0.481, Tmax = 0.599k = 89
4252 measured reflectionsl = 77
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.017Secondary atom site location: difference Fourier map
wR(F2) = 0.043 w = 1/[σ2(Fo2) + (0.0225P)2 + 0.1997P]
where P = (Fo2 + 2Fc2)/3
S = 1.10(Δ/σ)max < 0.001
656 reflectionsΔρmax = 0.67 e Å3
40 parametersΔρmin = 0.75 e Å3
Crystal data top
NaCd(AsO4)V = 382.47 (10) Å3
Mr = 274.32Z = 4
Orthorhombic, PnmaMo Kα radiation
a = 11.1585 (17) ŵ = 14.27 mm1
b = 6.550 (1) ÅT = 293 K
c = 5.2330 (8) Å0.06 × 0.04 × 0.04 mm
Data collection top
656 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2005)
615 reflections with I > 2σ(I)
Tmin = 0.481, Tmax = 0.599Rint = 0.030
4252 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01740 parameters
wR(F2) = 0.0430 restraints
S = 1.10Δρmax = 0.67 e Å3
656 reflectionsΔρmin = 0.75 e Å3
Special details top

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 (Å2) top
Cd0.21753 (2)0.25000.50818 (5)0.00974 (8)
Na0.00000.00000.00000.0140 (3)
As0.39805 (3)0.25000.06207 (6)0.00795 (9)
O10.32677 (15)0.0493 (3)0.2024 (3)0.0122 (3)
O20.0386 (2)0.25000.3191 (5)0.0127 (5)
O30.3944 (2)0.25000.7417 (5)0.0116 (5)
Atomic displacement parameters (Å2) top
Cd0.00894 (13)0.00954 (14)0.01073 (13)0.0000.00028 (7)0.000
Na0.0209 (7)0.0128 (8)0.0084 (7)0.0086 (6)0.0005 (5)0.0027 (5)
As0.00785 (16)0.00855 (18)0.00743 (16)0.0000.00039 (11)0.000
O10.0157 (8)0.0086 (8)0.0124 (8)0.0029 (6)0.0025 (6)0.0004 (6)
O20.0088 (10)0.0188 (14)0.0106 (11)0.0000.0015 (8)0.000
O30.0119 (11)0.0158 (13)0.0069 (10)0.0000.0003 (8)0.000
Geometric parameters (Å, º) top
Cd—O22.229 (3)Na—O3v2.3814 (18)
Cd—O1i2.2629 (18)Na—O3vi2.3814 (18)
Cd—O1ii2.2629 (18)Na—O1v2.5031 (18)
Cd—O32.321 (2)Na—O1vi2.5031 (18)
Cd—O12.4030 (18)As—O3vii1.677 (2)
Cd—O1iii2.4030 (18)As—O2viii1.687 (2)
Na—O22.3779 (19)As—O1iii1.7030 (17)
Na—O2iv2.3779 (19)As—O11.7030 (17)
O2—Cd—O1i90.21 (5)O3v—Na—O1vi98.07 (7)
O2—Cd—O1ii90.21 (5)O3vi—Na—O1vi81.93 (7)
O1i—Cd—O1ii120.07 (9)O1v—Na—O1vi180.00 (5)
O2—Cd—O3174.59 (9)O3vii—As—O2viii113.04 (12)
O1i—Cd—O387.09 (5)O3vii—As—O1iii114.82 (8)
O1ii—Cd—O387.09 (5)O2viii—As—O1iii105.97 (8)
O2—Cd—O199.14 (7)O3vii—As—O1114.82 (8)
O1i—Cd—O1152.11 (7)O2viii—As—O1105.97 (8)
O1ii—Cd—O186.33 (4)O1iii—As—O1101.06 (12)
O3—Cd—O185.37 (7)As—O1—Cdvi125.24 (9)
O2—Cd—O1iii99.14 (7)As—O1—Cd95.84 (8)
O1i—Cd—O1iii86.33 (4)Cdvi—O1—Cd131.47 (7)
O1ii—Cd—O1iii152.11 (7)As—O1—Naii90.41 (7)
O3—Cd—O1iii85.37 (7)Cdvi—O1—Naii109.66 (7)
O1—Cd—O1iii66.33 (8)Cd—O1—Naii92.76 (6)
O2—Na—O2iv180.00 (9)Asv—O2—Cd132.00 (14)
O2—Na—O3v89.37 (6)Asv—O2—Na95.20 (9)
O2iv—Na—O3v90.63 (6)Cd—O2—Na118.29 (8)
O2—Na—O3vi90.63 (6)Asv—O2—Naix95.20 (9)
O2iv—Na—O3vi89.37 (6)Cd—O2—Naix118.29 (8)
O3v—Na—O3vi180.00 (10)Na—O2—Naix87.04 (9)
O2—Na—O1v67.30 (7)Asx—O3—Cd123.17 (13)
O2iv—Na—O1v112.70 (7)Asx—O3—Naii121.26 (9)
O3v—Na—O1v81.93 (7)Cd—O3—Naii98.11 (8)
O3vi—Na—O1v98.07 (7)Asx—O3—Naxi121.26 (9)
O2—Na—O1vi112.70 (7)Cd—O3—Naxi98.11 (8)
O2iv—Na—O1vi67.30 (7)Naii—O3—Naxi86.89 (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) x1/2, y, z+1/2; (vi) x+1/2, y, z1/2; (vii) x, y, z1; (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.
Selected bond lengths (Å) top
Cd—O22.229 (3)Na—O3v2.3814 (18)
Cd—O1i2.2629 (18)Na—O3vi2.3814 (18)
Cd—O1ii2.2629 (18)Na—O1v2.5031 (18)
Cd—O32.321 (2)Na—O1vi2.5031 (18)
Cd—O12.4030 (18)As—O3vii1.677 (2)
Cd—O1iii2.4030 (18)As—O2viii1.687 (2)
Na—O22.3779 (19)As—O1iii1.7030 (17)
Na—O2iv2.3779 (19)As—O11.7030 (17)
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) x1/2, y, z+1/2; (vi) x+1/2, y, z1/2; (vii) x, y, z1; (viii) x+1/2, y, z+1/2.
Acknowledgements top

The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.