A sodium calcium arsenate, NaCa(AsO₄)

The title compound, NaCa(AsO₄), was synthesized using a hydro­thermal method at 633–643 K. It has a dense structure composed of alternating layers of distorted [CaO₆] octa­hedra and layers of [AsO₄] tetra­hedra and distorted [NaO₆] octa­hedra, stacked along the a axis. The As, Ca and two O atoms lie on the mirror plane at y = 1/4 (i.e. 4c), while the Na atom lies on an inversion centre (1/2, 1/2, 0) (i.e. 4b). Each distorted [CaO₆] octa­hedron shares four equatorial common O vertices with four neighboring octa­hedra, forming a layer parallel to (100), whereas each distorted [NaO₆] octa­hedron shares two opposite edges with two neighboring ones, forming a chain running along [010]. Each isolated [AsO₄] tetra­hedron shares two edges with two different [NaO₆] octa­hedra in one [NaO₆] chain and a vertex with another chain. Simultaneously the above [AsO₄] tetra­hedron located in a four-membered [CaO₆] ring shares one edge of its base facet with one [CaO₆] octa­hedron and three corners with three other [CaO₆] octa­hedra of one [CaO₆] layer, and the remaining apex is shared with another [CaO₆] layer. [NaO₆] octa­hedra and [CaO₆] octa­hedra are linked to each other by sharing edges and vertices.

Related literature

For general background, see: Smedley & Kinniburgh (2002); Zhu et al. (2006); Rodríguez et al. (2008). For related structures, see: Graia et al. (1999) for CaNa₂(AsO₄)₃; IJdo (1982) for NaCa(VO₄); Ben Amara et al. (1983) for buchwaldite, NaCa(PO₄); Bredig (1942) for NaCa(PO₄).

Experimental

Crystal data

• NaCa(AsO₄)

• M_r = 201.99

• Orthorhombic, P n m a

• a = 11.486 (2) Å

• b = 6.6615 (14) Å

• c = 5.2406 (11) Å

• V = 400.97 (15) ų

• Z = 4

• Mo Kα radiation

• μ = 9.73 mm⁻¹

• T = 173 K

• 0.18 × 0.12 × 0.10 mm

Data collection

• Bruker SMART CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2001) T_min = 0.273, T_max = 0.443

• 2163 measured reflections

• 505 independent reflections

• 498 reflections with I > 2σ(I)

• R_int = 0.035

Refinement

• R[F² > 2σ(F²)] = 0.021

• wR(F²) = 0.067

• S = 1.03

• 505 reflections

• 42 parameters

• Δρ_max = 0.83 e Å⁻³

• Δρ_min = −0.74 e Å⁻³

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2011) and ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

Arsenic, a highly toxic pollutant in surface and ground waters, poses serious health and environmental problems all around the world (Smedley & Kinniburgh, 2002). One common method for immobilization and remediation of arsenic contamination in aqueous environments is through co-precipitation with calcium by forming various Ca-arsenate compounds (Zhu et al., 2006). To date, more than 20 Ca-arsenate compounds have been reported, but only a few of them have their crystal structures determined (Rodríguez et al., 2008). Knowledge about the crystal structures of Ca-arsenate compounds is important for better understanding their stabilities and potential applications for remediation of arsenic contamination in aqueous environments. Herein we report on the hydrothermal synthesis and crystal structure of a new compound NaCa(AsO₄), which is the second sodium calcium arsenate after CaNa₂(AsO₄)₃ (Graia et al., 1999).

The crystal structure of the title compound differs from those of its phosphorus and vanadium counterparts (IJdo, 1982; Ben Amara et al., 1983 & Bredig, 1942). The basic structural features of the title compound include [NaO₆] octahedra, [AsO₄] tetrahedra and [CaO₆] octahedra. Sodium atoms are surrounded by six O-atoms forming distorted [NaO₆] octahedra, which share edges to form chains running along [010] (Fig. 1–2). These octahedral chains are linked by isolated [AsO₄] tetrahedra to form polyhedral sheets parallel to the (100) plane (Fig. 2). This linkage is made by each [AsO₄] tetrahedron sharing edges with two [NaO₆] octahedra in one chain and a vertex with another chain. These sheets are then linked together by [CaO₆] octahedra via sharing edges and vertices(Fig. 1–3).

Related literature

For general background, see: Smedley & Kinniburgh (2002); Zhu et al. (2006); Rodríguez et al. (2008). For related structures, see: Graia et al. (1999) for CaNa₂(AsO₄)₃; IJdo (1982) for NaCa(VO₄); Ben Amara et al. (1983) for buchwaldite, NaCa(PO₄); Bredig (1942) for NaCa(PO₄).

S2. Experimental

Single crystals of NaCa(AsO₄) were synthesized using a hydrothermal method. A mixture of 0.5 mmol calcium nitrate Ca(NO₃)₂.H₂O and 0.3 mmol sodium hydrogen arsenate heptahydrate Na₂HAsO₄.H₂O was added to 5 ml of 2M NaOH solution. This mixed solution was then transferred into a 22 ml pressure vessel from Parr Instrument Company and heated to and maintained at a temperature of 633–643 K for 13 days. Solid products were filtered, washed with deionized water, and dried in the air. Crystals with a rectangular morphology were selected with a polarizing microscope for the collection of single-crystal X-ray diffraction data at 173 K.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2011) and ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The crystal structure of NaCa(AsO4) plotted in projection along [001].
	Fig. 2. The crystal structure of NaCa(AsO4) plotted in projection along a direction near [010].
	Fig. 3. Coordination environment of Ca, Na, As and O atoms, with displacement ellipsoids drawn at the 50% probability level (symmetry codes: (i) -0.5+x, 0.5-y, 0.5-z; (ii) -0.5+x, 0.5-y, 1.5-z; (iii) -0.5+x, y, 1.5-z; (iv) 1-x, 0.5+y, 1-z; (v) 1-x, -y, 1-z; (vi) 1-x,0.5+y, -z; (vii) x, y, -1+z; (viii) x, 0.5-y,-1+z; (ix) 1-x, -0.5+y, -z; (x) 0.5+x, 0.5-y, 0.5-z; (xi) 1-x, -0.5+y, 1-z; (xii) 0.5+x, 0.5-y, 1.5-z; (xiii) x, 0.5-y, z; (xiv) x, y, z+1).

sodium calcium arsenate

Crystal data

NaCa(AsO4)	F(000) = 384
Mr = 201.99	Dx = 3.346 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 2163 reflections
a = 11.486 (2) Å	θ = 3.6–28.2°
b = 6.6615 (14) Å	µ = 9.73 mm^−1
c = 5.2406 (11) Å	T = 173 K
V = 400.97 (15) Å^3	Prism, colorless
Z = 4	0.18 × 0.12 × 0.10 mm

Data collection

Bruker SMART CCD area-detector diffractometer	505 independent reflections
Radiation source: fine-focus sealed tube	498 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.035
1200 images,Δω=1°, Exp time: 15 s. scans	θmax = 28.2°, θmin = 3.6°
Absorption correction: multi-scan (SADABS; Bruker, 2001)	h = −15→14
Tmin = 0.273, Tmax = 0.443	k = −5→8
2163 measured reflections	l = −6→6

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)] = 0.021	w = 1/[σ^2(Fo^2) + (0.0474P)^2 + 0.5926P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.067	(Δ/σ)max = 0.001
S = 1.03	Δρmax = 0.83 e Å^−3
505 reflections	Δρmin = −0.74 e Å^−3
42 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.017 (2)

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
As1	0.59797 (3)	0.2500	0.56866 (7)	0.0046 (2)
Na1	0.5000	0.5000	0.0000	0.0088 (3)
Ca1	0.28059 (6)	0.2500	0.49344 (14)	0.0053 (2)
O1	0.4611 (2)	0.2500	0.6849 (5)	0.0075 (5)
O2	0.6038 (2)	0.2500	0.2482 (6)	0.0068 (6)
O3	0.66769 (14)	0.0513 (3)	0.7038 (3)	0.0068 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.0014 (3)	0.0050 (3)	0.0073 (3)	0.000	0.00013 (11)	0.000
Na1	0.0076 (7)	0.0087 (7)	0.0101 (7)	0.0027 (6)	−0.0003 (6)	−0.0014 (6)
Ca1	0.0017 (4)	0.0051 (4)	0.0091 (4)	0.000	−0.0002 (2)	0.000
O1	0.0017 (10)	0.0096 (12)	0.0111 (12)	0.000	0.0023 (9)	0.000
O2	0.0042 (12)	0.0100 (13)	0.0062 (13)	0.000	0.0012 (8)	0.000
O3	0.0056 (8)	0.0048 (8)	0.0099 (8)	0.0014 (6)	−0.0024 (7)	0.0000 (7)

Geometric parameters (Å, º)

As1—O2	1.681 (3)	Na1—O3^v	2.4970 (18)
As1—O1	1.686 (2)	Na1—O3^vi	2.4970 (18)
As1—O3^i	1.7015 (17)	Ca1—O1	2.304 (3)
As1—O3	1.7015 (17)	Ca1—O3^vi	2.3346 (19)
Na1—O1^ii	2.3873 (19)	Ca1—O3^vii	2.3346 (19)
Na1—O1^iii	2.3874 (19)	Ca1—O2^viii	2.393 (3)
Na1—O2	2.426 (2)	Ca1—O3^ix	2.4393 (19)
Na1—O2^iv	2.426 (2)	Ca1—O3^x	2.4393 (19)
O2—As1—O1	113.48 (12)	O2^iv—Na1—O3^vi	81.96 (8)
O2—As1—O3^i	113.41 (8)	O1—Ca1—O3^vi	87.93 (6)
O1—As1—O3^i	106.76 (8)	O1—Ca1—O3^vii	87.93 (6)
O2—As1—O3	113.41 (8)	O3^vi—Ca1—O3^vii	118.57 (10)
O1—As1—O3	106.76 (8)	O1—Ca1—O2^viii	173.87 (10)
O3^i—As1—O3	102.14 (12)	O3^vi—Ca1—O2^viii	88.94 (6)
O1^ii—Na1—O2	89.08 (7)	O3^vii—Ca1—O2^viii	88.94 (6)
O1^iii—Na1—O2	90.92 (7)	O1—Ca1—O3^ix	101.26 (7)
O1^ii—Na1—O2^iv	90.92 (7)	O3^vi—Ca1—O3^ix	87.52 (4)
O1^iii—Na1—O2^iv	89.08 (7)	O3^vii—Ca1—O3^ix	152.85 (7)
O1^ii—Na1—O3^v	67.60 (7)	O2^viii—Ca1—O3^ix	83.86 (7)
O1^iii—Na1—O3^v	112.40 (7)	O1—Ca1—O3^x	101.26 (7)
O2—Na1—O3^v	81.96 (8)	O3^vi—Ca1—O3^x	152.85 (7)
O2^iv—Na1—O3^v	98.04 (8)	O3^vii—Ca1—O3^x	87.52 (4)
O1^ii—Na1—O3^vi	112.40 (7)	O2^viii—Ca1—O3^x	83.86 (7)
O1^iii—Na1—O3^vi	67.60 (7)	O3^ix—Ca1—O3^x	65.72 (8)
O2—Na1—O3^vi	98.04 (8)

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