NaSr(AsO₄)(H₂O)₉: the (Sr,As) analogue of nabaphite and nastrophite

Abstract

The crystal structure of the title compound, sodium strontium orthoarsenate(V) nona­hydrate, is isotypic with NaSr(PO₄)(H₂O)₉ and the minerals nabaphite [NaBa(PO₄)(H₂O)₉] and nastrophite [Na(Sr,Ba)(PO₄)(H₂O)₉]. The Na and Sr atoms are located on threefold rotation axes and are in the centres of slightly distorted Na(H₂O)₆ octa­hedra and Sr(H₂O)₉ tricapped trigonal prisms, respectively. A framework structure is established via edge-sharing of these polyhedra. Disordered AsO₄ tetra­hedra (with threefold symmetry) are situated in the inter­stitial space of the framework. Although reasonable H-atom positions of the water mol­ecules were not established, close O⋯O contacts between the disordered AsO₄ tetra­hedra and the water mol­ecules suggest strong O—H⋯O hydrogen bonding.

Related literature

For a previous study of the title compound that revealed cubic symmetry and the lattice parameters, see: Ariguib-Kbir & Guerin (1973 ▶). Isotypic structures have been reported for synthetic NaSr(PO₄)(H₂O)₉ (Takagi et al., 1982 ▶), nabaphite [NaBa(PO₄)(H₂O)₉] (Baturin et al., 1982 ▶) and nastrophite [Na(Sr,Ba)(PO₄)(H₂O)₉] (Baturin et al., 1981 ▶). For crystal structures in the Sr—As—O—(H) system, see: Mihajlovic & Effenberger (2006 ▶); Weil et al. (2009 ▶). As—O bond-length data for tetra­hedrally coordinated arsenic were compiled and computed by Baur (1981 ▶) and Schwendtner (2008 ▶). For ionic radii, see: Shannon (1976 ▶).

Experimental

Crystal data

• NaSr(AsO₄)(H₂O)₉

• M _r = 411.67

• Cubic,

• a = 10.6435 (1) Å

• V = 1205.74 (2) ų

• Z = 4

• Mo Kα radiation

• μ = 7.29 mm⁻¹

• T = 296 K

• 0.36 × 0.24 × 0.24 mm

Data collection

• Bruker APEXII CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2008 ▶) T _min = 0.179, T _max = 0.274

• 41355 measured reflections

• 3435 independent reflections

• 3272 reflections with I > 2σ(I)

• R _int = 0.037

Refinement

• R[F ² > 2σ(F ²)] = 0.023

• wR(F ²) = 0.056

• S = 1.07

• 3435 reflections

• 60 parameters

• H-atom parameters not refined

• Δρ_max = 1.73 e Å⁻³

• Δρ_min = −1.66 e Å⁻³

• Absolute structure: Flack (1983 ▶), 1537 Friedel pairs

• Flack parameter: −0.005 (6)

Data collection: APEX2 (Bruker, 2008 ▶); cell refinement: SAINT (Bruker, 2008 ▶); 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: SHELXL97.

Supplementary Material

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected interatomic distances (Å).

Sr—O1i	2.6326 (10)
Sr—O2	2.6558 (10)
Sr—O3ii	2.6994 (10)
Na—O3iii	2.3926 (12)
Na—O2iv	2.4086 (12)

O1⋯O5v	2.558 (7)
O1⋯O5	2.612 (8)
O1⋯O9vi	2.623 (8)
O1⋯O6vii	2.6640 (17)
O1⋯O4	2.7667 (14)
O1⋯O7	2.829 (5)
O1⋯O7vi	2.919 (5)
O2⋯O9vi	2.515 (9)
O2⋯O6vii	2.7488 (17)
O3⋯O8vi	2.562 (9)
O3⋯O6vi	2.6949 (17)
O3⋯O9vi	2.732 (9)
O3⋯O6	2.7492 (17)
O3⋯O5	2.757 (7)
O3⋯O7	2.776 (5)
O3⋯O7vi	2.930 (5)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

supplementary crystallographic information

Comment

A previous study of NaSr(AsO₄)(H₂O)₉ reports cubic symmetry and the lattice parameter as a = 10.70 Å (Ariguib-Kbir & Guerin, 1973). Moreover, isotypism with NaSr(PO₄)(H₂O)₉ (Takagi et al., 1982) was also revealed. Besides the synthetic phosphate analogue, NaSr(AsO₄)(H₂O)₉ is also isotypic with the minerals nabaphite [NaBa(PO₄)(H₂O)₉] (Baturin et al., 1982) and nastrophite [Na(Sr,Ba)(PO₄)(H₂O)₉] (Baturin et al., 1981).

The crystal structures consist of slightly distorted Na(OH₂)₆ octahedra (3 symmetry) and M(OH₂)₉ tricapped trigonal prisms (3 symmetry), where M = Sr, Ba. These polyhedra share edges and establish a framework structure. Disordered XO₄ tetrahedra (X = P, As; 3 symmetry) are situated in the interstitial space of the framework (Fig. 1).

The Na(H₂O)₆ octahedron is slightly distorted. The corresponding Na—O bond lenghts are in the usual range with an average of 2.40 Å, conform with the values in the isotypic compounds (NaSr(PO₄)(H₂O)₉: 2.41 Å; nabaphite: 2.42 Å; nastrophite: 2.42 Å) and the sum of the ionic radii of 2.41 Å, as calculated for six-coordinated Na (Shannon, 1976).

The Sr²⁺ ion is surrounded by 9 oxygen atoms with an average Sr—O distance of 2.663 Å, in good agreement with the phosphate analogue (2.668 Å; Takagi et al., 1982) and the sum of the ionic radii of 2.67 Å, as calculated for nine-coordinated Sr (Shannon, 1976).

The environment of the disordered AsO₄ group consists of 16 water molecules with donor (D) — acceptor (A) distances between 2.5 and 3.0 Å (see Table 2). The overcrowding of water molecules (only 12 surrounding O atoms are expected, considering an ordered tetrahedral configuration for the arsenate unit with three donator atoms) may be the reason for the disorder of the AsO₄ group. The average As—O bond length for the disordered AsO₄ group is 1.685 Å, a value in very good agreement with those of 1.682 Å (Baur, 1981) and 1.686 Å (Schwendtner, 2008).

Experimental

Crystals of the title compound were obtained during phase formation studies in the system Sr—As—O (Weil et al., 2009) from hydrous solutions. All chemicals used were of analytical grade and employed without further purification. To a saturated Sr(OH)₂^.8H₂O solution a diluted arsenic acid solution was dropwise added which resulted in a flocculent white precipitate (pH ca. 9). A concentrated NaOH solution was then added until a pH of 12 was reached. The resulting suspension was boiled for an hour. Then the precipitate was filtered off and the remaining solution was left to stand for several days. Besides few crystals of SrHAsO₄ (Mihajlovic & Effenberger, 2006), colourless columnar crystals of the title compound up to several mm in length were obtained after complete evaporation of water.

Refinement

The oxygen atoms O4 and O6 of the arsenate group were clearly discernible from Fourier maps. Consideration of full occupancy of these sites resulted in high electron densities of ca 4 e ų at a distance of ca 1.7 Å from As, indicating other disordered oxygen atoms. Therefore four additional O atoms were considered in the final model. Free refinement of the site occupancy factors (s.o.f.) of all six O atoms attached to arsenic resulted in a composition very close to the theoretical value. In the last refinement cycles the s.o.f.'s were constrained to meet the criterion for electroneutrality. The six disordered O atoms were finally refined with isotropic displacement parameters. No reasonable positions of the H atoms attached to the water molecules (O1, O2, O3) could be found in difference Fourier maps which may be due to the disorder of the AsO₄ tetrahedron and the resulting complex hydrogen bonding scheme. Therefore all H atoms were excluded from the refinement. The highest remaining peak in the final difference Fourier map is 0.49 Å from As and the deepest hole is 0.43 Å from the same atom.

Figures

Fig. 1.

The crystal structure of NaSr(PO4)(H2O)9 in a projection approximately along [001]. Sr atoms are represented as blue spheres and O atoms as white spheres; NaO6 octahedra are given in yellow and AsO4 tetrahedra in red. For clarity, Sr—O bonds are omitted and only one orientation of the disordered AsO4 group is given. Ellipsoids are drawn at the 90% probability level.

Crystal data

NaSr(AsO4)(H2O)9	Dx = 2.268 Mg m−3
Mr = 411.67	Mo Kα radiation, λ = 0.71073 Å
Cubic, P213	Cell parameters from 9782 reflections
Hall symbol: P 2ac 2ab 3	θ = 5.8–45.2°
a = 10.6435 (1) Å	µ = 7.29 mm−1
V = 1205.74 (2) Å3	T = 296 K
Z = 4	Fragment, colourless
F(000) = 816	0.36 × 0.24 × 0.24 mm

Data collection

Bruker APEXII CCD diffractometer	3435 independent reflections
Radiation source: fine-focus sealed tube	3272 reflections with I > 2σ(I)
graphite	Rint = 0.037
ω and φ scans	θmax = 45.7°, θmin = 5.4°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −21→21
Tmin = 0.179, Tmax = 0.274	k = −20→21
41355 measured reflections	l = −17→17

Refinement

Refinement on F2	H-atom parameters not refined
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0274P)2 + 0.5628P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.023	(Δ/σ)max = 0.001
wR(F2) = 0.056	Δρmax = 1.73 e Å−3
S = 1.07	Δρmin = −1.66 e Å−3
3435 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
60 parameters	Extinction coefficient: 0.0147 (10)
0 restraints	Absolute structure: Flack (1983), 1537 Friedel pairs
Primary atom site location: structure-invariant direct methods	Flack parameter: −0.005 (6)
Secondary atom site location: difference Fourier map

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	Occ. (<1)
Sr	0.055001 (8)	0.055001 (8)	0.055001 (8)	0.00459 (3)
As	0.421839 (11)	0.421839 (11)	0.421839 (11)	0.00709 (4)
Na	0.67077 (6)	0.67077 (6)	0.67077 (6)	0.01193 (15)
O1	0.39368 (12)	0.07909 (10)	0.35359 (10)	0.01687 (18)
O2	0.29270 (9)	0.13067 (11)	0.04892 (10)	0.01519 (16)
O3	0.60854 (10)	0.29633 (9)	0.14338 (10)	0.01243 (14)
O4	0.33000 (13)	0.33000 (13)	0.33000 (13)	0.0077 (3)*	0.60
O5	0.4046 (7)	0.3186 (7)	0.3016 (7)	0.0117 (10)*	0.13
O6	0.56151 (12)	0.44234 (13)	0.35120 (12)	0.00759 (17)*	0.60
O7	0.5256 (5)	0.3069 (5)	0.3904 (5)	0.0115 (7)*	0.20
O8	0.5623 (8)	0.3833 (8)	0.4407 (8)	0.0088 (11)*	0.10
O9	0.5627 (8)	0.3668 (8)	0.4973 (8)	0.0084 (11)*	0.10

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Sr	0.00459 (3)	0.00459 (3)	0.00459 (3)	0.00016 (2)	0.00016 (2)	0.00016 (2)
As	0.00709 (4)	0.00709 (4)	0.00709 (4)	−0.00024 (3)	−0.00024 (3)	−0.00024 (3)
Na	0.01193 (15)	0.01193 (15)	0.01193 (15)	0.00126 (17)	0.00126 (17)	0.00126 (17)
O1	0.0319 (5)	0.0088 (3)	0.0098 (3)	0.0043 (3)	−0.0045 (3)	−0.0005 (3)
O2	0.0109 (3)	0.0229 (4)	0.0118 (3)	−0.0039 (3)	0.0002 (3)	0.0007 (3)
O3	0.0130 (3)	0.0111 (3)	0.0132 (3)	0.0024 (3)	−0.0005 (3)	0.0015 (3)

Geometric parameters (Å, °)

Sr—O1i	2.6326 (10)	As—O9iv	1.799 (9)
Sr—O1ii	2.6326 (10)	As—O9v	1.799 (9)
Sr—O1iii	2.6326 (10)	Na—O3ix	2.3926 (12)
Sr—O2iv	2.6558 (10)	Na—O3x	2.3926 (12)
Sr—O2	2.6558 (10)	Na—O3xi	2.3926 (12)
Sr—O2v	2.6558 (10)	Na—O2xii	2.4086 (12)
Sr—O3vi	2.6994 (10)	Na—O2xiii	2.4086 (12)
Sr—O3vii	2.6994 (10)	Na—O2xiv	2.4086 (12)
Sr—O3viii	2.6994 (10)	O4—O5iv	0.858 (7)
As—O8iv	1.563 (8)	O4—O5	0.858 (7)
As—O8	1.563 (8)	O4—O5v	0.858 (7)
As—O8v	1.563 (8)	O5—O5v	1.440 (13)
As—O6	1.6801 (13)	O5—O5iv	1.440 (13)
As—O6v	1.6801 (13)	O5—O7	1.602 (9)
As—O6iv	1.6801 (13)	O6—O8	1.141 (9)
As—O7iv	1.682 (5)	O6—O9v	1.462 (9)
As—O7	1.682 (5)	O6—O7	1.549 (5)
As—O7v	1.682 (5)	O6—O9	1.750 (8)
As—O4	1.693 (2)	O7—O8	1.049 (10)
As—O5iv	1.697 (7)	O7—O9	1.362 (9)
As—O5	1.697 (7)	O8—O9	0.627 (11)
As—O5v	1.697 (7)	O9—O6iv	1.462 (9)
As—O9	1.799 (9)
O1···O5iv	2.558 (7)	O2···O6xvi	2.7488 (17)
O1···O5	2.612 (8)	O3···O8xv	2.562 (9)
O1···O9xv	2.623 (8)	O3···O6xv	2.6949 (17)
O1···O6xvi	2.6640 (17)	O3···O9xv	2.732 (9)
O1···O4	2.7667 (14)	O3···O6	2.7492 (17)
O1···O7	2.829 (5)	O3···O5	2.757 (7)
O1···O7xv	2.919 (5)	O3···O7	2.776 (5)
O2···O9xv	2.515 (9)	O3···O7xv	2.930 (5)
O1i—Sr—O1ii	80.79 (4)	O8v—As—O5iv	140.9 (4)
O1i—Sr—O1iii	80.79 (4)	O6—As—O5iv	128.5 (3)
O1ii—Sr—O1iii	80.79 (4)	O6v—As—O5iv	113.1 (3)
O1i—Sr—O2iv	86.96 (4)	O6iv—As—O5iv	80.9 (3)
O1ii—Sr—O2iv	134.39 (3)	O7iv—As—O5iv	56.6 (3)
O1iii—Sr—O2iv	140.21 (3)	O7—As—O5iv	81.6 (3)
O1i—Sr—O2	140.21 (3)	O7v—As—O5iv	106.7 (3)
O1ii—Sr—O2	86.96 (4)	O8iv—As—O5	140.9 (4)
O1iii—Sr—O2	134.39 (3)	O8—As—O5	91.7 (4)
O2iv—Sr—O2	75.03 (4)	O8v—As—O5	115.8 (4)
O1i—Sr—O2v	134.39 (3)	O6—As—O5	80.9 (3)
O1ii—Sr—O2v	140.21 (3)	O6v—As—O5	128.5 (3)
O1iii—Sr—O2v	86.96 (4)	O6iv—As—O5	113.1 (3)
O2iv—Sr—O2v	75.03 (4)	O7iv—As—O5	106.7 (3)
O2—Sr—O2v	75.03 (4)	O7—As—O5	56.6 (3)
O1i—Sr—O3vi	68.72 (3)	O7v—As—O5	81.6 (3)
O1ii—Sr—O3vi	68.05 (3)	O5iv—As—O5	50.2 (4)
O1iii—Sr—O3vi	138.94 (4)	O8iv—As—O9	80.4 (4)
O2iv—Sr—O3vi	66.55 (3)	O8v—As—O9	108.7 (4)
O2—Sr—O3vi	71.58 (3)	O8—As—O9	19.9 (4)
O2v—Sr—O3vi	134.06 (3)	O6v—As—O9	127.1 (3)
O1i—Sr—O3vii	68.05 (3)	O6—As—O9	60.3 (3)
O1ii—Sr—O3vii	138.94 (4)	O6iv—As—O9	49.6 (3)
O1iii—Sr—O3vii	68.72 (3)	O7iv—As—O9	104.4 (3)
O2iv—Sr—O3vii	71.58 (3)	O7—As—O9	45.9 (3)
O2—Sr—O3vii	134.06 (3)	O7v—As—O9	134.8 (3)
O2v—Sr—O3vii	66.55 (3)	O4—As—O9	123.4 (3)
O3vi—Sr—O3vii	119.992 (1)	O5iv—As—O9	110.0 (4)
O1i—Sr—O3viii	138.94 (4)	O5—As—O9	102.5 (4)
O1ii—Sr—O3viii	68.72 (3)	O5v—As—O9	152.0 (4)
O1iii—Sr—O3viii	68.05 (3)	O8iv—As—O9iv	19.9 (4)
O2iv—Sr—O3viii	134.06 (3)	O8v—As—O9iv	80.4 (4)
O2—Sr—O3viii	66.55 (3)	O8—As—O9iv	108.7 (4)
O2v—Sr—O3viii	71.58 (3)	O6v—As—O9iv	49.6 (3)
O3vi—Sr—O3viii	119.992 (1)	O6—As—O9iv	127.1 (3)
O3vii—Sr—O3viii	119.992 (1)	O6iv—As—O9iv	60.3 (3)
O8iv—As—O8	99.3 (4)	O7iv—As—O9iv	45.9 (3)
O8iv—As—O8v	99.3 (4)	O7—As—O9iv	134.8 (3)
O8—As—O8v	99.3 (4)	O7v—As—O9iv	104.4 (3)
O8iv—As—O6	130.1 (3)	O4—As—O9iv	123.4 (3)
O8v—As—O6	69.2 (3)	O5iv—As—O9iv	102.5 (4)
O8iv—As—O6v	69.2 (3)	O5—As—O9iv	152.0 (4)
O8—As—O6v	130.1 (3)	O5v—As—O9iv	110.0 (4)
O6—As—O6v	109.83 (4)	O9—As—O9iv	92.6 (4)
O8—As—O6iv	69.2 (3)	O8iv—As—O9v	108.7 (4)
O8v—As—O6iv	130.1 (3)	O8v—As—O9v	19.9 (4)
O6—As—O6iv	109.83 (4)	O8—As—O9v	80.4 (4)
O6v—As—O6iv	109.83 (4)	O6v—As—O9v	60.3 (3)
O8—As—O7iv	124.0 (4)	O6—As—O9v	49.6 (3)
O8v—As—O7iv	117.1 (3)	O6iv—As—O9v	127.1 (3)
O6—As—O7iv	164.45 (18)	O7iv—As—O9v	134.8 (3)
O6v—As—O7iv	76.41 (17)	O7—As—O9v	104.4 (3)
O6iv—As—O7iv	54.87 (18)	O7v—As—O9v	45.9 (3)
O8iv—As—O7	117.1 (3)	O4—As—O9v	123.4 (3)
O8v—As—O7	124.0 (4)	O5iv—As—O9v	152.0 (4)
O6—As—O7	54.87 (18)	O5—As—O9v	110.0 (4)
O6v—As—O7	164.45 (18)	O5v—As—O9v	102.5 (4)
O6iv—As—O7	76.41 (17)	O9—As—O9v	92.6 (4)
O7iv—As—O7	117.62 (9)	O9iv—As—O9v	92.6 (4)
O8iv—As—O7v	124.0 (4)	O3ix—Na—O3x	89.64 (5)
O8—As—O7v	117.1 (3)	O3ix—Na—O3xi	89.65 (5)
O6—As—O7v	76.41 (17)	O3x—Na—O3xi	89.64 (5)
O6v—As—O7v	54.87 (18)	O3ix—Na—O2xii	75.47 (4)
O6iv—As—O7v	164.45 (18)	O3x—Na—O2xii	155.21 (3)
O7iv—As—O7v	117.62 (9)	O3xi—Na—O2xii	109.62 (4)
O7—As—O7v	117.62 (9)	O3ix—Na—O2xiii	109.62 (4)
O8iv—As—O4	118.4 (3)	O3x—Na—O2xiii	75.47 (4)
O8—As—O4	118.4 (3)	O3xi—Na—O2xiii	155.21 (3)
O8v—As—O4	118.4 (3)	O2xii—Na—O2xiii	90.76 (5)
O6—As—O4	109.11 (5)	O3ix—Na—O2xiv	155.21 (3)
O6v—As—O4	109.11 (5)	O3x—Na—O2xiv	109.62 (4)
O6iv—As—O4	109.11 (5)	O3xi—Na—O2xiv	75.47 (4)
O7iv—As—O4	81.04 (17)	O2xii—Na—O2xiv	90.76 (5)
O7—As—O4	81.04 (17)	O2xiii—Na—O2xiv	90.76 (5)
O7v—As—O4	81.04 (17)	Naxvi—O2—Sr	103.37 (4)
O8iv—As—O5iv	91.7 (4)	Naxvii—O3—Srxviii	102.52 (4)
O8—As—O5iv	115.8 (4)

Symmetry codes: (i) −y, z−1/2, −x+1/2; (ii) −x+1/2, −y, z−1/2; (iii) z−1/2, −x+1/2, −y; (iv) y, z, x; (v) z, x, y; (vi) −y+1/2, −z, x−1/2; (vii) −z, x−1/2, −y+1/2; (viii) x−1/2, −y+1/2, −z; (ix) −y+1, z+1/2, −x+3/2; (x) z+1/2, −x+3/2, −y+1; (xi) −x+3/2, −y+1, z+1/2; (xii) y+1/2, −z+1/2, −x+1; (xiii) −z+1/2, −x+1, y+1/2; (xiv) −x+1, y+1/2, −z+1/2; (xv) −z+1, x−1/2, −y+1/2; (xvi) −x+1, y−1/2, −z+1/2; (xvii) −x+3/2, −y+1, z−1/2; (xviii) x+1/2, −y+1/2, −z.