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Acta Crystallogr Sect E Struct Rep Online. 2009 January 1; 65(Pt 1): i2.
Published online 2008 December 20. doi:  10.1107/S1600536808043171
PMCID: PMC2967843

Redetermination of the cubic struvite analogue Cs[Mg(OH2)6](AsO4)

Abstract

In contrast to the previous refinement from photographic data [Ferrari et al. (1955 [triangle]). Gazz. Chim. Ital. 84, 169–174], the present redetermination of the title compound, caesium hexa­aqua­magnesium arsenate(V), revealed the Cs atom to be on Wyckoff position 4d instead of Wyckoff position 4b of space group F An external file that holds a picture, illustration, etc.
Object name is e-65-000i2-efi2.jpg3m. The structure can be derived from the halite structure. The centres of the complex [Mg(OH2)6] octa­hedra and the AsO4 tetra­hedra (both with An external file that holds a picture, illustration, etc.
Object name is e-65-000i2-efi2.jpg3m symmetry) are on the respective Na and Cl positions. The building units are connected to each other by O—H(...)O hydrogen bonds. The Cs+ cations (An external file that holds a picture, illustration, etc.
Object name is e-65-000i2-efi2.jpg3m symmetry) are located in the voids of this arrangement and exhibit a regular cubocta­hedral 12-coordination to the O atoms of the water mol­ecules. The O atom bonded to As has 2mm site symmetry (Wyckoff position 24f) and the water-mol­ecule O atom has m site symmetry (Wyckoff position 48h).

Related literature

The crystal structure of struvite, NH4[Mg(OH2)6](PO4), was reported by Whitaker & Jeffery (1970a [triangle],b [triangle]). Crystal growth of struvite-type compounds using the gel diffusion technique was reported by Banks et al. (1975 [triangle]). For isotypic structures, see: Carver et al. (2006 [triangle]) for Cs[Fe(OH2)6](PO4) and Massa et al. (2003 [triangle]) for the cubic form of dimorphic Cs[Mg(OH2)6](PO4). Isotypic struvite-type phases as well as analogues were recently surveyed by Weil (2008 [triangle]).

Experimental

Crystal data

  • Cs[Mg(OH2)6](AsO4)
  • M r = 404.24
  • Cubic, An external file that holds a picture, illustration, etc.
Object name is e-65-000i2-efi5.jpg
  • a = 10.1609 (5) Å
  • V = 1049.05 (9) Å3
  • Z = 4
  • Mo Kα radiation
  • μ = 6.75 mm−1
  • T = 293 (2) K
  • 0.18 × 0.18 × 0.18 mm

Data collection

  • Enraf–Nonius CAD-4 diffractometer
  • Absorption correction: integration (SHELXTL; Sheldrick, 2008 [triangle]) T min = 0.341, T max = 0.382
  • 3332 measured reflections
  • 208 independent reflections
  • 207 reflections with I > 2σ(I)
  • R int = 0.037
  • 3 standard reflections frequency: 200 min intensity decay: none

Refinement

  • R[F 2 > 2σ(F 2)] = 0.016
  • wR(F 2) = 0.038
  • S = 1.14
  • 208 reflections
  • 15 parameters
  • 1 restraint
  • H atoms treated by a mixture of independent and constrained refinement
  • Δρmax = 0.50 e Å−3
  • Δρmin = −0.48 e Å−3
  • Absolute structure: Flack (1983 [triangle]), 90 Friedel pairs
  • Flack parameter: 0.01 (4)

Data collection: CAD-4 Software (Enraf–Nonius, 1989 [triangle]); cell refinement: CAD-4 Software; data reduction: HELENA implemented in PLATON (Spek, 2003 [triangle]); method used to solve structure: coordinates taken from an isotypic compound; program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: ATOMS (Dowty, 2004 [triangle]); software used to prepare material for publication: SHELXL97.

Table 1
Selected bond lengths (Å)
Table 2
Hydrogen-bond geometry (Å, °)

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808043171/hb2885sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808043171/hb2885Isup2.hkl

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

supplementary crystallographic information

Comment

Numerous compounds with general formula A[B(OH2)6]XO4, where A = alkali metal, NH4 or Tl, B = Mg or first row transition metal, and X = P or As, are known to crystallize in the orthorhombic struvite (NH4[Mg(OH2)6](PO4)) structure in space group Pmn21 (Whitaker & Jeffrey, 1970a, b). The title compound, (I), Cs[Mg(OH2)6](AsO4), is an isoformular analogue of struvite, but crystallizes in the cubic crystal system. All these structures can be described in terms of closed-packed layers with different stacking sequences (Massa et al., 2003). Phases that are isotypes of struvite, as well as struvite analogues were recently surveyed by Weil (2008). In comparison with the previous refinement from photographic data (Ferrari et al., 1955), the present redetermination of Cs[Mg(OH2)6](AsO4) revealed a different location of the Cs atom, the localization of the H atom and anisotropic displacement parameters for all non H-atoms.

The structure can be described as a derivative of the NaCl structure type (Massa et al., 2003). The centres of the regular complex [Mg(H2O)6] octahedra are situated on the respective Na positions, and the centres of the regular AsO4 tetrahedra are situated on the Cl positions (Fig. 1). Thus one [Mg(H2O)6] octahedron is surrounded by six AsO4 tetrahedra in an octahedral arrangement, and vice versa. The corresponding Mg—O and As—O distances are in the normal range (Table 1). These building units are linked via medium-strong hydrogen bonds (Table 2). Details and differences of the hydrogen bonding schemes in cubic, hexagonal and orthorhombic struvite-type structures were discussed in detail by Massa et al. (2003). The Cs+ cations are located in the voids of this arrangement and exhibit a regular cuboctahedral 12-coordination to the oxygen atoms of the water molecules (Table 1).

Experimental

Colourless octahedral crystals of Cs[Mg(OH2)6](AsO4) with an edge-length up to 1 mm were grown by means of the gel diffusion technique, following a slightly modified procedure as that given by Banks et al. (1975). Aqueous solutions of 0.025 M MgSO4 and 0.02 M Na4edta (edta = ethylenediaminetetraacetate) were adjusted to pH 10 with NaOH. Commercially available gelatine foils (5 g) were dissolved in the hot resulting 100 ml solution and allowed to form a gel inside a large test tube overnight. When the gel had set, an equivalent amount of a solution of 0.025 M CsH2AsO4 (50 ml) was carefully poured over the gel. This solution was then adjusted to pH 9 with NaOH. The test tube was covered with parafilm and the crystal growth proceeded at the gel-liquid interface and into the gel. Crystals large enough for conventional x-ray analysis grew within one week at room temperature. They were separated mechanically from the gel and were washed with a water/ethanol/acetone (1/3/1) mixture.

Refinement

The coordinates of the isotypic compound Cs[Fe(OH2)6](PO4) (Carver et al., 2006) were taken as starting parameters. The Cs atom in the original structure determination (Ferrari et al., 1955) was positioned on Wyckoff site 4b (1/2, 1/2, 1/2), whereas in the present refinement Cs is on position 4d (3/4, 3/4, 3/4). The position of the H atom was found from difference Fourier maps and was refined with a soft distance restraint of O—H = 0.85 (3) Å.

Figures

Fig. 1.
The crystal structure of Cs[Mg(H2O)6](AsO4) projected approximately along [001]. [Mg(OH2)6] octahedra are yellow, PO4 tetrahedra are red, Cs atoms are blue O atoms are white and H atoms are grey. For one of the Cs+ cations the Cs–O bonds are indicated. ...

Crystal data

Cs[Mg(H2O)6](AsO4)Dx = 2.559 Mg m3
Mr = 404.24Mo Kα radiation, λ = 0.71073 Å
Cubic, F43mCell parameters from 25 reflections
Hall symbol: F -4 2 3θ = 11.4–12.7°
a = 10.1609 (5) ŵ = 6.75 mm1
V = 1049.05 (9) Å3T = 293 K
Z = 4Octahedron, colourless
F(000) = 7680.18 × 0.18 × 0.18 mm

Data collection

Enraf–Nonius CAD-4 diffractometer207 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.037
graphiteθmax = 30.9°, θmin = 3.5°
ω/2θ scansh = −14→14
Absorption correction: integration (SHELXTL; Sheldrick, 2008)k = −14→14
Tmin = 0.341, Tmax = 0.382l = −14→14
3332 measured reflections3 standard reflections every 200 min
208 independent reflections intensity decay: none

Refinement

Refinement on F2H atoms treated by a mixture of independent and constrained refinement
Least-squares matrix: fullw = 1/[σ2(Fo2) + 5.0469P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.016(Δ/σ)max < 0.001
wR(F2) = 0.038Δρmax = 0.50 e Å3
S = 1.14Δρmin = −0.48 e Å3
208 reflectionsExtinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
15 parametersExtinction coefficient: 0.00144 (17)
1 restraintAbsolute structure: Flack (1983), 90 Friedel pairs
Primary atom site location: isomorphous structure methodsFlack parameter: 0.01 (4)

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 (Å2)

xyzUiso*/Ueq
Cs10.75000.75000.75000.0599 (3)
Mg10.00000.00000.00000.0250 (6)
As10.25000.25000.25000.0201 (2)
O10.2031 (3)0.00000.00000.0508 (8)
O20.34550 (19)0.34550 (19)0.34550 (19)0.0269 (7)
H10.249 (3)0.045 (2)0.045 (2)0.053 (12)*

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
Cs10.0599 (3)0.0599 (3)0.0599 (3)0.0000.0000.000
Mg10.0250 (6)0.0250 (6)0.0250 (6)0.0000.0000.000
As10.0201 (2)0.0201 (2)0.0201 (2)0.0000.0000.000
O10.0259 (13)0.0632 (13)0.0632 (13)0.0000.000−0.033 (2)
O20.0269 (7)0.0269 (7)0.0269 (7)−0.0036 (7)−0.0036 (7)−0.0036 (7)

Geometric parameters (Å, °)

Cs1—O1i3.6239 (5)Mg1—O1xiii2.064 (3)
Cs1—O1ii3.6239 (5)Mg1—O1xiv2.064 (3)
Cs1—O1iii3.6239 (5)Mg1—O1xv2.064 (3)
Cs1—O1iv3.6239 (5)Mg1—O1xvi2.064 (3)
Cs1—O1v3.6239 (5)Mg1—O1xvii2.064 (3)
Cs1—O1vi3.6239 (5)Mg1—O12.064 (3)
Cs1—O1vii3.6239 (5)As1—O21.681 (3)
Cs1—O1viii3.6239 (5)As1—O2xviii1.681 (3)
Cs1—O1ix3.6239 (5)As1—O2xix1.681 (3)
Cs1—O1x3.6239 (5)As1—O2xx1.681 (3)
Cs1—O1xi3.6239 (5)O1—H10.79 (2)
Cs1—O1xii3.6239 (5)
O1i—Cs1—O1ii47.49 (8)O1iii—Cs1—O1xi72.12 (8)
O1i—Cs1—O1iii47.49 (8)O1iv—Cs1—O1xi47.49 (8)
O1ii—Cs1—O1iii47.49 (8)O1v—Cs1—O1xi90.991 (13)
O1i—Cs1—O1iv164.89 (10)O1vi—Cs1—O1xi119.429 (8)
O1ii—Cs1—O1iv119.430 (7)O1vii—Cs1—O1xi72.12 (8)
O1iii—Cs1—O1iv119.429 (7)O1viii—Cs1—O1xi119.429 (7)
O1i—Cs1—O1v119.429 (8)O1ix—Cs1—O1xi47.49 (8)
O1ii—Cs1—O1v164.89 (10)O1x—Cs1—O1xi164.89 (10)
O1iii—Cs1—O1v119.429 (7)O1i—Cs1—O1xii72.12 (8)
O1iv—Cs1—O1v72.12 (8)O1ii—Cs1—O1xii119.429 (7)
O1i—Cs1—O1vi119.429 (8)O1iii—Cs1—O1xii90.991 (13)
O1ii—Cs1—O1vi119.429 (8)O1iv—Cs1—O1xii119.429 (8)
O1iii—Cs1—O1vi164.89 (10)O1v—Cs1—O1xii47.49 (8)
O1iv—Cs1—O1vi72.12 (8)O1vi—Cs1—O1xii90.991 (13)
O1v—Cs1—O1vi72.12 (8)O1vii—Cs1—O1xii47.49 (8)
O1i—Cs1—O1vii90.991 (13)O1viii—Cs1—O1xii119.429 (7)
O1ii—Cs1—O1vii119.430 (7)O1ix—Cs1—O1xii164.89 (10)
O1iii—Cs1—O1vii72.12 (8)O1x—Cs1—O1xii72.12 (8)
O1iv—Cs1—O1vii90.991 (13)O1xi—Cs1—O1xii119.429 (8)
O1v—Cs1—O1vii47.49 (8)O1xiii—Mg1—O1xiv180.0
O1vi—Cs1—O1vii119.429 (8)O1xiii—Mg1—O1xv90.0
O1i—Cs1—O1viii90.991 (13)O1xiv—Mg1—O1xv90.0
O1ii—Cs1—O1viii72.12 (8)O1xiii—Mg1—O1xvi90.0
O1iii—Cs1—O1viii119.430 (7)O1xiv—Mg1—O1xvi90.0
O1iv—Cs1—O1viii90.991 (13)O1xv—Mg1—O1xvi180.0
O1v—Cs1—O1viii119.429 (8)O1xiii—Mg1—O1xvii90.0
O1vi—Cs1—O1viii47.49 (8)O1xiv—Mg1—O1xvii90.0
O1vii—Cs1—O1viii164.89 (10)O1xv—Mg1—O1xvii90.0
O1i—Cs1—O1ix119.429 (7)O1xvi—Mg1—O1xvii90.0
O1ii—Cs1—O1ix72.12 (8)O1xiii—Mg1—O190.0
O1iii—Cs1—O1ix90.991 (13)O1xiv—Mg1—O190.0
O1iv—Cs1—O1ix47.49 (8)O1xv—Mg1—O190.0
O1v—Cs1—O1ix119.429 (8)O1xvi—Mg1—O190.0
O1vi—Cs1—O1ix90.991 (13)O1xvii—Mg1—O1180.0
O1vii—Cs1—O1ix119.429 (8)O2—As1—O2xviii109.5
O1viii—Cs1—O1ix72.12 (8)O2—As1—O2xix109.471 (1)
O1i—Cs1—O1x72.12 (8)O2xviii—As1—O2xix109.5
O1ii—Cs1—O1x90.991 (13)O2—As1—O2xx109.5
O1iii—Cs1—O1x119.430 (7)O2xviii—As1—O2xx109.5
O1iv—Cs1—O1x119.429 (8)O2xix—As1—O2xx109.5
O1v—Cs1—O1x90.991 (13)Mg1—O1—Cs1xxi97.56 (5)
O1vi—Cs1—O1x47.49 (8)Mg1—O1—Cs1xxii97.56 (5)
O1vii—Cs1—O1x119.429 (7)Cs1xxi—O1—Cs1xxii164.89 (10)
O1viii—Cs1—O1x47.49 (8)Mg1—O1—H1126 (3)
O1ix—Cs1—O1x119.429 (7)Cs1xxi—O1—H185.6 (3)
O1i—Cs1—O1xi119.430 (7)Cs1xxii—O1—H185.6 (3)
O1ii—Cs1—O1xi90.991 (13)

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

Hydrogen-bond geometry (Å, °)

D—H···AD—HH···AD···AD—H···A
O1—H1···O2xix0.79 (2)1.86 (2)2.650 (2)176 (4)

Symmetry codes: (xix) x, −y+1/2, −z+1/2.

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB2885).

References

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