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

Weil, M.

doi:10.1107/S1600536808043171

inorganic compounds

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Volume 65| Part 1| January 2009| Page i2

doi:10.1107/S1600536808043171

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

Matthias Weil ^a ^*

^aInstitute for Chemical Technologies and Analytics, Division of Structural Chemistry, Vienna University of Technology, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria
^*Correspondence e-mail: mweil@mail.zserv.tuwien.ac.at

(Received 18 December 2008; accepted 18 December 2008; online 20 December 2008)

In contrast to the previous refinement from photographic data [Ferrari et al. (1955 ). 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 $[\overline{4}]$ 3m. The structure can be derived from the halite structure. The centres of the complex [Mg(OH₂)₆] octahedra and the AsO₄ tetrahedra (both with $[\overline{4}]$ 3m 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 ( $[\overline{4}]$ 3m 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 ,b ). Crystal growth of struvite-type compounds using the gel diffusion technique was reported by Banks et al. (1975 ). For isotypic structures, see: Carver et al. (2006 ) for Cs[Fe(OH₂)₆](PO₄) and Massa et al. (2003 ) 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 ).

Experimental

Crystal data

Cs[Mg(OH₂)₆](AsO₄)
M_r = 404.24
Cubic, $[F \overline 43m ]$
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 ) 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[F² > 2σ(F²)] = 0.016
wR(F²) = 0.038
S = 1.14
208 reflections
15 parameters
1 restraint
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.50 e Å⁻³
Δρ_min = −0.48 e Å⁻³
Absolute structure: Flack (1983 ), 90 Friedel pairs
Flack parameter: 0.01 (4)

Table 1
Selected bond lengths (Å)

Cs1—O1ⁱ	3.6239 (5)
Mg1—O1	2.064 (3)
As1—O2	1.681 (3)
Symmetry code: (i) -x+1, -y+1, z+1.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯O2ⁱⁱ	0.79 (2)	1.86 (2)	2.650 (2)	176 (4)
Symmetry code: (ii) $[x, -y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ .

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

Supporting information

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

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

checkCIF report

Comment top

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).

Related literature top

The crystal structure of struvite, NH₄[Mg(OH₂)₆](PO₄), was reported by Whitaker & Jeffery (1970a,b). Crystal growth of struvite-type compounds using the gel diffusion technique was reported by Banks et al. (1975). For isotypic structures, see: Carver et al. (2006) for Cs[Fe(OH₂)₆](PO₄) and Massa et al. (2003) 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).

Experimental top

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.

Computing details top

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

Figures top

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.

Caesium hexaquamagnesium orthoarsenate(V) top

Crystal data top

Cs[Mg(H₂O)₆](AsO₄)	D_x = 2.559 Mg m⁻³
M_r = 404.24	Mo Kα radiation, λ = 0.71073 Å
Cubic, F43m	Cell parameters from 25 reflections
Hall symbol: F -4 2 3	θ = 11.4–12.7°
a = 10.1609 (5) Å	µ = 6.75 mm⁻¹
V = 1049.05 (9) Å³	T = 293 K
Z = 4	Octahedron, colourless
F(000) = 768	0.18 × 0.18 × 0.18 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	207 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.037
Graphite monochromator	θ_max = 30.9°, θ_min = 3.5°
ω/2θ scans	h = −14→14
Absorption correction: integration (SHELXTL; Sheldrick, 2008)	k = −14→14
T_min = 0.341, T_max = 0.382	l = −14→14
3332 measured reflections	3 standard reflections every 200 min
208 independent reflections	intensity decay: none

Refinement top

Refinement on F²	H atoms treated by a mixture of independent and constrained refinement
Least-squares matrix: full	w = 1/[σ²(F_o²) + 5.0469P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.016	(Δ/σ)_max < 0.001
wR(F²) = 0.038	Δρ_max = 0.50 e Å⁻³
S = 1.14	Δρ_min = −0.48 e Å⁻³
208 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
15 parameters	Extinction coefficient: 0.00144 (17)
1 restraint	Absolute structure: Flack (1983), 90 Friedel pairs
Primary atom site location: isomorphous structure methods	Absolute structure parameter: 0.01 (4)

Crystal data top

Cs[Mg(H₂O)₆](AsO₄)	Z = 4
M_r = 404.24	Mo Kα radiation
Cubic, F43m	µ = 6.75 mm⁻¹
a = 10.1609 (5) Å	T = 293 K
V = 1049.05 (9) Å³	0.18 × 0.18 × 0.18 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	207 reflections with I > 2σ(I)
Absorption correction: integration (SHELXTL; Sheldrick, 2008)	R_int = 0.037
T_min = 0.341, T_max = 0.382	3 standard reflections every 200 min
3332 measured reflections	intensity decay: none
208 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.016	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.038	Δρ_max = 0.50 e Å⁻³
S = 1.14	Δρ_min = −0.48 e Å⁻³
208 reflections	Absolute structure: Flack (1983), 90 Friedel pairs
15 parameters	Absolute structure parameter: 0.01 (4)
1 restraint

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

	x	y	z	U_iso*/U_eq
Cs1	0.7500	0.7500	0.7500	0.0599 (3)
Mg1	0.0000	0.0000	0.0000	0.0250 (6)
As1	0.2500	0.2500	0.2500	0.0201 (2)
O1	0.2031 (3)	0.0000	0.0000	0.0508 (8)
O2	0.34550 (19)	0.34550 (19)	0.34550 (19)	0.0269 (7)
H1	0.249 (3)	0.045 (2)	0.045 (2)	0.053 (12)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cs1	0.0599 (3)	0.0599 (3)	0.0599 (3)	0.000	0.000	0.000
Mg1	0.0250 (6)	0.0250 (6)	0.0250 (6)	0.000	0.000	0.000
As1	0.0201 (2)	0.0201 (2)	0.0201 (2)	0.000	0.000	0.000
O1	0.0259 (13)	0.0632 (13)	0.0632 (13)	0.000	0.000	−0.033 (2)
O2	0.0269 (7)	0.0269 (7)	0.0269 (7)	−0.0036 (7)	−0.0036 (7)	−0.0036 (7)

Geometric parameters (Å, º) top

Cs1—O1ⁱ	3.6239 (5)	Mg1—O1^xiii	2.064 (3)
Cs1—O1ⁱⁱ	3.6239 (5)	Mg1—O1^xiv	2.064 (3)
Cs1—O1ⁱⁱⁱ	3.6239 (5)	Mg1—O1^xv	2.064 (3)
Cs1—O1^iv	3.6239 (5)	Mg1—O1^xvi	2.064 (3)
Cs1—O1^v	3.6239 (5)	Mg1—O1^xvii	2.064 (3)
Cs1—O1^vi	3.6239 (5)	Mg1—O1	2.064 (3)
Cs1—O1^vii	3.6239 (5)	As1—O2	1.681 (3)
Cs1—O1^viii	3.6239 (5)	As1—O2^xviii	1.681 (3)
Cs1—O1^ix	3.6239 (5)	As1—O2^xix	1.681 (3)
Cs1—O1^x	3.6239 (5)	As1—O2^xx	1.681 (3)
Cs1—O1^xi	3.6239 (5)	O1—H1	0.79 (2)
Cs1—O1^xii	3.6239 (5)

O1ⁱ—Cs1—O1ⁱⁱ	47.49 (8)	O1ⁱⁱⁱ—Cs1—O1^xi	72.12 (8)
O1ⁱ—Cs1—O1ⁱⁱⁱ	47.49 (8)	O1^iv—Cs1—O1^xi	47.49 (8)
O1ⁱⁱ—Cs1—O1ⁱⁱⁱ	47.49 (8)	O1^v—Cs1—O1^xi	90.991 (13)
O1ⁱ—Cs1—O1^iv	164.89 (10)	O1^vi—Cs1—O1^xi	119.429 (8)
O1ⁱⁱ—Cs1—O1^iv	119.430 (7)	O1^vii—Cs1—O1^xi	72.12 (8)
O1ⁱⁱⁱ—Cs1—O1^iv	119.429 (7)	O1^viii—Cs1—O1^xi	119.429 (7)
O1ⁱ—Cs1—O1^v	119.429 (8)	O1^ix—Cs1—O1^xi	47.49 (8)
O1ⁱⁱ—Cs1—O1^v	164.89 (10)	O1^x—Cs1—O1^xi	164.89 (10)
O1ⁱⁱⁱ—Cs1—O1^v	119.429 (7)	O1ⁱ—Cs1—O1^xii	72.12 (8)
O1^iv—Cs1—O1^v	72.12 (8)	O1ⁱⁱ—Cs1—O1^xii	119.429 (7)
O1ⁱ—Cs1—O1^vi	119.429 (8)	O1ⁱⁱⁱ—Cs1—O1^xii	90.991 (13)
O1ⁱⁱ—Cs1—O1^vi	119.429 (8)	O1^iv—Cs1—O1^xii	119.429 (8)
O1ⁱⁱⁱ—Cs1—O1^vi	164.89 (10)	O1^v—Cs1—O1^xii	47.49 (8)
O1^iv—Cs1—O1^vi	72.12 (8)	O1^vi—Cs1—O1^xii	90.991 (13)
O1^v—Cs1—O1^vi	72.12 (8)	O1^vii—Cs1—O1^xii	47.49 (8)
O1ⁱ—Cs1—O1^vii	90.991 (13)	O1^viii—Cs1—O1^xii	119.429 (7)
O1ⁱⁱ—Cs1—O1^vii	119.430 (7)	O1^ix—Cs1—O1^xii	164.89 (10)
O1ⁱⁱⁱ—Cs1—O1^vii	72.12 (8)	O1^x—Cs1—O1^xii	72.12 (8)
O1^iv—Cs1—O1^vii	90.991 (13)	O1^xi—Cs1—O1^xii	119.429 (8)
O1^v—Cs1—O1^vii	47.49 (8)	O1^xiii—Mg1—O1^xiv	180.0
O1^vi—Cs1—O1^vii	119.429 (8)	O1^xiii—Mg1—O1^xv	90.0
O1ⁱ—Cs1—O1^viii	90.991 (13)	O1^xiv—Mg1—O1^xv	90.0
O1ⁱⁱ—Cs1—O1^viii	72.12 (8)	O1^xiii—Mg1—O1^xvi	90.0
O1ⁱⁱⁱ—Cs1—O1^viii	119.430 (7)	O1^xiv—Mg1—O1^xvi	90.0
O1^iv—Cs1—O1^viii	90.991 (13)	O1^xv—Mg1—O1^xvi	180.0
O1^v—Cs1—O1^viii	119.429 (8)	O1^xiii—Mg1—O1^xvii	90.0
O1^vi—Cs1—O1^viii	47.49 (8)	O1^xiv—Mg1—O1^xvii	90.0
O1^vii—Cs1—O1^viii	164.89 (10)	O1^xv—Mg1—O1^xvii	90.0
O1ⁱ—Cs1—O1^ix	119.429 (7)	O1^xvi—Mg1—O1^xvii	90.0
O1ⁱⁱ—Cs1—O1^ix	72.12 (8)	O1^xiii—Mg1—O1	90.0
O1ⁱⁱⁱ—Cs1—O1^ix	90.991 (13)	O1^xiv—Mg1—O1	90.0
O1^iv—Cs1—O1^ix	47.49 (8)	O1^xv—Mg1—O1	90.0
O1^v—Cs1—O1^ix	119.429 (8)	O1^xvi—Mg1—O1	90.0
O1^vi—Cs1—O1^ix	90.991 (13)	O1^xvii—Mg1—O1	180.0
O1^vii—Cs1—O1^ix	119.429 (8)	O2—As1—O2^xviii	109.5
O1^viii—Cs1—O1^ix	72.12 (8)	O2—As1—O2^xix	109.471 (1)
O1ⁱ—Cs1—O1^x	72.12 (8)	O2^xviii—As1—O2^xix	109.5
O1ⁱⁱ—Cs1—O1^x	90.991 (13)	O2—As1—O2^xx	109.5
O1ⁱⁱⁱ—Cs1—O1^x	119.430 (7)	O2^xviii—As1—O2^xx	109.5
O1^iv—Cs1—O1^x	119.429 (8)	O2^xix—As1—O2^xx	109.5
O1^v—Cs1—O1^x	90.991 (13)	Mg1—O1—Cs1^xxi	97.56 (5)
O1^vi—Cs1—O1^x	47.49 (8)	Mg1—O1—Cs1^xxii	97.56 (5)
O1^vii—Cs1—O1^x	119.429 (7)	Cs1^xxi—O1—Cs1^xxii	164.89 (10)
O1^viii—Cs1—O1^x	47.49 (8)	Mg1—O1—H1	126 (3)
O1^ix—Cs1—O1^x	119.429 (7)	Cs1^xxi—O1—H1	85.6 (3)
O1ⁱ—Cs1—O1^xi	119.430 (7)	Cs1^xxii—O1—H1	85.6 (3)
O1ⁱⁱ—Cs1—O1^xi	90.991 (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, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O2^xix	0.79 (2)	1.86 (2)	2.650 (2)	176 (4)

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

Experimental details

Crystal data
Chemical formula	Cs[Mg(H₂O)₆](AsO₄)
M_r	404.24
Crystal system, space group	Cubic, F43m
Temperature (K)	293
a (Å)	10.1609 (5)
V (Å³)	1049.05 (9)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	6.75
Crystal size (mm)	0.18 × 0.18 × 0.18

Data collection
Diffractometer	Enraf–Nonius CAD-4 diffractometer
Absorption correction	Integration (SHELXTL; Sheldrick, 2008)
T_min, T_max	0.341, 0.382
No. of measured, independent and observed [I > 2σ(I)] reflections	3332, 208, 207
R_int	0.037
(sin θ/λ)_max (Å⁻¹)	0.723

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.016, 0.038, 1.14
No. of reflections	208
No. of parameters	15
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.50, −0.48
Absolute structure	Flack (1983), 90 Friedel pairs
Absolute structure parameter	0.01 (4)

Computer programs: CAD-4 Software (Enraf–Nonius, 1989), HELENA implemented in PLATON (Spek, 2003), coordinates taken from an isotypic compound, SHELXL97 (Sheldrick, 2008), ATOMS (Dowty, 2004).

Selected bond lengths (Å) top

Cs1—O1ⁱ	3.6239 (5)	As1—O2	1.681 (3)
Mg1—O1	2.064 (3)

Symmetry code: (i) −x+1, −y+1, z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O2ⁱⁱ	0.79 (2)	1.86 (2)	2.650 (2)	176 (4)

Symmetry code: (ii) x, −y+1/2, −z+1/2.

References

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ISSN: 2056-9890

Volume 65| Part 1| January 2009| Page i2

doi:10.1107/S1600536808043171

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access