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

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 F3m. The structure can be derived from the halite structure. The centres of the complex [Mg(OH₂)₆] octa­hedra and the AsO₄ tetra­hedra (both with 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 (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).

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Mg-O) = 0.003 Å
• R factor = 0.016
• wR factor = 0.038
• Data-to-parameter ratio = 13.9

checkCIF/PLATON results

No syntax errors found



Alert level C
            Value of measurement temperature given =   293.000
            Value of melting point given =     0.000
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       0.50 Ratio

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           30.93
           From the CIF: _reflns_number_total                208
           Count of symmetry unique reflns          115
           Completeness (_total/calc)            180.87%
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured        93
           Fraction of Friedel pairs measured     0.809
           Are heavy atom types Z>Si present        yes
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature .        293 K

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   4 ALERT level G = General alerts; check

   5 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. 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).

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).

S2. 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.

S3. 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.

Computing details

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

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)

Crystal data

Cs[Mg(H2O)6](AsO4)	Dx = 2.559 Mg m^−3
Mr = 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) Å^3	T = 293 K
Z = 4	Octahedron, colourless
F(000) = 768	0.18 × 0.18 × 0.18 mm

Data collection

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

Refinement

Refinement on F^2	H atoms treated by a mixture of independent and constrained refinement
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + 5.0469P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.016	(Δ/σ)max < 0.001
wR(F^2) = 0.038	Δρmax = 0.50 e Å^−3
S = 1.14	Δρmin = −0.48 e Å^−3
208 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/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)

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
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 (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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)

Cs1—O1^i	3.6239 (5)	Mg1—O1^xiii	2.064 (3)
Cs1—O1^ii	3.6239 (5)	Mg1—O1^xiv	2.064 (3)
Cs1—O1^iii	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)
O1^i—Cs1—O1^ii	47.49 (8)	O1^iii—Cs1—O1^xi	72.12 (8)
O1^i—Cs1—O1^iii	47.49 (8)	O1^iv—Cs1—O1^xi	47.49 (8)
O1^ii—Cs1—O1^iii	47.49 (8)	O1^v—Cs1—O1^xi	90.991 (13)
O1^i—Cs1—O1^iv	164.89 (10)	O1^vi—Cs1—O1^xi	119.429 (8)
O1^ii—Cs1—O1^iv	119.430 (7)	O1^vii—Cs1—O1^xi	72.12 (8)
O1^iii—Cs1—O1^iv	119.429 (7)	O1^viii—Cs1—O1^xi	119.429 (7)
O1^i—Cs1—O1^v	119.429 (8)	O1^ix—Cs1—O1^xi	47.49 (8)
O1^ii—Cs1—O1^v	164.89 (10)	O1^x—Cs1—O1^xi	164.89 (10)
O1^iii—Cs1—O1^v	119.429 (7)	O1^i—Cs1—O1^xii	72.12 (8)
O1^iv—Cs1—O1^v	72.12 (8)	O1^ii—Cs1—O1^xii	119.429 (7)
O1^i—Cs1—O1^vi	119.429 (8)	O1^iii—Cs1—O1^xii	90.991 (13)
O1^ii—Cs1—O1^vi	119.429 (8)	O1^iv—Cs1—O1^xii	119.429 (8)
O1^iii—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)
O1^i—Cs1—O1^vii	90.991 (13)	O1^viii—Cs1—O1^xii	119.429 (7)
O1^ii—Cs1—O1^vii	119.430 (7)	O1^ix—Cs1—O1^xii	164.89 (10)
O1^iii—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
O1^i—Cs1—O1^viii	90.991 (13)	O1^xiv—Mg1—O1^xv	90.0
O1^ii—Cs1—O1^viii	72.12 (8)	O1^xiii—Mg1—O1^xvi	90.0
O1^iii—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
O1^i—Cs1—O1^ix	119.429 (7)	O1^xvi—Mg1—O1^xvii	90.0
O1^ii—Cs1—O1^ix	72.12 (8)	O1^xiii—Mg1—O1	90.0
O1^iii—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)
O1^i—Cs1—O1^x	72.12 (8)	O2^xviii—As1—O2^xix	109.5
O1^ii—Cs1—O1^x	90.991 (13)	O2—As1—O2^xx	109.5
O1^iii—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)
O1^i—Cs1—O1^xi	119.430 (7)	Cs1^xxii—O1—H1	85.6 (3)
O1^ii—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)

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.