Redetermination of the hexa­gonal struvite analogue Cs[Mg(OH₂)₆](PO₄)

The structure of the hexa­gonal modification of caesium hexa­aqua­magnesium phosphate has been redetermined from single-crystal X-ray data. The previous refinement from photographic data [Ferrari, Calvaca & Nardelli (1955). Gazz. Chim. Ital. 85, 1232–1238] was basically confirmed, but with all H atoms located and with all non H-atoms refined with anisotropic displacement parameters. The structure can be derived from the NiAs structure type: the PO₄ tetra­hedra (3m. symmetry) are on the Ni positions and the complex [Mg(OH₂)₆] octa­hedra (3m. symmetry) are on the As positions. The building units are connected to each other by hydrogen bonds. The Cs⁺ cations (3m. symmetry) are located in the voids of this arrangement and exhibit a distorted cubocta­hedral 12-coordination by the O atoms of the water mol­ecules.

Related literature

The crystal structure of struvite, NH₄[Mg(OH₂)₆](PO₄), was reported by Whitaker & Jeffery (1970a,b). Structure determinations of the hexa­gonal and cubic forms of Cs[Mg(OH₂)₆](PO₄) were performed by Ferrari et al. (1955) and Massa et al. (2003), respectively. Crystal growth of struvite-like compounds using the gel diffusion technique was reported by Banks et al. (1975). For general background, see: Flack (1983).

Experimental

Crystal data

• Cs[Mg(H₂O)₆](PO₄)

• M_r = 360.29

• Hexagonal, P 6₃ m c

• a = 6.8827 (8) Å

• c = 11.9188 (16) Å

• V = 488.97 (10) ų

• Z = 2

• Mo Kα radiation

• μ = 4.04 mm⁻¹

• T = 293 (2) K

• 0.46 × 0.38 × 0.38 mm

Data collection

• Stoe IPDS diffractometer

• Absorption correction: numerical (HABITUS; Herrendorf, 1997) T_min = 0.213, T_max = 0.327

• 5412 measured reflections

• 368 independent reflections

• 367 reflections with I > 2σ(I)

• R_int = 0.025

Refinement

• R[F² > 2σ(F²)] = 0.016

• wR(F²) = 0.034

• S = 1.29

• 368 reflections

• 39 parameters

• 4 restraints

• H atoms treated by a mixture of independent and constrained refinement

• Δρ_max = 0.40 e Å⁻³

• Δρ_min = −0.31 e Å⁻³

• Absolute structure: Flack (1983), with 164 Friedel pairs

• Flack parameter: −0.01 (3)

Table 1
Selected bond lengths (Å)

Cs—O3^i	3.469 (5)
Cs—O4^i	3.470 (4)
Cs—O4^ii	3.742 (4)
Mg—O3	2.054 (4)
Mg—O4	2.082 (3)
P—O1	1.536 (3)
P—O2	1.539 (6)
Symmetry codes: (i) -y, x-y, z; (ii) .

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O3—H1⋯O2	0.93 (4)	1.71 (4)	2.643 (5)	180 (6)
O3—H2⋯O1^iii	0.88 (4)	1.76 (5)	2.630 (5)	168 (7)
O4—H3⋯O1^iv	0.83 (4)	1.85 (4)	2.672 (3)	177 (8)
Symmetry codes: (iii) ; (iv) -x+y, -x, z.

Data collection: EXPOSE in IPDS Software (Stoe & Cie, 1998); cell refinement: CELL in IPDS Software; data reduction: INTEGRATE in IPDS Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97.

Supporting information

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 & Jeffery, 1979a,b). However, for the isoformular compounds Cs[Mg(OH₂)₆](XO₄), where X = P and As, cubic and hexagonal forms were reported and structurally characterized (Ferrari et al., 1955; Massa et al., 2003). The cubic polymorph forms under hydrothermal conditions, whereas the hexagonal form is obtained under normal pressure and temperatures. All these structures can be described in terms of closed-packed layers with different stacking sequences (Massa et al., 2003).

In this communication the redetermination of hexagonal Cs[Mg(OH₂)₆](PO₄) is reported. The previous refinement from photographic data (Ferrari et al., 1955) was basically confirmed, but with all H atoms located and with all non H-atoms refined with anisotropic displacement parameters.

In addition to the description of the hexagonal Cs[Mg(OH₂)₆](PO₄) structure in terms of closed-packed layers (Massa et al., 2003), the structure can be described as a derivative of the NiAs structure type. The centres of the slightly distorted PO₄ tetrahedra (3m. symmetry) are situated on the Ni positions, whereas the centres of the likewise slightly distorted complex [Mg(H₂O)₆] octahedra (3m. symmetry) are situated on the As positions (Fig. 1). Thus one [Mg(H₂O)₆] octahedron is surrounded by six PO₄ tetrahedra in a distorted trigonal–prismatic arrangement, whereas one PO₄ tetrahedron is surrounded by six [Mg(H₂O)₆] octahedra in a distorted octahedral arrangement. The corresponding P—O and Mg—O distances are in the normal range (Table 1). These building units are linked via medium-strong hydrogen bonds (Table 2, Fig. 2). Details and differences of the hydrogen bonding schemes in cubic, hexagonal and struvite-type structures were discussed by Massa et al. (2003). The Cs⁺ cations are located in the voids of this arrangement and exhibit a distorted cuboctahedral 12-coordination [9+3] 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). Structure determinations of the hexagonal and cubic forms of Cs[Mg(OH₂)₆](PO₄) were performed by Ferrari et al. (1955) and Massa et al. (2003), respectively. Crystal growth of struvite-like compounds using the gel diffusion technique was reported by Banks et al. (1975). For general background, see: Flack (1983).

S2. Experimental

Colourless crystals of Cs[Mg(OH₂)₆](PO₄) with an edge-length up to 2 mm and mostly spherical habit 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 CsH₂PO₄ (50 ml) was carefully poured over the gel. This solution was then adjusted to pH 8.5 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 positions of the H atoms were found from difference Fourier maps and were refined with soft distance restraints (d(O—H) = 0.90 (5) Å) and a common U_iso parameter.

Computing details

Data collection: EXPOSE in IPDS Software (Stoe & Cie, 1998); cell refinement: CELL in IPDS Software (Stoe & Cie, 1998); data reduction: INTEGRATE in IPDS Software (Stoe & Cie, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); 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. Projection of the crystal structure of Cs[Mg(H2O)6](PO4) 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.
	Fig. 2. The [Mg(OH2)6] octahedron with six surrounding PO4 tetrahedra emphasizing the hydrogen bonding scheme (green dashed lines). The anisotropic displacement parameters are given at the 74% probability level. H atoms are given as spheres of arbitrary radius. [Symmetry codes: (i) y, x, z+1/2; (ii) -x+1, -y+1, -z+1.]

Caesium hexaaquamagnesium orthophosphate

Crystal data

Cs[Mg(H2O)6](PO4)	Dx = 2.447 Mg m^−3
Mr = 360.29	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mc	Cell parameters from 1544 reflections
Hall symbol: P 6c -2c	θ = 3.4–25.9°
a = 6.8827 (8) Å	µ = 4.04 mm^−1
c = 11.9188 (16) Å	T = 293 K
V = 488.97 (10) Å^3	Block, colourless
Z = 2	0.46 × 0.38 × 0.38 mm
F(000) = 348

Data collection

Stoe IPDS diffractometer	368 independent reflections
Radiation source: fine-focus sealed tube	367 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
ω scans	θmax = 25.7°, θmin = 3.4°
Absorption correction: numerical (HABITUS; Herrendorf, 1997)	h = −8→8
Tmin = 0.213, Tmax = 0.327	k = −8→8
5412 measured reflections	l = −12→13

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.016	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.034	w = 1/[σ^2(Fo^2) + (0.0029P)^2 + 0.8141P] where P = (Fo^2 + 2Fc^2)/3
S = 1.29	(Δ/σ)max < 0.001
368 reflections	Δρmax = 0.40 e Å^−3
39 parameters	Δρmin = −0.31 e Å^−3
4 restraints	Absolute structure: Flack (1983), with 164 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.01 (3)

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
Cs	0.3333	0.6667	0.02181 (4)	0.03126 (15)
Mg	0.6667	0.3333	0.1436 (2)	0.0159 (5)
P	0.0000	0.0000	0.29985 (14)	0.0137 (5)
O1	0.1220 (2)	0.2440 (4)	0.3413 (3)	0.0173 (6)
O2	0.0000	0.0000	0.1707 (5)	0.0189 (13)
O3	0.3747 (6)	0.1873 (3)	0.0521 (3)	0.0265 (9)
O4	0.5229 (3)	0.0458 (5)	0.2427 (3)	0.0282 (8)
H1	0.243 (8)	0.122 (4)	0.094 (5)	0.048 (9)*
H2	0.350 (12)	0.175 (6)	−0.021 (4)	0.048 (9)*
H3	0.397 (8)	−0.010 (14)	0.272 (5)	0.048 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.02671 (16)	0.02671 (16)	0.0404 (3)	0.01335 (8)	0.000	0.000
Mg	0.0158 (8)	0.0158 (8)	0.0160 (16)	0.0079 (4)	0.000	0.000
P	0.0139 (5)	0.0139 (5)	0.0134 (13)	0.0069 (2)	0.000	0.000
O1	0.0172 (9)	0.0139 (13)	0.0195 (17)	0.0070 (7)	−0.0023 (6)	−0.0046 (12)
O2	0.0177 (19)	0.0177 (19)	0.021 (4)	0.0089 (10)	0.000	0.000
O3	0.0142 (14)	0.0406 (14)	0.016 (3)	0.0071 (7)	−0.0023 (11)	−0.0012 (5)
O4	0.0191 (11)	0.0252 (19)	0.042 (2)	0.0126 (10)	0.0096 (7)	0.0191 (15)

Geometric parameters (Å, º)

Cs—O3^i	3.469 (5)	Mg—O3	2.054 (4)
Cs—O3^ii	3.469 (5)	Mg—O3^v	2.054 (4)
Cs—O3^iii	3.469 (5)	Mg—O4^ix	2.082 (3)
Cs—O3^iv	3.469 (5)	Mg—O4	2.082 (3)
Cs—O3^v	3.469 (5)	Mg—O4^v	2.082 (3)
Cs—O3	3.469 (5)	Mg—Cs^x	4.2306 (10)
Cs—O4^i	3.470 (4)	Mg—Cs^xi	4.2306 (10)
Cs—O4^iv	3.470 (4)	Mg—Cs^xii	4.508 (3)
Cs—O4^v	3.470 (4)	P—O1	1.536 (3)
Cs—O4^vi	3.742 (4)	P—O1^iv	1.536 (3)
Cs—O4^vii	3.742 (4)	P—O1^xiii	1.536 (3)
Cs—O4^viii	3.742 (4)	P—O2	1.539 (6)
Mg—O3^ix	2.054 (4)
O3^i—Cs—O3^ii	51.51 (11)	O3^v—Cs—O4^vii	97.18 (5)
O3^i—Cs—O3^iii	67.77 (11)	O3—Cs—O4^vii	118.02 (7)
O3^ii—Cs—O3^iii	118.93 (2)	O4^i—Cs—O4^vii	112.101 (19)
O3^i—Cs—O3^iv	118.93 (2)	O4^iv—Cs—O4^vii	145.46 (3)
O3^ii—Cs—O3^iv	165.52 (11)	O4^v—Cs—O4^vii	145.46 (3)
O3^iii—Cs—O3^iv	51.51 (11)	O4^vi—Cs—O4^vii	46.74 (8)
O3^i—Cs—O3^v	118.93 (2)	O3^i—Cs—O4^viii	118.02 (7)
O3^ii—Cs—O3^v	67.77 (11)	O3^ii—Cs—O4^viii	97.18 (5)
O3^iii—Cs—O3^v	165.52 (11)	O3^iii—Cs—O4^viii	118.02 (7)
O3^iv—Cs—O3^v	118.93 (2)	O3^iv—Cs—O4^viii	97.18 (5)
O3^i—Cs—O3	165.52 (11)	O3^v—Cs—O4^viii	71.50 (7)
O3^ii—Cs—O3	118.93 (2)	O3—Cs—O4^viii	71.50 (7)
O3^iii—Cs—O3	118.93 (2)	O4^i—Cs—O4^viii	145.46 (3)
O3^iv—Cs—O3	67.77 (11)	O4^iv—Cs—O4^viii	145.46 (3)
O3^v—Cs—O3	51.51 (11)	O4^v—Cs—O4^viii	112.101 (19)
O3^i—Cs—O4^i	48.58 (7)	O4^vi—Cs—O4^viii	46.74 (8)
O3^ii—Cs—O4^i	48.58 (7)	O4^vii—Cs—O4^viii	46.74 (8)
O3^iii—Cs—O4^i	88.12 (6)	O3^ix—Mg—O3	94.42 (16)
O3^iv—Cs—O4^i	117.22 (7)	O3^ix—Mg—O3^v	94.42 (16)
O3^v—Cs—O4^i	88.12 (6)	O3—Mg—O3^v	94.42 (16)
O3—Cs—O4^i	117.22 (7)	O3^ix—Mg—O4^ix	87.27 (10)
O3^i—Cs—O4^iv	88.12 (6)	O3—Mg—O4^ix	177.5 (2)
O3^ii—Cs—O4^iv	117.22 (7)	O3^v—Mg—O4^ix	87.27 (10)
O3^iii—Cs—O4^iv	48.58 (7)	O3^ix—Mg—O4	87.27 (10)
O3^iv—Cs—O4^iv	48.58 (7)	O3—Mg—O4	87.27 (10)
O3^v—Cs—O4^iv	117.22 (7)	O3^v—Mg—O4	177.5 (2)
O3—Cs—O4^iv	88.12 (6)	O4^ix—Mg—O4	90.98 (19)
O4^i—Cs—O4^iv	68.67 (8)	O3^ix—Mg—O4^v	177.5 (2)
O3^i—Cs—O4^v	117.22 (7)	O3—Mg—O4^v	87.27 (10)
O3^ii—Cs—O4^v	88.12 (6)	O3^v—Mg—O4^v	87.27 (10)
O3^iii—Cs—O4^v	117.22 (7)	O4^ix—Mg—O4^v	90.98 (19)
O3^iv—Cs—O4^v	88.12 (6)	O4—Mg—O4^v	90.98 (19)
O3^v—Cs—O4^v	48.58 (7)	O1—P—O1^iv	110.17 (14)
O3—Cs—O4^v	48.58 (7)	O1—P—O1^xiii	110.17 (14)
O4^i—Cs—O4^v	68.67 (8)	O1^iv—P—O1^xiii	110.17 (14)
O4^iv—Cs—O4^v	68.67 (8)	O1—P—O2	108.76 (14)
O3^i—Cs—O4^vi	97.18 (5)	O1^iv—P—O2	108.76 (14)
O3^ii—Cs—O4^vi	118.02 (7)	O1^xiii—P—O2	108.76 (14)
O3^iii—Cs—O4^vi	71.50 (7)	Mg—O3—Cs^x	96.63 (6)
O3^iv—Cs—O4^vi	71.50 (7)	Mg—O3—Cs	96.63 (6)
O3^v—Cs—O4^vi	118.02 (7)	Cs^x—O3—Cs	165.52 (11)
O3—Cs—O4^vi	97.18 (5)	Mg—O3—H1	116 (4)
O4^i—Cs—O4^vi	145.46 (3)	Mg—O3—H2	132 (5)
O4^iv—Cs—O4^vi	112.101 (19)	H1—O3—H2	113 (6)
O4^v—Cs—O4^vi	145.46 (3)	Mg—O4—Cs^x	96.07 (16)
O3^i—Cs—O4^vii	71.50 (7)	Mg—O4—Cs^xii	97.31 (14)
O3^ii—Cs—O4^vii	71.50 (7)	Cs^x—O4—Cs^xii	166.63 (10)
O3^iii—Cs—O4^vii	97.18 (5)	Mg—O4—H3	125 (4)
O3^iv—Cs—O4^vii	118.02 (7)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H1···O2	0.93 (4)	1.71 (4)	2.643 (5)	180 (6)
O3—H2···O1^xiv	0.88 (4)	1.76 (5)	2.630 (5)	168 (7)
O4—H3···O1^xiii	0.83 (4)	1.85 (4)	2.672 (3)	177 (8)

Symmetry codes: (xiii) −x+y, −x, z; (xiv) y, −x+y, z−1/2.