A new cubic form of caesium hexa­aqua­magnesium phosphate, Cs[Mg(H₂O)₆](PO₄)

A new cubic form (space group F3m) of the title compound has been found which is isostructural with the analogous arsenate. [Mg(H₂O)₆]²⁺ cations and phosphate anions are connected by hydrogen bonds, forming a sphalerite-like three-dimensional framework.

S1. Comment

In the course of investigations of magnesium phosphate minerals, such as bakchisaraitsevite, Na₂(Mg_4.5Fe_0.5)[PO₄]₄·7H₂O (Yakubovich et al., 2000), and their synthetic relatives, we have studied the products of soft hydrothermal syntheses in the Cs₃PO₄–Mg(OH)₂–H₂O system and found crystals of a new polymorph of Cs[Mg(H₂O)₆](PO₄). Its structure has been determined by single-crystal X-ray diffraction. A hexagonal form was already known for this compound (Ferrari et al., 1955), but a cubic structure was deduced from powder diffraction data for the As analogue, Cs[Mg(H₂O)₆](AsO₄).

Our crystal of Cs[Mg(H₂O)₆](PO₄) belongs to the cubic F-43m space group. According to the high site symmetry (−43m) of the 4 d position, the Mg²⁺ ions occupy regular octahedra formed by the O2 atoms, with Mg—O distances of 2.065 (3) Å (Fig. 1). H atoms were localized and refined on the 48t h position, resulting in a sensible geometry for the aqua ligands.

Regular PO₄ tetrahedra are connected `face-to-face' to four neighbouring [Mg(H₂O)₆]²⁺ cations by three strong hydrogen bonds [O2···O1 = 2.649 (2) Å and O2—H···O1 173 (5)°; Figs. 1 and 2, and Table 1]. Conversely, the octahedral cations are tetrahedrally surrounded by four anions. Thus, a sphalerite-like three-dimensional framework is formed, with [Mg(H₂O)₆]²⁺ and [PO₄]³⁻ complexes in the positions of the Zn²⁺ and S²⁻ ions. The Cs⁺ cations center the volume and the edges of the cubic unit cell (Fig. 2). They are in the center of a cuboctahedron of 12 H₂O-ligands from four [Mg(H₂O)₆]²⁺ cations. If the contacts of the Cs⁺ anions to these ligands are included, the O2 atoms are in an approximate trigonal-pyramidal environment. The two cations and the phosphate anion form cubic F-centered sublattices with shifted origins in 0,0,0 (Mg), 1/4,1/4,1/4 (P) and 3/4,3/4,3/4 (Cs), respectively. Thus, this simple cubic structure can also be derived from a rock-salt type '{Cs(PO₄)}²⁻' structure, with [Mg(H₂O)₆]²⁺ cations in every second center of the Cs₄(PO₄)₄ sub-cubes. This structure corresponds to that proposed for the arsenate analogue, Cs[Mg(H₂O)₆](AsO₄), on the basis of powder data (Ferrari et al., 1955). The larger atomic radius of As compared with P is consistent with the enlargement of the unit-cell volume by about 6% with respect to the phosphate. For the latter, these authors found a hexagonal form [space group P6₃mc; a = 6.939 (2) Å and c = 11.896 (2) Å]. The volume per formula unit in the cubic polymorph [252.3 (1) ų] is slightly larger (1.7%) than that of the hexagonal form (248.0 ų), pointing to metastability of the cubic form.

The differences between the two structural forms can be described in terms of different sequences of close-packed layers of the Cs⁺, [Mg(H₂O)₆]²⁺ and PO₄³⁻ components.

In the cubic form, each sublattice is arranged along the [111] direction in a cubic close packing (CCP), as indicated in Fig. 3(a) by the ABC (PO₄), bca ([Mg(H₂O)₆]) and γαβ (Cs) sequences. In addition, the sequence of these alternating layers is also of the CCP-type (Abγ Bcα Caβ). In the hexagonal form, the [Mg(H₂O)₆]²⁺ and Cs⁺ cations form hexagonal close-packed sequences (bcb and γβγ) along the c axis. The phosphate anions are aligned along c but with alternating orientation (AA'A; Fig. 3 b).

A similar pseudohexagonal unit cell can be identified in the orthorhombic structures (space group Pmn2₁) of two analogues of the title compound, viz. the mineral struvite, (NH₄)Mg(H₂O)₆[PO₄], which seems to form as a result of bacterial attack on organic material (Whitaker & Jefferey, 1970; Dickens & Brown, 1972; Abbona et al., 1986; Ferraris et al., 1986), and a synthetic phase, KMg(H₂O)₆[PO₄] (Mathew & Schröder, 1979). According to Dickens & Brown (1972), the {h0l} reflections of struvite show pseudohexagonal symmetry. As shown in Fig. 3c, layers of [PO₄] tetrahedra are now stacked in a hexagonal primitive sequence (AAA). In contrast to the Cs compound, mixed layers of [Mg(H₂O)₆]²⁺ and ammonium cations follow so that, together with the phosphate anions, a cubic sequence results (AbcAbc).

The different packings in these related compounds correlate with significant differences in the hydrogen bonding networks. In the cubic Cs compound, the [Mg(H₂O)₆]²⁺ cations are tetrahedrally surrounded by four phosphate anions with four `face-to-face' triple hydrogen bonds. Although no H-atom positions are reported for the hexagonal form, it can be concluded from the O···O distances that the [Mg(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> cations are surrounded by six PO<sub>4</sub> anions in a trigonal-prismatic arrangement and are connected by three `edge-to-corner' and three `corner-to-edge' hydrogen bonds. A similar trigonal prismatic coordination is found in the struvite structure, which is connected by two `face-to-face', one `edge-to-edge' and three `corner-to-corner' hydrogen bonds. In addition, an O—H···N hydrogen bond links? one of the aqua ligands to the ammonium cation.

S2. Experimental

Cs[Mg(H₂O)₆](PO₄) was formed by hydrothermal synthesis in the Cs₃PO₄–Mg(OH)₂–H₂O system (T = 280°C, P = 70 bar, t = 20 days, ratio Cs₃PO₄/Mg(OH)₂ = 1:1) in 7 ml Cu tubes in a steel autoclave. Very small (<0.05 mm) colourless octahedral crystals often formed aggregates of 15–20 individuals. The presence of Cs, P and Mg atoms in the samples was confirmed by qualitative X-ray spectral analysis (CamScan 4DV and EDA Link AN 1000).

Computing details

Data collection: Win-Xpose in X-AREA (Stoe & Cie, 2000); cell refinement: Win-Cell in X-AREA; data reduction: Win-Integrate in X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999).

Figures

	Fig. 1. : A DIAMOND (Brandenburg, 1999) drawing of the [Mg(H2O)6] cation and two neighbouring phosphate anions, with the atomic numbering scheme. Displacement ellipsoids have been drawn at the 50% probability level, and hydrogen bonds are shown as dotted lines.
	Fig. 2. : A view of the unit cell of Cs[Mg(H2O)6](PO4). Octahedra, tetrahedra and spheres represent [Mg(H2O)6]^2+ cations, [PO4]^3− anions and Cs^+ cations, respectively.
	Fig. 3. : A comparison of the stacking layers in (a) cubic Cs[Mg(H2O)6](PO4), (b) hexagonal Cs[Mg(H2O)6](PO4) and (c) struvite, NH4[Mg(H2O)6](PO4). Large spheres represent [Mg(H2O)6]^2+ cations, small spheres represent Cs^+ or NH4^+ cations, and tetrahedra represent PO4^3− anions.

caesium hexaaquamagnesium phosphate

Crystal data

Cs[Mg(H2O)6](PO4)	Mo Kα radiation, λ = 0.71073 Å
Mr = 360.29	Cell parameters from 2068 reflections
Cubic, F43m	θ = 3.5–29.6°
a = 10.0308 (14) Å	µ = 3.92 mm^−1
V = 1009.3 (2) Å^3	T = 295 K
Z = 4	Octahedron, colourless
F(000) = 696	0.04 × 0.04 × 0.04 mm
Dx = 2.371 Mg m^−3

Data collection

Stoe IPDS-II diffractometer	176 independent reflections
Radiation source: fine-focus sealed tube	163 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
Detector resolution: 150 pixels mm^-1	θmax = 29.7°, θmin = 3.5°
ω scans	h = −13→13
Absorption correction: multi-scan XPREP in SHELXTL (Sheldrick, 1996)	k = −12→12
Tmin = 0.774, Tmax = 0.831	l = −13→12
1751 measured reflections

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)] = 0.017	All H-atom parameters refined
wR(F^2) = 0.035	w = 1/[σ^2(Fo^2) + (0.02P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max < 0.001
176 reflections	Δρmax = 0.19 e Å^−3
14 parameters	Δρmin = −0.21 e Å^−3
0 restraints	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.04 (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	0.7500	0.7500	0.7500	0.0560 (2)
Mg	0.0000	0.0000	0.0000	0.0307 (5)
P	0.2500	0.2500	0.2500	0.0279 (3)
O2	0.2059 (3)	0.0000	0.0000	0.0565 (8)
O1	0.33857 (17)	0.33857 (17)	0.33857 (17)	0.0346 (6)
H	0.252 (5)	0.047 (2)	0.047 (2)	0.052 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.0560 (2)	0.0560 (2)	0.0560 (2)	0.000	0.000	0.000
Mg	0.0307 (5)	0.0307 (5)	0.0307 (5)	0.000	0.000	0.000
P	0.0279 (3)	0.0279 (3)	0.0279 (3)	0.000	0.000	0.000
O2	0.0335 (12)	0.0679 (12)	0.0679 (12)	0.000	0.000	−0.0360 (18)
O1	0.0346 (6)	0.0346 (6)	0.0346 (6)	−0.0032 (6)	−0.0032 (6)	−0.0032 (6)

Geometric parameters (Å, º)

Cs—O2^i	3.5740 (6)	Mg—O2^xv	2.065 (3)
Cs—O2^ii	3.5740 (6)	Mg—O2^xvi	2.065 (3)
Cs—O2^iii	3.5740 (6)	Mg—O2^xvii	2.065 (3)
Cs—O2^iv	3.5740 (6)	Mg—Cs^xviii	4.3435 (6)
Cs—O2^v	3.5740 (6)	Mg—Cs^xix	4.3435 (6)
Cs—O2^vi	3.5740 (6)	Mg—Cs^xx	4.3435 (6)
Cs—O2^vii	3.5739 (6)	Mg—Cs^xxi	4.3435 (6)
Cs—O2^viii	3.5739 (6)	P—O1	1.539 (3)
Cs—O2^ix	3.5740 (6)	P—O1^xxii	1.539 (3)
Cs—O2^x	3.5739 (6)	P—O1^xxiii	1.539 (3)
Cs—O2^xi	3.5739 (6)	P—O1^xxiv	1.539 (3)
Cs—O2^xii	3.5740 (6)	O2—Cs^xix	3.5740 (6)
Mg—O2	2.065 (3)	O2—Cs^xx	3.5740 (6)
Mg—O2^xiii	2.065 (3)	O2—H	0.82 (4)
Mg—O2^xiv	2.065 (3)
O2^i—Cs—O2^ii	48.23 (8)	O2^viii—Cs—O2^xii	119.494 (6)
O2^i—Cs—O2^iii	48.23 (8)	O2^ix—Cs—O2^xii	165.77 (9)
O2^ii—Cs—O2^iii	48.23 (8)	O2^x—Cs—O2^xii	71.43 (7)
O2^i—Cs—O2^iv	165.77 (9)	O2^xi—Cs—O2^xii	119.494 (6)
O2^ii—Cs—O2^iv	119.494 (6)	O2—Mg—O2^xiii	90.0
O2^iii—Cs—O2^iv	119.494 (6)	O2—Mg—O2^xiv	90.0
O2^i—Cs—O2^v	119.494 (6)	O2^xiii—Mg—O2^xiv	180.0
O2^ii—Cs—O2^v	165.77 (9)	O2—Mg—O2^xv	90.0
O2^iii—Cs—O2^v	119.494 (6)	O2^xiii—Mg—O2^xv	90.0
O2^iv—Cs—O2^v	71.43 (7)	O2^xiv—Mg—O2^xv	90.0
O2^i—Cs—O2^vi	119.494 (6)	O2—Mg—O2^xvi	90.0
O2^ii—Cs—O2^vi	119.494 (6)	O2^xiii—Mg—O2^xvi	90.0
O2^iii—Cs—O2^vi	165.77 (9)	O2^xiv—Mg—O2^xvi	90.0
O2^iv—Cs—O2^vi	71.43 (7)	O2^xv—Mg—O2^xvi	180.0
O2^v—Cs—O2^vi	71.43 (7)	O2—Mg—O2^xvii	180.0
O2^i—Cs—O2^vii	90.879 (11)	O2^xiii—Mg—O2^xvii	90.0
O2^ii—Cs—O2^vii	119.494 (6)	O2^xiv—Mg—O2^xvii	90.0
O2^iii—Cs—O2^vii	71.43 (7)	O2^xv—Mg—O2^xvii	90.0
O2^iv—Cs—O2^vii	90.879 (11)	O2^xvi—Mg—O2^xvii	90.0
O2^v—Cs—O2^vii	48.23 (8)	O2—Mg—Cs^xviii	125.3
O2^vi—Cs—O2^vii	119.494 (6)	O2^xiii—Mg—Cs^xviii	54.7
O2^i—Cs—O2^viii	90.879 (12)	O2^xiv—Mg—Cs^xviii	125.3
O2^ii—Cs—O2^viii	71.43 (7)	O2^xv—Mg—Cs^xviii	54.7
O2^iii—Cs—O2^viii	119.494 (6)	O2^xvi—Mg—Cs^xviii	125.3
O2^iv—Cs—O2^viii	90.879 (11)	O2^xvii—Mg—Cs^xviii	54.7
O2^v—Cs—O2^viii	119.494 (6)	O2—Mg—Cs^xix	54.7
O2^vi—Cs—O2^viii	48.23 (8)	O2^xiii—Mg—Cs^xix	54.7
O2^vii—Cs—O2^viii	165.77 (9)	O2^xiv—Mg—Cs^xix	125.3
O2^i—Cs—O2^ix	119.494 (6)	O2^xv—Mg—Cs^xix	125.3
O2^ii—Cs—O2^ix	71.43 (7)	O2^xvi—Mg—Cs^xix	54.7
O2^iii—Cs—O2^ix	90.879 (11)	O2^xvii—Mg—Cs^xix	125.3
O2^iv—Cs—O2^ix	48.23 (8)	Cs^xviii—Mg—Cs^xix	109.5
O2^v—Cs—O2^ix	119.494 (7)	O2—Mg—Cs^xx	54.7
O2^vi—Cs—O2^ix	90.879 (11)	O2^xiii—Mg—Cs^xx	125.3
O2^vii—Cs—O2^ix	119.494 (7)	O2^xiv—Mg—Cs^xx	54.7
O2^viii—Cs—O2^ix	71.43 (7)	O2^xv—Mg—Cs^xx	54.736 (1)
O2^i—Cs—O2^x	71.43 (7)	O2^xvi—Mg—Cs^xx	125.3
O2^ii—Cs—O2^x	90.879 (12)	O2^xvii—Mg—Cs^xx	125.3
O2^iii—Cs—O2^x	119.494 (6)	Cs^xviii—Mg—Cs^xx	109.5
O2^iv—Cs—O2^x	119.494 (6)	Cs^xix—Mg—Cs^xx	109.471 (1)
O2^v—Cs—O2^x	90.879 (11)	O2—Mg—Cs^xxi	125.3
O2^vi—Cs—O2^x	48.23 (8)	O2^xiii—Mg—Cs^xxi	125.3
O2^vii—Cs—O2^x	119.494 (6)	O2^xiv—Mg—Cs^xxi	54.7
O2^viii—Cs—O2^x	48.23 (8)	O2^xv—Mg—Cs^xxi	125.3
O2^ix—Cs—O2^x	119.494 (6)	O2^xvi—Mg—Cs^xxi	54.7
O2^i—Cs—O2^xi	119.494 (6)	O2^xvii—Mg—Cs^xxi	54.736 (1)
O2^ii—Cs—O2^xi	90.879 (11)	Cs^xviii—Mg—Cs^xxi	109.5
O2^iii—Cs—O2^xi	71.43 (7)	Cs^xix—Mg—Cs^xxi	109.471 (1)
O2^iv—Cs—O2^xi	48.23 (8)	Cs^xx—Mg—Cs^xxi	109.5
O2^v—Cs—O2^xi	90.879 (12)	O1—P—O1^xxii	109.471 (1)
O2^vi—Cs—O2^xi	119.494 (6)	O1—P—O1^xxiii	109.5
O2^vii—Cs—O2^xi	71.43 (7)	O1^xxii—P—O1^xxiii	109.5
O2^viii—Cs—O2^xi	119.494 (6)	O1—P—O1^xxiv	109.5
O2^ix—Cs—O2^xi	48.23 (8)	O1^xxii—P—O1^xxiv	109.471 (1)
O2^x—Cs—O2^xi	165.77 (9)	O1^xxiii—P—O1^xxiv	109.5
O2^i—Cs—O2^xii	71.43 (7)	Mg—O2—Cs^xix	97.12 (4)
O2^ii—Cs—O2^xii	119.494 (6)	Mg—O2—Cs^xx	97.12 (4)
O2^iii—Cs—O2^xii	90.879 (11)	Cs^xix—O2—Cs^xx	165.77 (9)
O2^iv—Cs—O2^xii	119.494 (7)	Mg—O2—H	125 (3)
O2^v—Cs—O2^xii	48.23 (8)	Cs^xix—O2—H	85.9 (3)
O2^vi—Cs—O2^xii	90.879 (11)	Cs^xx—O2—H	85.9 (3)
O2^vii—Cs—O2^xii	48.23 (8)	H^xxv—O2—H	110.46 (2)

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, y−1, z−1; (xix) x−1/2, y−1/2, z−1; (xx) x−1/2, y−1, z−1/2; (xxi) x−1, y−1/2, z−1/2; (xxii) x, −y+1/2, −z+1/2; (xxiii) −x+1/2, y, −z+1/2; (xxiv) −x+1/2, −y+1/2, z; (xxv) x, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H···O1^xxii	0.82 (4)	1.83 (3)	2.649 (2)	173 (5)

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