Acta Cryst. (2008). E64, i50 [ doi:10.1107/S1600536808023283 ]
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.
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.
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 Uiso parameter.
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).
![[Figure 1]](./br2080fig1thm.gif)
| 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. |
![[Figure 2]](./br2080fig2thm.gif)
| 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.] |
| Cs[Mg(H2O1)6](PO4) | Z = 2 |
| Mr = 360.29 | F000 = 348 |
| Hexagonal, P63mc | Dx = 2.447 Mg m−3 |
| Hall symbol: P 6c -2c | Mo Kα radiation λ = 0.71073 Å |
| a = 6.8827 (8) Å | Cell parameters from 1544 reflections |
| b = 6.8827 (8) Å | θ = 3.4–25.9º |
| c = 11.9188 (16) Å | µ = 4.04 mm−1 |
| α = 90º | T = 293 (2) K |
| β = 90º | Block, colourless |
| γ = 120º | 0.46 × 0.38 × 0.38 mm |
| V = 488.97 (10) Å3 |
| Stoe IPDS diffractometer | 368 independent reflections |
| Radiation source: fine-focus sealed tube | 367 reflections with I > 2σ(I) |
| Monochromator: graphite | Rint = 0.025 |
| T = 293(2) K | θmax = 25.7º |
| ω scans | θmin = 3.4º |
| 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 on F2 | Hydrogen site location: difference Fourier map |
| Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
| R[F2 > 2σ(F2)] = 0.016 | w = 1/[σ2(Fo2) + (0.0029P)2 + 0.8141P] where P = (Fo2 + 2Fc2)/3 |
| wR(F2) = 0.034 | (Δ/σ)max < 0.001 |
| S = 1.29 | Δρmax = 0.40 e Å−3 |
| 368 reflections | Δρmin = −0.31 e Å−3 |
| 39 parameters | Extinction correction: none |
| 4 restraints | Absolute structure: Flack (1983), with 164 Friedel pairs |
| Primary atom site location: structure-invariant direct methods | Flack parameter: −0.01 (3) |
| Secondary atom site location: difference Fourier map |
| Cs[Mg(H2O1)6](PO4) | γ = 120º |
| Mr = 360.29 | V = 488.97 (10) Å3 |
| Hexagonal, P63mc | Z = 2 |
| a = 6.8827 (8) Å | Mo Kα |
| b = 6.8827 (8) Å | µ = 4.04 mm−1 |
| c = 11.9188 (16) Å | T = 293 (2) K |
| α = 90º | 0.46 × 0.38 × 0.38 mm |
| β = 90º |
| Stoe IPDS diffractometer | 368 independent reflections |
| Absorption correction: numerical (HABITUS; Herrendorf, 1997) | 367 reflections with I > 2σ(I) |
| Tmin = 0.213, Tmax = 0.327 | Rint = 0.025 |
| 5412 measured reflections |
| R[F2 > 2σ(F2)] = 0.016 | H atoms treated by a mixture of independent and constrained refinement |
| wR(F2) = 0.034 | Δρmax = 0.40 e Å−3 |
| S = 1.29 | Δρmin = −0.31 e Å−3 |
| 368 reflections | Absolute structure: Flack (1983), with 164 Friedel pairs |
| 39 parameters | Flack parameter: −0.01 (3) |
| 4 restraints |
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. |
| 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)* |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| 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) |
| Cs—O3i | 3.469 (5) | Mg—O3 | 2.054 (4) |
| Cs—O3ii | 3.469 (5) | Mg—O3v | 2.054 (4) |
| Cs—O3iii | 3.469 (5) | Mg—O4ix | 2.082 (3) |
| Cs—O3iv | 3.469 (5) | Mg—O4 | 2.082 (3) |
| Cs—O3v | 3.469 (5) | Mg—O4v | 2.082 (3) |
| Cs—O3 | 3.469 (5) | Mg—Csx | 4.2306 (10) |
| Cs—O4i | 3.470 (4) | Mg—Csxi | 4.2306 (10) |
| Cs—O4iv | 3.470 (4) | Mg—Csxii | 4.508 (3) |
| Cs—O4v | 3.470 (4) | P—O1 | 1.536 (3) |
| Cs—O4vi | 3.742 (4) | P—O1iv | 1.536 (3) |
| Cs—O4vii | 3.742 (4) | P—O1xiii | 1.536 (3) |
| Cs—O4viii | 3.742 (4) | P—O2 | 1.539 (6) |
| Mg—O3ix | 2.054 (4) | ||
| O3i—Cs—O3ii | 51.51 (11) | O3v—Cs—O4vii | 97.18 (5) |
| O3i—Cs—O3iii | 67.77 (11) | O3—Cs—O4vii | 118.02 (7) |
| O3ii—Cs—O3iii | 118.93 (2) | O4i—Cs—O4vii | 112.101 (19) |
| O3i—Cs—O3iv | 118.93 (2) | O4iv—Cs—O4vii | 145.46 (3) |
| O3ii—Cs—O3iv | 165.52 (11) | O4v—Cs—O4vii | 145.46 (3) |
| O3iii—Cs—O3iv | 51.51 (11) | O4vi—Cs—O4vii | 46.74 (8) |
| O3i—Cs—O3v | 118.93 (2) | O3i—Cs—O4viii | 118.02 (7) |
| O3ii—Cs—O3v | 67.77 (11) | O3ii—Cs—O4viii | 97.18 (5) |
| O3iii—Cs—O3v | 165.52 (11) | O3iii—Cs—O4viii | 118.02 (7) |
| O3iv—Cs—O3v | 118.93 (2) | O3iv—Cs—O4viii | 97.18 (5) |
| O3i—Cs—O3 | 165.52 (11) | O3v—Cs—O4viii | 71.50 (7) |
| O3ii—Cs—O3 | 118.93 (2) | O3—Cs—O4viii | 71.50 (7) |
| O3iii—Cs—O3 | 118.93 (2) | O4i—Cs—O4viii | 145.46 (3) |
| O3iv—Cs—O3 | 67.77 (11) | O4iv—Cs—O4viii | 145.46 (3) |
| O3v—Cs—O3 | 51.51 (11) | O4v—Cs—O4viii | 112.101 (19) |
| O3i—Cs—O4i | 48.58 (7) | O4vi—Cs—O4viii | 46.74 (8) |
| O3ii—Cs—O4i | 48.58 (7) | O4vii—Cs—O4viii | 46.74 (8) |
| O3iii—Cs—O4i | 88.12 (6) | O3ix—Mg—O3 | 94.42 (16) |
| O3iv—Cs—O4i | 117.22 (7) | O3ix—Mg—O3v | 94.42 (16) |
| O3v—Cs—O4i | 88.12 (6) | O3—Mg—O3v | 94.42 (16) |
| O3—Cs—O4i | 117.22 (7) | O3ix—Mg—O4ix | 87.27 (10) |
| O3i—Cs—O4iv | 88.12 (6) | O3—Mg—O4ix | 177.5 (2) |
| O3ii—Cs—O4iv | 117.22 (7) | O3v—Mg—O4ix | 87.27 (10) |
| O3iii—Cs—O4iv | 48.58 (7) | O3ix—Mg—O4 | 87.27 (10) |
| O3iv—Cs—O4iv | 48.58 (7) | O3—Mg—O4 | 87.27 (10) |
| O3v—Cs—O4iv | 117.22 (7) | O3v—Mg—O4 | 177.5 (2) |
| O3—Cs—O4iv | 88.12 (6) | O4ix—Mg—O4 | 90.98 (19) |
| O4i—Cs—O4iv | 68.67 (8) | O3ix—Mg—O4v | 177.5 (2) |
| O3i—Cs—O4v | 117.22 (7) | O3—Mg—O4v | 87.27 (10) |
| O3ii—Cs—O4v | 88.12 (6) | O3v—Mg—O4v | 87.27 (10) |
| O3iii—Cs—O4v | 117.22 (7) | O4ix—Mg—O4v | 90.98 (19) |
| O3iv—Cs—O4v | 88.12 (6) | O4—Mg—O4v | 90.98 (19) |
| O3v—Cs—O4v | 48.58 (7) | O1—P—O1iv | 110.17 (14) |
| O3—Cs—O4v | 48.58 (7) | O1—P—O1xiii | 110.17 (14) |
| O4i—Cs—O4v | 68.67 (8) | O1iv—P—O1xiii | 110.17 (14) |
| O4iv—Cs—O4v | 68.67 (8) | O1—P—O2 | 108.76 (14) |
| O3i—Cs—O4vi | 97.18 (5) | O1iv—P—O2 | 108.76 (14) |
| O3ii—Cs—O4vi | 118.02 (7) | O1xiii—P—O2 | 108.76 (14) |
| O3iii—Cs—O4vi | 71.50 (7) | Mg—O3—Csx | 96.63 (6) |
| O3iv—Cs—O4vi | 71.50 (7) | Mg—O3—Cs | 96.63 (6) |
| O3v—Cs—O4vi | 118.02 (7) | Csx—O3—Cs | 165.52 (11) |
| O3—Cs—O4vi | 97.18 (5) | Mg—O3—H1 | 116 (4) |
| O4i—Cs—O4vi | 145.46 (3) | Mg—O3—H2 | 132 (5) |
| O4iv—Cs—O4vi | 112.101 (19) | H1—O3—H2 | 113 (6) |
| O4v—Cs—O4vi | 145.46 (3) | Mg—O4—Csx | 96.07 (16) |
| O3i—Cs—O4vii | 71.50 (7) | Mg—O4—Csxii | 97.31 (14) |
| O3ii—Cs—O4vii | 71.50 (7) | Csx—O4—Csxii | 166.63 (10) |
| O3iii—Cs—O4vii | 97.18 (5) | Mg—O4—H3 | 125 (4) |
| O3iv—Cs—O4vii | 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. |
| 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···O1xiv | 0.88 (4) | 1.76 (5) | 2.630 (5) | 168 (7) |
| O4—H3···O1xiii | 0.83 (4) | 1.85 (4) | 2.672 (3) | 177 (8) |
| Symmetry codes: (xiv) y, −x+y, z−1/2; (xiii) −x+y, −x, z. |
| Cs—O3i | 3.469 (5) | Mg—O4 | 2.082 (3) |
| Cs—O4i | 3.470 (4) | P—O1 | 1.536 (3) |
| Cs—O4ii | 3.742 (4) | P—O2 | 1.539 (6) |
| Mg—O3 | 2.054 (4) |
| Symmetry codes: (i) −y, x−y, z; (ii) y, −x+y+1, z−1/2. |
| 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···O1iii | 0.88 (4) | 1.76 (5) | 2.630 (5) | 168 (7) |
| O4—H3···O1iv | 0.83 (4) | 1.85 (4) | 2.672 (3) | 177 (8) |
| Symmetry codes: (iii) y, −x+y, z−1/2; (iv) −x+y, −x, z. |
The author thanks B. Müller (University of Ulm, Germany) for collecting the intensity data.
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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).