inorganic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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

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 16 July 2008; accepted 23 July 2008; online 31 July 2008)

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[Ferrari, A., Calvaca, L. & Nardelli, M. (1955). Gazz. Chim. Ital. 85, 1232-1238.]). 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[Whitaker, A. & Jeffery, J. W. (1970a). Acta Cryst. B26, 1429-1440.],b[Whitaker, A. & Jeffery, J. W. (1970b). Acta Cryst. B26, 1440-1444.]). Structure determinations of the hexa­gonal and cubic forms of Cs[Mg(OH2)6](PO4) were performed by Ferrari et al. (1955[Ferrari, A., Calvaca, L. & Nardelli, M. (1955). Gazz. Chim. Ital. 85, 1232-1238.]) and Massa et al. (2003[Massa, W., Yakubovich, O. V. & Dimitrova, O. V. (2003). Acta Cryst. C59, i83-i85.]), respectively. Crystal growth of struvite-like compounds using the gel diffusion technique was reported by Banks et al. (1975[Banks, E., Chianelli, R. & Korenstein, R. (1975). Inorg. Chem. 14, 1634-1639.]). For general background, see: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]).

Experimental

Crystal data
  • Cs[Mg(H2O)6](PO4)

  • Mr = 360.29

  • Hexagonal, P 63 m c

  • a = 6.8827 (8) Å

  • c = 11.9188 (16) Å

  • V = 488.97 (10) Å3

  • 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[Herrendorf, W. (1997). HABITUS. University of Giessen, Germany.]) Tmin = 0.213, Tmax = 0.327

  • 5412 measured reflections

  • 368 independent reflections

  • 367 reflections with I > 2σ(I)

  • Rint = 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

  • Δρmax = 0.40 e Å−3

  • Δρmin = −0.31 e Å−3

  • Absolute structure: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]), with 164 Friedel pairs

  • Flack parameter: −0.01 (3)

Table 1
Selected bond lengths (Å)

Cs—O3i 3.469 (5)
Cs—O4i 3.470 (4)
Cs—O4ii 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) [y, -x+y+1, z-{\script{1\over 2}}].

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA 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-{\script{1\over 2}}]; (iv) -x+y, -x, z.

Data collection: EXPOSE in IPDS Software (Stoe & Cie, 1998[Stoe & Cie (1998). IPDS Software. Stoe & Cie, Darmstadt, Germany.]); cell refinement: CELL in IPDS Software; data reduction: INTEGRATE in IPDS Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: ATOMS (Dowty, 2004[Dowty, E. (2004). ATOMS for Windows. Shape Software, Kingsport, Tennessee, USA.]); software used to prepare material for publication: SHELXL97.

Supporting information


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 & 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 top

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 top

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.

Refinement top

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.

Computing details top

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 top
[Figure 1] 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] 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 top
Crystal data top
Cs[Mg(H2O)6](PO4)Dx = 2.447 Mg m3
Mr = 360.29Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mcCell parameters from 1544 reflections
Hall symbol: P 6c -2cθ = 3.4–25.9°
a = 6.8827 (8) ŵ = 4.04 mm1
c = 11.9188 (16) ÅT = 293 K
V = 488.97 (10) Å3Block, colourless
Z = 20.46 × 0.38 × 0.38 mm
F(000) = 348
Data collection top
Stoe IPDS
diffractometer
368 independent reflections
Radiation source: fine-focus sealed tube367 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.025
ω scansθmax = 25.7°, θmin = 3.4°
Absorption correction: numerical
(HABITUS; Herrendorf, 1997)
h = 88
Tmin = 0.213, Tmax = 0.327k = 88
5412 measured reflectionsl = 1213
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.016H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.034 w = 1/[σ2(Fo2) + (0.0029P)2 + 0.8141P]
where P = (Fo2 + 2Fc2)/3
S = 1.29(Δ/σ)max < 0.001
368 reflectionsΔρmax = 0.40 e Å3
39 parametersΔρmin = 0.31 e Å3
4 restraintsAbsolute structure: Flack (1983), with 164 Friedel pairs
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.01 (3)
Crystal data top
Cs[Mg(H2O)6](PO4)Z = 2
Mr = 360.29Mo Kα radiation
Hexagonal, P63mcµ = 4.04 mm1
a = 6.8827 (8) ÅT = 293 K
c = 11.9188 (16) Å0.46 × 0.38 × 0.38 mm
V = 488.97 (10) Å3
Data collection top
Stoe IPDS
diffractometer
368 independent reflections
Absorption correction: numerical
(HABITUS; Herrendorf, 1997)
367 reflections with I > 2σ(I)
Tmin = 0.213, Tmax = 0.327Rint = 0.025
5412 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.016H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.034Δρmax = 0.40 e Å3
S = 1.29Δρmin = 0.31 e Å3
368 reflectionsAbsolute structure: Flack (1983), with 164 Friedel pairs
39 parametersAbsolute structure parameter: 0.01 (3)
4 restraints
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
xyzUiso*/Ueq
Cs0.33330.66670.02181 (4)0.03126 (15)
Mg0.66670.33330.1436 (2)0.0159 (5)
P0.00000.00000.29985 (14)0.0137 (5)
O10.1220 (2)0.2440 (4)0.3413 (3)0.0173 (6)
O20.00000.00000.1707 (5)0.0189 (13)
O30.3747 (6)0.1873 (3)0.0521 (3)0.0265 (9)
O40.5229 (3)0.0458 (5)0.2427 (3)0.0282 (8)
H10.243 (8)0.122 (4)0.094 (5)0.048 (9)*
H20.350 (12)0.175 (6)0.021 (4)0.048 (9)*
H30.397 (8)0.010 (14)0.272 (5)0.048 (9)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cs0.02671 (16)0.02671 (16)0.0404 (3)0.01335 (8)0.0000.000
Mg0.0158 (8)0.0158 (8)0.0160 (16)0.0079 (4)0.0000.000
P0.0139 (5)0.0139 (5)0.0134 (13)0.0069 (2)0.0000.000
O10.0172 (9)0.0139 (13)0.0195 (17)0.0070 (7)0.0023 (6)0.0046 (12)
O20.0177 (19)0.0177 (19)0.021 (4)0.0089 (10)0.0000.000
O30.0142 (14)0.0406 (14)0.016 (3)0.0071 (7)0.0023 (11)0.0012 (5)
O40.0191 (11)0.0252 (19)0.042 (2)0.0126 (10)0.0096 (7)0.0191 (15)
Geometric parameters (Å, º) top
Cs—O3i3.469 (5)Mg—O32.054 (4)
Cs—O3ii3.469 (5)Mg—O3v2.054 (4)
Cs—O3iii3.469 (5)Mg—O4ix2.082 (3)
Cs—O3iv3.469 (5)Mg—O42.082 (3)
Cs—O3v3.469 (5)Mg—O4v2.082 (3)
Cs—O33.469 (5)Mg—Csx4.2306 (10)
Cs—O4i3.470 (4)Mg—Csxi4.2306 (10)
Cs—O4iv3.470 (4)Mg—Csxii4.508 (3)
Cs—O4v3.470 (4)P—O11.536 (3)
Cs—O4vi3.742 (4)P—O1iv1.536 (3)
Cs—O4vii3.742 (4)P—O1xiii1.536 (3)
Cs—O4viii3.742 (4)P—O21.539 (6)
Mg—O3ix2.054 (4)
O3i—Cs—O3ii51.51 (11)O3v—Cs—O4vii97.18 (5)
O3i—Cs—O3iii67.77 (11)O3—Cs—O4vii118.02 (7)
O3ii—Cs—O3iii118.93 (2)O4i—Cs—O4vii112.101 (19)
O3i—Cs—O3iv118.93 (2)O4iv—Cs—O4vii145.46 (3)
O3ii—Cs—O3iv165.52 (11)O4v—Cs—O4vii145.46 (3)
O3iii—Cs—O3iv51.51 (11)O4vi—Cs—O4vii46.74 (8)
O3i—Cs—O3v118.93 (2)O3i—Cs—O4viii118.02 (7)
O3ii—Cs—O3v67.77 (11)O3ii—Cs—O4viii97.18 (5)
O3iii—Cs—O3v165.52 (11)O3iii—Cs—O4viii118.02 (7)
O3iv—Cs—O3v118.93 (2)O3iv—Cs—O4viii97.18 (5)
O3i—Cs—O3165.52 (11)O3v—Cs—O4viii71.50 (7)
O3ii—Cs—O3118.93 (2)O3—Cs—O4viii71.50 (7)
O3iii—Cs—O3118.93 (2)O4i—Cs—O4viii145.46 (3)
O3iv—Cs—O367.77 (11)O4iv—Cs—O4viii145.46 (3)
O3v—Cs—O351.51 (11)O4v—Cs—O4viii112.101 (19)
O3i—Cs—O4i48.58 (7)O4vi—Cs—O4viii46.74 (8)
O3ii—Cs—O4i48.58 (7)O4vii—Cs—O4viii46.74 (8)
O3iii—Cs—O4i88.12 (6)O3ix—Mg—O394.42 (16)
O3iv—Cs—O4i117.22 (7)O3ix—Mg—O3v94.42 (16)
O3v—Cs—O4i88.12 (6)O3—Mg—O3v94.42 (16)
O3—Cs—O4i117.22 (7)O3ix—Mg—O4ix87.27 (10)
O3i—Cs—O4iv88.12 (6)O3—Mg—O4ix177.5 (2)
O3ii—Cs—O4iv117.22 (7)O3v—Mg—O4ix87.27 (10)
O3iii—Cs—O4iv48.58 (7)O3ix—Mg—O487.27 (10)
O3iv—Cs—O4iv48.58 (7)O3—Mg—O487.27 (10)
O3v—Cs—O4iv117.22 (7)O3v—Mg—O4177.5 (2)
O3—Cs—O4iv88.12 (6)O4ix—Mg—O490.98 (19)
O4i—Cs—O4iv68.67 (8)O3ix—Mg—O4v177.5 (2)
O3i—Cs—O4v117.22 (7)O3—Mg—O4v87.27 (10)
O3ii—Cs—O4v88.12 (6)O3v—Mg—O4v87.27 (10)
O3iii—Cs—O4v117.22 (7)O4ix—Mg—O4v90.98 (19)
O3iv—Cs—O4v88.12 (6)O4—Mg—O4v90.98 (19)
O3v—Cs—O4v48.58 (7)O1—P—O1iv110.17 (14)
O3—Cs—O4v48.58 (7)O1—P—O1xiii110.17 (14)
O4i—Cs—O4v68.67 (8)O1iv—P—O1xiii110.17 (14)
O4iv—Cs—O4v68.67 (8)O1—P—O2108.76 (14)
O3i—Cs—O4vi97.18 (5)O1iv—P—O2108.76 (14)
O3ii—Cs—O4vi118.02 (7)O1xiii—P—O2108.76 (14)
O3iii—Cs—O4vi71.50 (7)Mg—O3—Csx96.63 (6)
O3iv—Cs—O4vi71.50 (7)Mg—O3—Cs96.63 (6)
O3v—Cs—O4vi118.02 (7)Csx—O3—Cs165.52 (11)
O3—Cs—O4vi97.18 (5)Mg—O3—H1116 (4)
O4i—Cs—O4vi145.46 (3)Mg—O3—H2132 (5)
O4iv—Cs—O4vi112.101 (19)H1—O3—H2113 (6)
O4v—Cs—O4vi145.46 (3)Mg—O4—Csx96.07 (16)
O3i—Cs—O4vii71.50 (7)Mg—O4—Csxii97.31 (14)
O3ii—Cs—O4vii71.50 (7)Csx—O4—Csxii166.63 (10)
O3iii—Cs—O4vii97.18 (5)Mg—O4—H3125 (4)
O3iv—Cs—O4vii118.02 (7)
Symmetry codes: (i) x, y+1, z; (ii) y+1, xy+1, z; (iii) x+y, x+1, z; (iv) y, xy, z; (v) x+y+1, x+1, z; (vi) y, x+y+1, z1/2; (vii) x+1, y+1, z1/2; (viii) xy, x, z1/2; (ix) y+1, xy, z; (x) x, y1, z; (xi) x+1, y, z; (xii) xy+1, x, z+1/2; (xiii) x+y, x, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H1···O20.93 (4)1.71 (4)2.643 (5)180 (6)
O3—H2···O1xiv0.88 (4)1.76 (5)2.630 (5)168 (7)
O4—H3···O1xiii0.83 (4)1.85 (4)2.672 (3)177 (8)
Symmetry codes: (xiii) x+y, x, z; (xiv) y, x+y, z1/2.

Experimental details

Crystal data
Chemical formulaCs[Mg(H2O)6](PO4)
Mr360.29
Crystal system, space groupHexagonal, P63mc
Temperature (K)293
a, c (Å)6.8827 (8), 11.9188 (16)
V3)488.97 (10)
Z2
Radiation typeMo Kα
µ (mm1)4.04
Crystal size (mm)0.46 × 0.38 × 0.38
Data collection
DiffractometerStoe IPDS
diffractometer
Absorption correctionNumerical
(HABITUS; Herrendorf, 1997)
Tmin, Tmax0.213, 0.327
No. of measured, independent and
observed [I > 2σ(I)] reflections
5412, 368, 367
Rint0.025
(sin θ/λ)max1)0.611
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.016, 0.034, 1.29
No. of reflections368
No. of parameters39
No. of restraints4
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.40, 0.31
Absolute structureFlack (1983), with 164 Friedel pairs
Absolute structure parameter0.01 (3)

Computer programs: EXPOSE in IPDS Software (Stoe & Cie, 1998), CELL in IPDS Software (Stoe & Cie, 1998), INTEGRATE in IPDS Software (Stoe & Cie, 1998), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ATOMS (Dowty, 2004).

Selected bond lengths (Å) top
Cs—O3i3.469 (5)Mg—O42.082 (3)
Cs—O4i3.470 (4)P—O11.536 (3)
Cs—O4ii3.742 (4)P—O21.539 (6)
Mg—O32.054 (4)
Symmetry codes: (i) y, xy, z; (ii) y, x+y+1, z1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O3—H1···O20.93 (4)1.71 (4)2.643 (5)180 (6)
O3—H2···O1iii0.88 (4)1.76 (5)2.630 (5)168 (7)
O4—H3···O1iv0.83 (4)1.85 (4)2.672 (3)177 (8)
Symmetry codes: (iii) y, x+y, z1/2; (iv) x+y, x, z.
 

Acknowledgements

The author thanks B. Müller (University of Ulm, Germany) for collecting the intensity data.

References

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