Acta Cryst. (2009). E65, i16-i17 [ doi:10.1107/S1600536809005054 ]
The apatite-type compound, pentastrontium tris[arsenate(V)] chloride, Sr5(AsO4)3Cl, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite [Pb5(AsO4)3Cl] with SrCO3 as a by-product. The results of the Rietveld refinement, based on high resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the same structure as other halogenoapatites with general formula A5(YO4)3X (A = divalent cation, Y = pentavalent cation, and X = F, Cl or Br) in the space group P63/m. The structure consists of isolated tetrahedral AsO43- anions (the As atom and two O atoms have m symmetry), separated by two crystallographically independent Sr2+ cations that are located on mirror planes and threefold rotation axes, respectively. One Sr atom is coordinated by nine O atoms and the other by six. The chloride anions (site symmetry 
) are at the 2a sites and are located in the channels of the structure.
This work was part of an attempt to synthesize analogues of Pb5(AsO4)3Cl (mimetite) with Pb2+ substituted by alkaline earth cations. All starting materials were well crystallized solids. Pb5(AsO4)3Cl was precipitated by titration of 0.1M Na2HAsO4 into a well stirred, saturated PbCl2 solution at room temperature (procedure modified from methods of Baker (1966) and Essington (1988)). The molar ratio of Pb:As was slightly greater than 5:3, allowing for excess PbCl2 during the precipitation. A very fine-grained pure solid formed immediately, which was then separated, washed, and dried. Typically, five de-ionized water washes were needed to reduce the conductivity of the wash water to < 50 µS.cm-1. Sr5(AsO4)3Cl was successfully synthesized by ion exchange of Pb5(AsO4)3Cl with molten SrCl2 at 1258 K (modified from the method given by Kreidler & Hummel (1970)). Two fusions were required to completely eliminate formation of Pb containing solid solutions and to yield the Pb free title compound. Excess metal in the form of SrCl2 was removed from the solids by repeated washing with de-ionized water followed by centrifugation and filtration to separate the solid from the solution.
The main Bragg reflections of the high resolution synchrotron X-ray powder diffraction pattern could be indexed in space group P63/m with similar lattice parameters to those of the published powder diffraction data (Kreidler & Hummel, 1970). Some broad and weak Bragg reflections were matched by the pattern of SrCO3 in space group Pmcn.
Initial lattice parameters for the two phases were refined using CELREF (Laugier & Bochu, 2003). The P63/m crystal structure of Ba5(AsO4)3Cl (Bell et al., 2008) was used as a starting model for the Rietveld (Rietveld, 1969) refinement of the structure of Sr5(AsO4)3Cl. The crystal structure of strontianite (de Villiers et al., 1971) was used as a starting model for refinement of the structure of SrCO3. Isotropic atomic displacement parameters were used for both phases. For the Sr5(AsO4)3Cl phase soft constraints were used for the As—O distances in the AsO4 tetrahedral units. These distances were restrained to those for mimetite (Dai et al., 1991). For the SrCO3 phase only the coordinates and the atomic displacement parameters for Sr were refined, the C and O coordinates were fixed to those in the starting model and the C and O atomic displacement parameters were fixed at zero. Proportions of the two phases were refined as 76.6 (1) wt.% Sr5(AsO4)3Cl and 23.4 (1) wt.% SrCO3.
Data collection: local software; cell refinement: CELREF (Laugier & Bochu, 2003); data reduction: local software; program(s) used to solve structure: coordinates taken from a related compound; program(s) used to refine structure: GSAS (Larson & Von Dreele (2004) and EXPGUI (Toby, 2001); molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: publCIF (Westrip, 2009).
![[Figure 1]](./br2096fig1thm.gif)
| Fig. 1. Rietveld difference plot for the multi-phase refinement of Sr5(AsO4)3Cl and SrCO3. The red crosses, and green and pink lines show respectively the observed, calculated and difference plots. Calculated Bragg reflection positions are indicated by black lines for the Sr5(AsO4)3Cl phase and by red lines for the SrCO3 phase. |
![[Figure 2]](./br2096fig2thm.gif)
| Fig. 2. The crystal structure of Sr5(AsO4)3Cl. Pink tetrahedra show AsO4 units with As5+ cations as orange spheres and O2- anions as red spheres. Large blue spheres represent Sr2+ cations and small green spheres Cl- anions. |
| Sr5(AsO4)3Cl | Z = 2 |
| Mr = 890.31 | Dx = 4.510 (1) Mg m−3 |
| Hexagonal, P63/m | Synchrotron radiation λ = 0.998043 Å |
| a = 10.1969 (1) Å | T = 298 K |
| b = 10.1969 (1) Å | Specimen shape: cylinder |
| c = 7.28108 (9) Å | 40 × 0.7 × 0.7 mm |
| α = 90º | Specimen prepared at 100 kPa |
| β = 90º | Specimen prepared at 1258 K |
| γ = 120º | Particle morphology: powder, white |
| V = 655.63 (2) Å3 |
| In-house design diffractometer | Scan method: step |
| Monochromator: Si(111) channel-cut crystal | T = 298 K |
| Specimen mounting: capillary | 2θmin = 2, 2θmax = 60º |
| Specimen mounted in transmission mode | Increment in 2θ = 0.01º |
| Rp = 0.052 | Profile function: Pseudo Voigt |
| Rwp = 0.066 | 16 parameters |
| Rexp = 0.047 | 4 constraints |
| RB = 0.090 | ? |
| S = 2.00 | (Δ/σ)max = 0.001 |
| Wavelength of incident radiation: 0.998043 Å | Preferred orientation correction: None |
| Excluded region(s): 2-6° 2θ |
| Sr5(AsO4)3Cl | Z = 2 |
| Mr = 890.31 | Synchrotron radiation λ = 0.998043 Å |
| Hexagonal, P63/m | µ = ? mm−1 |
| a = 10.1969 (1) Å | T = 298 K |
| b = 10.1969 (1) Å | Specimen shape: cylinder |
| c = 7.28108 (9) Å | 40 × 0.7 × 0.7 mm |
| α = 90º | Specimen prepared at 100 kPa |
| β = 90º | Specimen prepared at 1258 K |
| γ = 120º | Particle morphology: powder, white |
| V = 655.63 (2) Å3 |
| In-house design diffractometer | Scan method: step |
| Specimen mounting: capillary | 2θmin = 2, 2θmax = 60º |
| Specimen mounted in transmission mode | Increment in 2θ = 0.01º |
| Rp = 0.052 | Wavelength of incident radiation: 0.998043 Å |
| Rwp = 0.066 | Excluded region(s): 2-6° 2θ |
| Rexp = 0.047 | Profile function: Pseudo Voigt |
| RB = 0.090 | 16 parameters |
| S = 2.00 | Preferred orientation correction: None |
Experimental. Absorption correction fixed at zero, all attempts to refine this term in GSAS were unsuccessful so this term was fixed at zero. CELREF was used for initial lattice parameter determinations before Rietveld refinement. Lattice parameters from GSAS refinement are quoted in the paper. |
| x | y | z | Uiso*/Ueq | ||
| Sr1 | 0.33333 | 0.66667 | 0.008 (1) | 0.0246 (9) | |
| Sr2 | 0.2496 (5) | 0.9936 (6) | 0.25 | 0.0246 (9) | |
| As1 | 0.4057 (5) | 0.3718 (5) | 0.25 | 0.029 (2) | |
| O1 | 0.337 (3) | 0.496 (2) | 0.25 | 0.015 (4) | |
| O2 | 0.598 (2) | 0.464 (2) | 0.25 | 0.015 (4) | |
| O3 | 0.354 (2) | 0.284 (2) | 0.063 (2) | 0.015 (4) | |
| Cl1 | 0.0000 | 0.0000 | 0.0000 | 0.031 (5) |
| Sr1—O1i | 2.49 (2) | Sr2—O3vi | 2.44 (1) |
| Sr1—O1ii | 2.49 (2) | Sr2—O3vii | 2.94 (1) |
| Sr1—O1 | 2.49 (2) | Sr2—O3viii | 2.94 (1) |
| Sr1—O2iii | 2.59 (2) | Sr2—O1ii | 3.02 (2) |
| Sr1—O2iv | 2.59 (2) | Sr2—Cl1viii | 3.156 (3) |
| Sr1—O2v | 2.59 (2) | Sr2—Cl1ix | 3.156 (3) |
| Sr1—O3iv | 3.01 (1) | As1—O3 | 1.57 (1) |
| Sr1—O3iii | 3.01 (1) | As1—O3x | 1.57 (1) |
| Sr1—O3v | 3.01 (1) | As1—O1 | 1.72 (2) |
| Sr2—O2i | 2.53 (2) | As1—O2 | 1.70 (2) |
| Sr2—O3iv | 2.44 (1) | ||
| O3—As1—O3x | 121 (1) | O3—As1—O2 | 106.3 (6) |
| O3—As1—O1 | 105.8 (7) | O3x—As1—O2 | 106.3 (6) |
| O3x—As1—O1 | 105.8 (7) | O1—As1—O2 | 112 (1) |
| Symmetry codes: (i) −y+1, x−y+1, z; (ii) −x+y, −x+1, z; (iii) x−y, x, −z; (iv) y, −x+y+1, −z; (v) −x+1, −y+1, −z; (vi) y, −x+y+1, z+1/2; (vii) x, y+1, −z+1/2; (viii) x, y+1, z; (ix) −x, −y+1, z+1/2; (x) x, y, −z+1/2. |
| Sr1—O1 | 2.49 (2) | Sr2—O1v | 3.02 (2) |
| Sr1—O2i | 2.59 (2) | Sr2—Cl1iv | 3.156 (3) |
| Sr1—O3i | 3.01 (1) | As1—O3 | 1.57 (1) |
| Sr2—O2ii | 2.53 (2) | As1—O1 | 1.72 (2) |
| Sr2—O3iii | 2.44 (1) | As1—O2 | 1.70 (2) |
| Sr2—O3iv | 2.94 (1) | ||
| O3—As1—O3vi | 121 (1) | O3—As1—O2 | 106.3 (6) |
| O3—As1—O1 | 105.8 (7) | O1—As1—O2 | 112 (1) |
| Symmetry codes: (i) x−y, x, −z; (ii) −y+1, x−y+1, z; (iii) y, −x+y+1, −z; (iv) x, y+1, z; (v) −x+y, −x+1, z; (vi) x, y, −z+1/2. |
AMTB acknowledges the use of the EPSRC's Chemical Database Service at Daresbury (Fletcher et al., 1996).
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Apatites are minerals and synthetic compounds with general formula A5(YO4)3X, containing tetrahedrally coordinated YO43- anions (Y = pentavalent cation) and a monovalent anion X such as F-, Cl- or OH-. The divalent cations frequently belong to the alkaline earth group, but other cations like Pb2+ are also known. For a review of the structures and crystal-chemistry of these materials, see Mercier et al. (2005), White & ZhiLi (2003) and Wu et al., (2003). Apatites containing arsenic (As-apatites) are of interest as hosts for storage of arsenic removed from contaminated water (Harrison et al., 2002). Powder diffraction data for the Sr containing As-apatite Sr5(AsO4)3Cl (Kreidler & Hummel, 1970) was indexed in space group P63/m. Related crystal structures have also been reported for Ca5(AsO4)3Cl (Wardojo and Hwu, 1996) and for Sr5(AsO4)3F and (Sr1.66Ba0.34)(Ba2.61Sr0.39)(AsO4)3Cl (Đordević et al., 2008). The crystal structure of Sr5(AsO4)3Cl in space group P63/m is reported in the present communication. We recently reported the related crystal structure of Ba5(AsO4)3Cl (Bell et al., 2008).
Table 1 shows refined interatomic distances and angles for the Sr5(AsO4)3Cl structure. The averaged Sr1—O and Sr2—O distances of respectively 2.70 Å and 2.72 Å, compare with Sr1—O and Sr2—O distances in: Sr5(AsO4)3F (Đordević et al. 2008) of 2.71 Å and 2.62 Å; 2.71 Å and 2.63 Å for Sr5(VO4)3Cl (Beck et al., 2006); 2.67 Å and 2.62 Å for Sr5(PO4)3Cl (Sudarsanan and Young, 1974); and 2.67 Å and 2.59 Å for Sr5(PO4)3F (Swafford and Holt, 2002). The As—O distances are characteristic for tetrahedral AsO4 units. The O—As—O angles deviate significantly from the ideal tetrahedral angle of 109.5°, indicating a strong distortion.
The refined lattice parameters for Sr5(AsO4)3Cl are similar to the previously published parameters of a = 10.18 Å, c = 7.28 Å given by Kreidler & Hummel (1970). Fig. 1 shows the Rietveld difference plot for the present refinement. The crystal structure of Sr5(AsO4)3Cl, showing the isolated tetrahedral AsO43- anions separated by Sr2+ cations and Cl- anions, is displayed in Fig. 2.