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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 A 5(YO4)3 X (A = divalent cation, Y = pentavalent cation, and X = F, Cl or Br) in the space group P63/m. The structure consists of isolated tetrahedral AsO4 3− 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.
For crystal chemistry of apatites, see: Mercier et al. (2005 ); White & ZhiLi (2003 ); Wu et al. (2003 ). For powder diffraction data on Sr As-apatite, see: Kreidler & Hummel (1970 ). Atomic coordinates as starting parameters for the Rietveld (Rietveld, 1969 ) refinement of the present phases were taken from Bell et al. (2008 ); Dai et al. (1991 ); de Villiers et al. (1971 ). For related Sr—Cl-apatites, see: Đordević et al. (2008 ); Sudarsanan & Young, (1974 , 1980 ); Beck et al. (2006 ); Noetzold et al. (1995 ); Noetzold & Wulff (1996 , 1997 , 1998 ); Swafford & Holt (2002 ); Wardojo & Hwu (1996 ). For synthetic work, see: Baker (1966 ); Essington (1988 ); Harrison et al. (2002 ).
Data collection: local software; cell refinement: CELREF (Laugier & Bochu, 2003 ) and GSAS (Larson & Von Dreele (2004 ); data reduction: local software; method used to solve structure: coordinates taken from a related compound; program(s) used to refine structure: GSAS and EXPGUI (Toby, 2001 ); molecular graphics: VESTA (Momma & Izumi, 2008 ); software used to prepare material for publication: publCIF (Westrip, 2009 ).
Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809005054/br2096sup1.cif
Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536809005054/br2096Isup2.rtv
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.
|Sr5(AsO4)3Cl||Dx = 4.510 (1) Mg m−3|
|Mr = 890.31||Synchrotron radiation, λ = 0.998043 Å|
|Hexagonal, P63/m||T = 298 K|
|a = 10.1969 (1) Å||Particle morphology: powder|
|c = 7.28108 (9) Å||white|
|V = 655.63 (2) Å3||cylinder, 40 × 0.7 mm|
|Z = 2||Specimen preparation: Prepared at 1258 K and 100 kPa|
|In-house design diffractometer||Data collection mode: transmission|
|Radiation source: Synchrotron||Scan method: step|
|Si(111) channel-cut crystal||2θmin = 6°, 2θmax = 60°, 2θstep = 0.01°|
|Specimen mounting: capillary|
|Rp = 0.052||Profile function: Pseudo Voigt|
|Rwp = 0.066||16 parameters|
|Rexp = 0.047||0 restraints|
|RBragg = 0.090||4 constraints|
|R(F2) = 0.33148||(Δ/σ)max = 0.001|
|χ2 = 3.992||Background function: Cosine Fourier Series|
|5801 data points||Preferred orientation correction: None|
|Excluded region(s): 2-6° 2θ|
|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.|
|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)|
|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)|
|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.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BR2096).