Rietveld refinement of Sr5(AsO4)3Cl from high-resolution synchrotron data

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.

The apatite-type compound, pentastrontium tris[arsenate(V)] chloride, Sr 5 (AsO 4 ) 3 Cl, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite [Pb 5 (AsO 4 ) 3 Cl] with SrCO 3 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 (YO 4 ) 3 X (A = divalent cation, Y = pentavalent cation, and X = F, Cl or Br) in the space group P6 3 /m. The structure consists of isolated tetrahedral AsO 4 3À anions (the As atom and two O atoms have m symmetry), separated by two crystallographically independent Sr 2+ 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 3) are at the 2a sites and are located in the channels of the structure.

Comment
Apatites are minerals and synthetic compounds with general formula A 5 (YO 4 ) 3 X, containing tetrahedrally coordinated YO 4 3anions (Y = pentavalent cation) and a monovalent anion X such as F -, Clor OH -. The divalent cations frequently belong to the alkaline earth group, but other cations like Pb 2+ 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 (Asapatites) 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 Sr 5 (AsO 4 ) 3 Cl (Kreidler & Hummel, 1970) was indexed in space group P6 3 /m. Related crystal structures have also been reported for Ca 5 (AsO 4 ) 3 Cl (Wardojo and Hwu, 1996) (Beck et al., 2006); 2.67 Å and 2.62 Å for Sr 5 (PO 4 ) 3 Cl (Sudarsanan and Young, 1974); and 2.67 Å and 2.59 Å for Sr 5 (PO 4 ) 3 F (Swafford and Holt, 2002). The As-O distances are characteristic for tetrahedral AsO 4 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 Sr 5 (AsO 4 ) 3 Cl 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 Sr 5 (AsO 4 ) 3 Cl, showing the isolated tetrahedral AsO 4 3anions separated by Sr 2+ cations and Clanions, is displayed in Fig. 2.

Experimental
This work was part of an attempt to synthesize analogues of Pb 5 (AsO 4 ) 3 Cl (mimetite) with Pb 2+ substituted by alkaline earth cations. All starting materials were well crystallized solids. Pb 5 (AsO 4 ) 3 Cl was precipitated by titration of 0.1M Na 2 HAsO 4 into a well stirred, saturated PbCl 2 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 PbCl 2 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 . Sr 5 (AsO 4 ) 3 Cl was successfully synthesized by ion exchange of Pb 5 (AsO 4 ) 3 Cl with molten SrCl 2 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 supplementary materials sup-2 yield the Pb free title compound. Excess metal in the form of SrCl 2 was removed from the solids by repeated washing with de-ionized water followed by centrifugation and filtration to separate the solid from the solution.

Refinement
The main Bragg reflections of the high resolution synchrotron X-ray powder diffraction pattern could be indexed in space group P6 3 /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 SrCO 3 in space group Pmcn.
Initial lattice parameters for the two phases were refined using CELREF (Laugier & Bochu, 2003). The P6 3 /m crystal structure of Ba 5 (AsO 4 ) 3 Cl (Bell et al., 2008) was used as a starting model for the Rietveld (Rietveld, 1969) refinement of the structure of Sr 5 (AsO 4 ) 3 Cl. The crystal structure of strontianite (de Villiers et al., 1971) was used as a starting model for refinement of the structure of SrCO 3 . Isotropic atomic displacement parameters were used for both phases. For the Sr 5 (AsO 4 ) 3 Cl phase soft constraints were used for the As-O distances in the AsO 4 tetrahedral units. These distances were restrained to those for mimetite (Dai et al., 1991). For the SrCO 3 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.% Sr 5 (AsO 4 ) 3 Cl and 23.4 (1) wt.% SrCO 3 . Fig. 1

Special details
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.