Rietveld refinement of Ba5(AsO4)3Cl from high-resolution synchrotron data

The apatite-type compound Ba5(AsO4)3Cl, pentabarium tris[arsenate(V)] chloride, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite (Pb5(AsO4)3Cl) with BaCO3 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, X = Cl, Br) in space group P63/m. The structure consists of isolated tetrahedral AsO4 3− anions (m symmetry), separated by two crystallographically independent Ba2+ cations that are located on mirror planes and threefold rotation axes, respectively. The Cl− anions are at the 2b sites ( symmetry) and are located in the channels of the structure.

The apatite-type compound Ba 5 (AsO 4 ) 3 Cl, pentabarium tris[arsenate(V)] chloride, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite (Pb 5 (AsO 4 ) 3 Cl) with BaCO 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, X = Cl, Br) in space group P6 3 /m. The structure consists of isolated tetrahedral AsO 4 3À anions (m symmetry), separated by two crystallographically independent Ba 2+ cations that are located on mirror planes and threefold rotation axes, respectively. The Cl À anions are at the 2b sites (3 symmetry) and are located in the channels of the structure.
AMTB acknowledges the use of the EPSRC's Chemical Database Service at Daresbury (Fletcher et al., 1996). AMTB also acknowledges the referees and Co-editor whose suggestions and comments helped to improve this paper.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2188).

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) and White & Dong (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 Ba containing As-apatites Ba 5 (AsO 4 ) 3 Cl (Kreidler & Hummel, 1970)  The refined lattice parameters for Ba 5 (AsO 4 ) 3 Cl are similar to the previously published parameters of a = 10.54 Å, c = 7.73 Å given by Kreidler & Hummel (1970). A study of 108 existing and predicted apatites with different compositions made use of elemental radii to calculate their lattice parameters (Wu et al., 2003). Only 52 of these compositions had known lattice parameters. The predicted lattice parameters for Ba 5 (AsO 4 ) 3 Cl were a = 10.3979 Å, c = 7.6105 Å. These predicted parameters are respectively 1.51% and 1.66% smaller than the measured lattice parameters, and only 2 of the 52 apatite compositions had bigger differences between observed and calculated lattice parameters.

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 supplementary materials sup-2 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 reduced the conductivity of the wash water to < 50 µS . cm -1 . Ba 5 (AsO 4 ) 3 Cl was successfully synthesized by ion exchange of Pb 5 (AsO 4 ) 3 Cl with molten BaCl 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 yield the Pb free title compound. Excess metal in the form of BaCl 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 powdered sample was loaded into a 0.7 mm diameter borosilicate capillary, prior to high-resolution synchrotron X-ray powder diffraction data collection using station 9.1 of the Daresbury Synchrotron Radiation Source. The beam on the sample was 13 mm wide and 1.2 mm high. 9 powder datasets were collected, all were with a step with of 0.01°/2θ and a counting time of 2 s per point. Three of these datasets were collected between 5-70°/2θ, two between 30-70°/2θ, two between 40-70°/2θ, one between 31.73-70°/2θ and one between 2-13.2°/2θ. All of these data were summed and normalized to account for decay of the synchrotron beam with time. The main Bragg reflections of the 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 BaCO 3 in space group Pmcn. The synchrotron X-ray wavelength was calibrated as 0.998043Å with an external NIST 640c silicon standard reference material.
Initial lattice parameters for the two phases were refined using CELREF (Laugier & Bochu, 2003). The P6 3 /m crystal structure of Ba 5 (PO 4 ) 3 (OH) (Chengjun et al., 2005) was used as a starting model for the Rietveld (Rietveld, 1969) refinement of the structure of Ba 5 (AsO 4 ) 3 Cl. The crystal structure of witherite (de Villiers et al., 1971) was used as a starting model for refinement of the structure of BaCO 3 . Isotropic atomic displacement parameters were used for both phases. For the Ba 5 (AsO 4 ) 3 Cl phase the As-O distances in the AsO 4 tetrahedral units were constrained to those for mimetite (Dai et al., 1991). For the BaCO 3 phase the C-O distances of the trigonal carbonate anion were constrained to those in witherite, and the U iso factors for all atoms in the carbonate anion were constrained to be the same. As the Ba 5 (AsO 4 ) 3 Cl phase was prepared by ion-exchange of Pb 5 (AsO 4 ) 3 Cl, Rietveld refinements were done with the metal sites partially occupied by both Pb and Ba. However, this resulted in the refined Pb occupancies falling to zero. Therefore the occupancies of the metal sites were fixed as fully occupied by Ba and no Pb was included for the final refinement of the Ba 5 (AsO 4 ) 3 Cl phase. Proportions of the two phases were refined as 64.7 (9) wt.% Ba 5 (AsO 4 ) 3 Cl and 35.3 (9) wt.% BaCO 3 . Fig. 1 Geometric parameters (Å, °)