Acta Cryst. (2008). E64, i63-i64 [ doi:10.1107/S1600536808026901 ]
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 A5(YO4)3X (A = divalent cation, Y = pentavalent cation, X = Cl, Br) in space group P63/m. The structure consists of isolated tetrahedral AsO43- 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.
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 reduced the conductivity of the wash water to < 50 µS.cm-1. Ba5(AsO4)3Cl was successfully synthesized by ion exchange of Pb5(AsO4)3Cl with molten BaCl2 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 BaCl2 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 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 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 BaCO3 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 P63/m crystal structure of Ba5(PO4)3(OH) (Chengjun et al., 2005) was used as a starting model for the Rietveld (Rietveld, 1969) refinement of the structure of Ba5(AsO4)3Cl. The crystal structure of witherite (de Villiers et al., 1971) was used as a starting model for refinement of the structure of BaCO3. Isotropic atomic displacement parameters were used for both phases. For the Ba5(AsO4)3Cl phase the As—O distances in the AsO4 tetrahedral units were constrained to those for mimetite (Dai et al., 1991). For the BaCO3 phase the C—O distances of the trigonal carbonate anion were constrained to those in witherite, and the Uiso factors for all atoms in the carbonate anion were constrained to be the same. As the Ba5(AsO4)3Cl phase was prepared by ion-exchange of Pb5(AsO4)3Cl, 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 Ba5(AsO4)3Cl phase. Proportions of the two phases were refined as 64.7 (9) wt.% Ba5(AsO4)3Cl and 35.3 (9) wt.% BaCO3.
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: TOPAS (Coelho, 2000); molecular graphics: Balls and Sticks (Kang & Ozawa, 2003); software used to prepare material for publication: publCIF (Westrip, 2008).
![[Figure 1]](./wm2188fig1thm.gif)
| Fig. 1. Rietveld difference plot for the multi-phase refinement of Ba5(AsO4)3Cl and BaCO3. The black dots, and grey and black lines show respectively the observed, calculated and difference plots. Calculated Bragg reflection positions are indicated by triangles for the Ba5(AsO4)3Cl phase and by crosses for the BaCO3 phase. |
![[Figure 2]](./wm2188fig2thm.gif)
| Fig. 2. The crystal structure of Ba5(AsO4)3Cl. Pink tetrahedra show AsO4 units with As5+ cations as yellow spheres and O2- anions as red spheres. Large blue spheres represent Ba2+ cations and small green spheres Cl- anions. |
| As3Ba5Cl1O12 | Z = 2 |
| Mr = 1138.85 | Dx = 5.063 (1) Mg m−3 |
| Hexagonal, P63/m | Synchrotron radiation λ = 0.998043 Å |
| a = 10.5570 (1) Å | µ = 56.07 (1) mm−1 |
| b = 10.5570 (1) Å | T = 298 K |
| c = 7.73912 (8) Å | Specimen shape: cylinder |
| α = 90º | 40 × 0.7 × 0.7 mm |
| β = 90º | Specimen prepared at 100 kPa |
| γ = 120º | Specimen prepared at 1258 K |
| V = 746.98 (1) Å3 | Particle morphology: powder, white |
| In-house design diffractometer | T = 298 K |
| Monochromator: Si(111) channel-cut crystal | 2θmin = 2, 2θmax = 70º |
| Specimen mounting: capillary | Increment in 2θ = 0.01º |
| Specimen mounted in transmission mode | Increment in 2θ = 0.01º |
| Scan method: step |
| Rp = 0.059 | Profile function: Fundamental Parameters |
| Rwp = 0.082 | 21 parameters |
| Rexp = 0.067 | 3 constraints |
| RB = 0.090 | ? |
| S = 1.23 | (Δ/σ)max = 0.001 |
| Wavelength of incident radiation: 0.998043 Å | Preferred orientation correction: None |
| Excluded region(s): 2-6 degrees 2θ. |
| As3Ba5Cl1O12 | Z = 2 |
| Mr = 1138.85 | Synchrotron radiation λ = 0.998043 Å |
| Hexagonal, P63/m | µ = 56.07 (1) mm−1 |
| a = 10.5570 (1) Å | T = 298 K |
| b = 10.5570 (1) Å | Specimen shape: cylinder |
| c = 7.73912 (8) 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 = 746.98 (1) Å3 |
| In-house design diffractometer | 2θmin = 2, 2θmax = 70º |
| Specimen mounting: capillary | Increment in 2θ = 0.01º |
| Specimen mounted in transmission mode | Increment in 2θ = 0.01º |
| Scan method: step |
| Rp = 0.059 | Wavelength of incident radiation: 0.998043 Å |
| Rwp = 0.082 | Excluded region(s): 2-6 degrees 2θ. |
| Rexp = 0.067 | Profile function: Fundamental Parameters |
| RB = 0.090 | 21 parameters |
| S = 1.23 | Preferred orientation correction: None |
| x | y | z | Uiso*/Ueq | ||
| Ba1 | 0.3333 | 0.6667 | 0.0061 (9) | 0.059 (1) | |
| Ba2 | 0.2445 (4) | 0.9874 (6) | 0.2500 | 0.065 (1) | |
| As1 | 0.4047 (7) | 0.3716 (7) | 0.2500 | 0.059 (2) | |
| O1 | 0.347 (7) | 0.495 (6) | 0.2500 | 0.13 (2) | |
| O2 | 0.591 (4) | 0.473 (4) | 0.2500 | 0.08 (1) | |
| O3 | 0.354 (2) | 0.280 (3) | 0.068 (3) | 0.065 (8) | |
| Cl1 | 0.0000 | 0.0000 | 0.0000 | 0.070 (6) |
| Ba1—O1i | 2.67 (5) | Ba2—O3vi | 2.62 (4) |
| Ba1—O1ii | 2.67 (5) | Ba2—O3vii | 3.05 (4) |
| Ba1—O1 | 2.67 (5) | Ba2—O3viii | 3.05 (4) |
| Ba1—O2iii | 2.81 (4) | Ba2—O1ii | 3.14 (4) |
| Ba1—O2iv | 2.81 (4) | Ba2—Cl1viii | 3.281 (5) |
| Ba1—O2v | 2.81 (4) | Ba2—Cl1ix | 3.281 (5) |
| Ba1—O3iv | 3.12 (3) | As1—O3 | 1.64 (2) |
| Ba1—O3iii | 3.12 (3) | As1—O3x | 1.64 (2) |
| Ba1—O3v | 3.12 (3) | As1—O1 | 1.70 (8) |
| Ba2—O2i | 2.59 (4) | As1—O2 | 1.70 (4) |
| Ba2—O3iv | 2.62 (4) | ||
| O3—As1—O3x | 118 (2) | O3—As1—O2 | 108 (2) |
| O3—As1—O1 | 108 (1) | O3x—As1—O2 | 108 (2) |
| O3x—As1—O1 | 108 (1) | O1—As1—O2 | 106 (2) |
| 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. |
| Ba1—O1 | 2.67 (5) | Ba2—O1v | 3.14 (4) |
| Ba1—O2i | 2.81 (4) | Ba2—Cl1iv | 3.281 (5) |
| Ba1—O3i | 3.12 (3) | As1—O3 | 1.64 (2) |
| Ba2—O2ii | 2.59 (4) | As1—O1 | 1.70 (8) |
| Ba2—O3iii | 2.62 (4) | As1—O2 | 1.70 (4) |
| Ba2—O3iv | 3.05 (4) | ||
| O3—As1—O3vi | 118 (2) | O3—As1—O2 | 108 (2) |
| O3—As1—O1 | 108 (1) | O1—As1—O2 | 106 (2) |
| 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). AMTB also acknowledges the referees and Co-editor whose suggestions and comments helped to improve this paper.
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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) 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 Ba5(AsO4)3Cl (Kreidler & Hummel, 1970) and for (Ba2.25Ca1.65Pb1.16Fe0.06Mg0.06)[(AsO4)2.56(PO4)0.3]Cl1.09 (mineral name morelandite; Dunn & Rouse, 1978) were indexed in space group P63/m. Related crystal structures have also been reported for Ba5(AsO4)2SO4S (Schiff-Francois et al., 1979) and (Sr1.66Ba0.34)(Ba2.61Sr0.39)(AsO4)3Cl (Dordevic et al., 2008). The crystal structure of Ba5(AsO4)3Cl in space group P63/m is reported in the present communication.
Table 1 shows refined interatomic distances and angles for the Ba5(AsO4)3Cl structure. The averaged Ba1—O and Ba2—O distances of respectively 2.87 Å and 2.84 Å are similar to those in other Ba and Cl containing apatites. In comparison, the average Ba1—O and Ba2—O distances are 2.84 Å and 2.78 Å for Ba5(VO4)3Cl (Roh & Hong, 2005), 2.83 Å and 2.79 Å for Ba5(PO4)3Cl (Hata et al., 1979) and 2.83 Å and 2.76 Å for Ba5(MnO4)3Cl (Reinen et al., 1986). 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 Ba5(AsO4)3Cl 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 Ba5(AsO4)3Cl 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.
Fig. 1 shows the Rietveld difference plot for the present refinement. The crystal structure of Ba5(AsO4)3Cl, showing the isolated tetrahedral AsO43- anions separated by Ba2+ cations and Cl- anions, is displayed in Fig. 2.