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 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.

Related literature

For crystal chemistry of apatites, see: Mercier et al. (2005); White & ZhiLi (2003); Wu et al. (2003). For powder diffraction data on Ba-containing As-apatites, see: Kreidler & Hummel (1970); Dunn & Rouse (1978). Atomic coordinates as starting parameters for the Rietveld (Rietveld, 1969) refinement of the present phases were taken from Chengjun et al. (2005); Dai et al. (1991); de Villiers et al. (1971). For related Ba—Cl-apatites, see: Đordevic et al. (2008); Hata et al. (1979); Reinen et al.(1986); Roh & Hong (2005); Schiff-Francois et al. (1979). For synthetic work, see: Baker (1966); Essington (1988); Harrison et al. (2002).

Experimental

Crystal data

• As₃Ba₅ClO₁₂

• M_r = 1138.85

• Hexagonal, P 6₃ /m

• a = 10.5570 (1) Å

• c = 7.73912 (8) Å

• V = 746.98 (1) ų

• Z = 2

• Synchrotron radiation

• λ = 0.998043 Å

• μ = 56.07 (1) mm⁻¹

• T = 298 K

• Specimen shape: cylinder

• 40 × 0.7 × 0.7 mm

• Specimen prepared at 100 kPa

• Specimen prepared at 1258 K

• Particle morphology: powder, white

Data collection

• In-house design diffractometer

• Specimen mounting: capillary

• Specimen mounted in transmission mode

• Scan method: step

• Absorption correction: none

• 2θ_min = 2, 2θ_max = 70°

• Increment in 2θ = 0.01°

Refinement

• R_p = 0.059

• R_wp = 0.082

• R_exp = 0.067

• R_B = 0.090

• S = 1.23

• Excluded region(s): 2-6 degrees 2θ.

• Profile function: Fundamental Parameters

• 464 Bragg reflections

• 21 parameters

• Preferred orientation correction: none

Table 1
Selected geometric parameters (Å, °)

Ba1—O1	2.67 (5)
Ba1—O2^i	2.81 (4)
Ba1—O3^i	3.12 (3)
Ba2—O2^ii	2.59 (4)
Ba2—O3^iii	2.62 (4)
Ba2—O3^iv	3.05 (4)
Ba2—O1^v	3.14 (4)
Ba2—Cl1^iv	3.281 (5)
As1—O3	1.64 (2)
As1—O1	1.70 (8)
As1—O2	1.70 (4)

O3—As1—O3^vi	118 (2)
O3—As1—O1	108 (1)
O3—As1—O2	108 (2)
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) .

Data collection: local software; cell refinement: CELREF (Laugier & Bochu, 2003); data reduction: local software; method 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).

Supporting information

S1. Comment

Apatites are minerals and synthetic compounds with general formula A₅(YO₄)₃X, containing tetrahedrally coordinated YO₄^3- 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 (Ba_2.25Ca_1.65Pb_1.16Fe_0.06Mg_0.06)[(AsO₄)_2.56(PO₄)_0.3]Cl_1.09 (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 (Sr_1.66Ba_0.34)(Ba_2.61Sr_0.39)(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 AsO₄^3- anions separated by Ba²⁺ cations and Cl^- anions, is displayed in Fig. 2.

Related literature

For crystal chemistry of apatites, see: Mercier et al. (2005); White & Dong (2003); Wu et al. (2003). For powder diffraction data on Ba containing As-apatites, see: Kreidler & Hummel (1970); Dunn & Rouse (1978). Atomic coordinates as starting parameters for the Rietveld (Rietveld, 1969) refinement of the present phases were taken from Chengjun et al. (2005); Dai et al. (1991); de Villiers et al. (1971). For related Ba—Cl-apatites, see: Dordevic et al. (2008); Hata et al. (1979); Reinen et al.(1986); Roh & Hong (2005); Schiff-Francois et al. (1979). For synthetic work, see: Baker (1966); Essington (1988); Harrison et al. (2002).

S2. Experimental

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.

S3. 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 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 U_iso 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.

Computing details

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).

Figures

	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.
	Fig. 2. The crystal structure of Ba5(AsO4)3Cl. Pink tetrahedra show AsO4 units with As^5+ cations as yellow spheres and O^2- anions as red spheres. Large blue spheres represent Ba^2+ cations and small green spheres Cl^- anions.

pentabarium tris(arsenate(V)) chloride

Crystal data

As3Ba5ClO12	Synchrotron radiation, λ = 0.998043 Å
Mr = 1138.85	µ = 56.07 (1) mm^−1
Hexagonal, P63/m	T = 298 K
a = 10.5570 (1) Å	Particle morphology: powder
c = 7.73912 (8) Å	white
V = 746.98 (1) Å^3	cylinder, 40 × 0.7 mm
Z = 2	Specimen preparation: Prepared at 1258 K and 100 kPa
Dx = 5.063 (1) Mg m^−3

Data collection

In-house design diffractometer	Data collection mode: transmission
Radiation source: Synchrotron	Scan method: step
Si(111) channel-cut crystal monochromator	2θmin = 6°, 2θmax = 70°, 2θstep = 0.01°
Specimen mounting: capillary

Refinement

Rp = 0.059	Profile function: Fundamental Parameters
Rwp = 0.082	21 parameters
Rexp = 0.067	0 restraints
RBragg = 0.090	3 constraints
R(F) = 0.090	(Δ/σ)max = 0.001
χ^2 = 1.506	Background function: Chebychev
6801 data points	Preferred orientation correction: None
Excluded region(s): 2-6 degrees 2θ.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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)

Geometric parameters (Å, º)

Ba1—O1^i	2.67 (5)	Ba2—O3^vi	2.62 (4)
Ba1—O1^ii	2.67 (5)	Ba2—O3^vii	3.05 (4)
Ba1—O1	2.67 (5)	Ba2—O3^viii	3.05 (4)
Ba1—O2^iii	2.81 (4)	Ba2—O1^ii	3.14 (4)
Ba1—O2^iv	2.81 (4)	Ba2—Cl1^viii	3.281 (5)
Ba1—O2^v	2.81 (4)	Ba2—Cl1^ix	3.281 (5)
Ba1—O3^iv	3.12 (3)	As1—O3	1.64 (2)
Ba1—O3^iii	3.12 (3)	As1—O3^x	1.64 (2)
Ba1—O3^v	3.12 (3)	As1—O1	1.70 (8)
Ba2—O2^i	2.59 (4)	As1—O2	1.70 (4)
Ba2—O3^iv	2.62 (4)
O3—As1—O3^x	118 (2)	O3—As1—O2	108 (2)
O3—As1—O1	108 (1)	O3^x—As1—O2	108 (2)
O3^x—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.