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 A₅(YO₄)₃X (A = divalent cation, Y = penta­valent cation, and X = F, Cl or Br) in the space group P6₃/m. The structure consists of isolated tetra­hedral AsO₄³⁻ 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.

Related literature

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).

Experimental

Crystal data

• Sr₅(AsO₄)₃Cl

• M_r = 890.31

• Hexagonal, P 6₃ /m

• a = 10.1969 (1) Å

• c = 7.28108 (9) Å

• V = 655.63 (2) ų

• Z = 2

• Synchrotron radiation

• λ = 0.998043 Å

• 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: fixed at 0

• 2θ_min = 2, 2θ_max = 60°

• Increment in 2θ = 0.01°

Refinement

• R_p = 0.052

• R_wp = 0.066

• R_exp = 0.047

• R_B = 0.090

• S = 2.00

• Excluded region(s): 2-6° 2θ

• Profile function: Pseudo Voigt

• 225 reflections

• 16 parameters

• Preferred orientation correction: none

Table 1
Selected geometric parameters (Å, °)

Sr1—O1	2.49 (2)
Sr1—O2^i	2.59 (2)
Sr1—O3^i	3.01 (1)
Sr2—O2^ii	2.53 (2)
Sr2—O3^iii	2.44 (1)
Sr2—O3^iv	2.94 (1)
Sr2—O1^v	3.02 (2)
Sr2—Cl1^iv	3.156 (3)
As1—O3	1.57 (1)
As1—O1	1.72 (2)
As1—O2	1.70 (2)

O3—As1—O3^vi	121 (1)
O3—As1—O1	105.8 (7)
O3—As1—O2	106.3 (6)
O1—As1—O2	112 (1)
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) 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).

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), White & ZhiLi (2003) and Wu et al., (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 Sr containing As-apatite Sr₅(AsO₄)₃Cl (Kreidler & Hummel, 1970) was indexed in space group P6₃/m. Related crystal structures have also been reported for Ca₅(AsO₄)₃Cl (Wardojo and Hwu, 1996) and for Sr₅(AsO₄)₃F and (Sr_1.66Ba_0.34)(Ba_2.61Sr_0.39)(AsO₄)₃Cl (Đordević et al., 2008). The crystal structure of Sr₅(AsO₄)₃Cl in space group P6₃/m is reported in the present communication. We recently reported the related crystal structure of Ba₅(AsO₄)₃Cl (Bell et al., 2008).

Table 1 shows refined interatomic distances and angles for the Sr₅(AsO₄)₃Cl structure. The averaged Sr1—O and Sr2—O distances of respectively 2.70 Å and 2.72 Å, compare with Sr1—O and Sr2—O distances in: Sr₅(AsO₄)₃F (Đordević et al. 2008) of 2.71 Å and 2.62 Å; 2.71 Å and 2.63 Å for Sr₅(VO₄)₃Cl (Beck et al., 2006); 2.67 Å and 2.62 Å for Sr₅(PO₄)₃Cl (Sudarsanan and Young, 1974); and 2.67 Å and 2.59 Å for Sr₅(PO₄)₃F (Swafford and Holt, 2002). 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 Sr₅(AsO₄)₃Cl 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 Sr₅(AsO₄)₃Cl, showing the isolated tetrahedral AsO₄^3- anions separated by Sr²⁺ cations and Cl^- anions, is displayed in Fig. 2.

Related literature

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).

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 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.

S3. Refinement

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.

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: GSAS (Larson & Von Dreele (2004) and EXPGUI (Toby, 2001); molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: publCIF (Westrip, 2009).

Figures

	Fig. 1. Rietveld difference plot for the multi-phase refinement of Sr5(AsO4)3Cl and SrCO3. The red crosses, and green and pink lines show respectively the observed, calculated and difference plots. Calculated Bragg reflection positions are indicated by black lines for the Sr5(AsO4)3Cl phase and by red lines for the SrCO3 phase.
	Fig. 2. The crystal structure of Sr5(AsO4)3Cl. Pink tetrahedra show AsO4 units with As^5+ cations as orange spheres and O^2- anions as red spheres. Large blue spheres represent Sr^2+ cations and small green spheres Cl^- anions.

pentastrontium tris[arsenate(V)] chloride

Crystal data

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) Å^3	cylinder, 40 × 0.7 mm
Z = 2	Specimen preparation: Prepared at 1258 K and 100 kPa

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 = 60°, 2θstep = 0.01°
Specimen mounting: capillary

Refinement

Rp = 0.052	Profile function: Pseudo Voigt
Rwp = 0.066	16 parameters
Rexp = 0.047	0 restraints
RBragg = 0.090	4 constraints
R(F^2) = 0.33148	(Δ/σ)max = 0.001
χ^2 = 3.992	Background function: Cosine Fourier Series
5801 data points	Preferred orientation correction: None
Excluded region(s): 2-6° 2θ

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.

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

	x	y	z	Uiso*/Ueq
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)
Cl1	0.0000	0.0000	0.0000	0.031 (5)

Geometric parameters (Å, º)

Sr1—O1^i	2.49 (2)	Sr2—O3^vi	2.44 (1)
Sr1—O1^ii	2.49 (2)	Sr2—O3^vii	2.94 (1)
Sr1—O1	2.49 (2)	Sr2—O3^viii	2.94 (1)
Sr1—O2^iii	2.59 (2)	Sr2—O1^ii	3.02 (2)
Sr1—O2^iv	2.59 (2)	Sr2—Cl1^viii	3.156 (3)
Sr1—O2^v	2.59 (2)	Sr2—Cl1^ix	3.156 (3)
Sr1—O3^iv	3.01 (1)	As1—O3	1.57 (1)
Sr1—O3^iii	3.01 (1)	As1—O3^x	1.57 (1)
Sr1—O3^v	3.01 (1)	As1—O1	1.72 (2)
Sr2—O2^i	2.53 (2)	As1—O2	1.70 (2)
Sr2—O3^iv	2.44 (1)
O3—As1—O3^x	121 (1)	O3—As1—O2	106.3 (6)
O3—As1—O1	105.8 (7)	O3^x—As1—O2	106.3 (6)
O3^x—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.