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Acta Cryst. (2013). E69, i23    [ doi:10.1107/S1600536813007848 ]

Rietveld refinement of AgCa10(PO4)7 from X-ray powder data

N. Y. Strutynska, I. V. Zatovsky, I. V. Ogorodnyk and N. S. Slobodyanik

Abstract top

Polycrystalline silver(I) deca­calcium heptakis(orthophos­phate), AgCa10(PO4)7, was obtained by solid-state reaction. It is isotopic with members of the series MCa10(PO4)7 (M = Li, Na, K and Cs), and is closely related to the structure of [beta]-Ca3(PO4)2. The crystal structure of the title compound is built up from a framework of [CaO9] and two [CaO8] polyhedra, one [CaO6] octa­hedron (site symmetry 3.) and three PO4 tetra­hedra (one with site symmetry 3.). The Ag+ cation is likewise located on a threefold rotation axis and resides in the cavities of the rigid [Ca10(PO4)7]- framework. It is surrounded by three O atoms in an almost regular triangular environment.

Comment top

In recent years phosphates which are isotypic with β-Ca3(PO4)2 (whitlockite; Calvo & Gopal, 1975; Yashima et al., 2003) or whitlockite-related structures have attracted a growing interest due to their ferroelectric (Lazoryak et al., 2004), non-linear optical (Teterskii et al., 2005) or luminescent (Dou et al., 2011; Enhai et al., 2011; Zhang et al., 2011) properties. The structure of β-Ca3(PO4)2 contains three phosphorus (P1—P3) and five metal (M1—M5) sites, that are amenable to different types of substitutions, thus yielding a large number of closely related compounds. The Ca sites in the M1 and M2 positions (6a) are prone to substitution by univalent metals under formation of MCa10(PO4)7 compounds (M = Li, Na, K, Cs; Morozov et al., 2000; Sandström & Boström, 2006; Zatovsky et al., 2011), or by trivalent metals under formation of Ca9M(PO4)7 (M = Cr, Fe, In; Lazoryak et al., 1996; Morozov et al., 2002; Zatovsky et al., 2007), or combinations of univalent and trivalent metals (Zatovsky et al., 2010; Zatovsky et al., 2011). The new title compound AgCa10(PO4)7, (I), is likewise isotopic to the family of MCa10(PO4)7 (M = Li, Na, K, Cs) phosphates.

In the crystal structure of (I) four types of Ca sites (three in general positions 18b and one in special position 6a), three P sites (two in 18b one in 6a), ten O atoms (nine in 18b and one in 6a) and one Ag in 6a are present (Fig. 1).

The anionic framework [Ca10(PO4)7]- of (I) is formed by interconnection of four types of [CaOx] and [PO4] tetrahedra (Fig. 2). The silver cations reside in cavities and compensate the charge of the rigid framework.

The Ca—O distances in the three types of [CaOx] polyhedra (one [CaO9] (Ca4) and two [CaO8] (Ca2, Ca3)) are in the range 2.28 (4)–2.97 (4) Å which is close to that in the series of MCa10(PO4)7 structures (M = K, Cs; Sandström & Boström, 2006; Zatovsky et al., 2011). The polyhedron [CaO6] (Ca1) is more irregular with Ca—O distances spread over the range 2.17 (4) to 2.40 (4) Å. In the case of MCa10(PO4)7 (M = K, Cs), the corresponding distances are 2.23–2.31 Å. The nearest oxygen environment of the Ag site corresponds to an almost regular triangular arrangement. The position of the Ag site is slightly shifted by 0.30 (3) Å from the plane of the O3 triangle (Fig. 3). On both sides from the central triangular plane two further groups of Ag—O contacts can be observed. Three O2 atoms, which belong to a single orthophosphate tetrahedron, coordinate the Ag atom from one side of the plane and three O9 atoms, which belong to three different orthophosphate tetrahera, complete the other part of the [AgO9] coordination sphere. Such kind of arrangement of O atoms can be described as a distorted three-capped triangular antiprism (Fig. 3). The lengths of Ag—O contacts are 2.476 (19), 3.15 (4) and 3.35 (4) Å. In comparison with MCa10(PO4)7 (M = Na, Cs) the corresponding M—O distances are: d(Na—O) = 2.452, 2.981, 3.362 (Morozov et al., 2000) and d(Cs—O) = 2.803, 3.200, 3.252 Å (Zatovsky et al., 2011) and the coordination numbers of the alkaline metal are six for Na and nine for Cs (Fig. 4(b,c)). For the Ag atom, the Ag—O2 distance (3.15 (4) Å) significantly exceeds that of Ag—O10 (2.471 (15) Å) thus indicating that the coordination number should rather be described as [3 + 6] (Fig. 4a). Bond valence calculations (Brown, 2002) of the Ag+ cation resulted in 0.60 valence units considering the three close O atoms, and 0.67 v.u. considering also the six remote O atoms, thus indicating a rather low contribution to the overall bonding of the latter O atoms.

Related literature top

For the structure of the mineral whitlockite, see: Calvo & Gopal (1975); Yashima et al. (2003). For powder diffraction studies and Rietveld refinements of phosphate-based whitlockite-related compounds, see: Lazoryak et al. (1996); Morozov et al. (2000, 2002); Zatovsky et al. (2007, 2010, 2011). For physical properties of these materials, see: Dou et al. (2011); Enhai et al. (2011); Lazoryak et al. (2004); Teterskii et al. (2005); Zhang et al. (2011). For the crystal structure of isotypic KCa10(PO4)7, see: Sandström & Boström (2006). For bond-valence calculations, see: Brown (2002).

Experimental top

The title compound has been prepared by solid state reactions from a mixture of Ag3PO4, CaCO3 and CaHPO4 in the stoichiometric molar ratio Ag:Ca:P = 1:10:7. The starting components were finely ground in an agate mortar and then placed in a porcelain crucible. The thermal treatment has been carried out in two steps. The first one included preheating to 873 K to decompose the carbonate and calcium hydrogen phosphate. After that, the mixture was annealed at 1173 K for 20 h. The final product was a white powder.

Refinement top

Structure refinement was performed using KCa10(PO4)7 (Sandström & Boström, 2006) as a starting model. For profile refinement Pearson VII function was used. For the oxygen atoms of each orthophosphate group the isotropic temperature factors were restrained as equal. The result of the final Rietveld refinement is given in Fig. 5.

Computing details top

Data collection: PCXRD (Shimadzu, 2006); cell refinement: Dicvol (Boultif & Louër, 2004); data reduction: FULLPROF (Rodriguez-Carvajal, 2006); program(s) used to solve structure: FULLPROF (Rodriguez-Carvajal, 2006); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 2006); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: PLATON (Spek, 2009) and enCIFer (Allen et al., 2004).

Figures top
[Figure 1] Fig. 1. The asymmetric unit of the structure of compound (I).
[Figure 2] Fig. 2. The environment of the four different Ca sites (violet plane corresponds to Ca2 sites, the green plane to Ca3 sites and the blue plane to Ca4 sites) and the Ag+ cation in the structure of (I). PO4 groups are represented as purple tetrahedra.
[Figure 3] Fig. 3. The coordination environment of the Ag+ cation. [Symmetry codes: (i) -x + y, -x, z; (ii) -y, x-y, z; (iii) -2/3 + x, -1/3 + x-y, 1/6 + z; (iv) 1/3 - y, 2/3 - x, 1/6 + z; (v) 1/3 - x + y, -1/3 + y, 1/6 + z].
[Figure 4] Fig. 4. Comparison of the coordination environment of MI in the series MCa10(PO4)7; a) M = Ag; b) M = Na; c) M = Cs.
[Figure 5] Fig. 5. Rietveld refinement of AgCa10(PO4)7. Experimental (dots), calculated (red curve) and difference (blue curve) data for 2θ range 9–72°.
Silver(I) decacalcium heptakis(orthophosphate) top
Crystal data top
AgCa10(PO4)7Dx = 3.319 Mg m3
Mr = 1173.46Cu Kα radiation, λ = 1.540560 Å
Trigonal, R3cT = 293 K
Hall symbol: R 3 -2"cParticle morphology: isometric
a = 10.43723 (5) Åwhite
c = 37.3379 (7) Åflat sheet, 25 × 25 mm
V = 3522.50 (7) Å3Specimen preparation: Prepared at 293 K and 101.3 kPa
Z = 6
Data collection top
Shimadzu LabX XRD-6000
Data collection mode: reflection
Radiation source: X-ray tube, X-rayScan method: step
Graphite monochromator2θmin = 9.045°, 2θmax = 100.045°, 2θstep = 0.020°
Specimen mounting: glass container
Refinement top
Rp = 0.094150 parameters
Rwp = 0.1253 restraints
Rexp = 0.0423 constraints
RBragg = 0.051 Standard least squares refinement
R(F) = 0.038(Δ/σ)max = 0.001
χ2 = 8.821Background function: Linear Interpolation between a set background points with refinable heights
4551 data pointsPreferred orientation correction: March-Dollase Numeric Multiaxial Function
Profile function: Pearson VII
Crystal data top
AgCa10(PO4)7V = 3522.50 (7) Å3
Mr = 1173.46Z = 6
Trigonal, R3cCu Kα radiation, λ = 1.540560 Å
a = 10.43723 (5) ÅT = 293 K
c = 37.3379 (7) Åflat sheet, 25 × 25 mm
Data collection top
Shimadzu LabX XRD-6000
Scan method: step
Specimen mounting: glass container2θmin = 9.045°, 2θmax = 100.045°, 2θstep = 0.020°
Data collection mode: reflection
Refinement top
Rp = 0.094χ2 = 8.821
Rwp = 0.1254551 data points
Rexp = 0.042150 parameters
RBragg = 0.0513 restraints
R(F) = 0.038
Special details top

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
Ag10.000000.000000.1780 (8)0.042 (2)*
Ca10.333330.666670.1632 (9)0.002 (2)*
Ca20.4650 (10)0.5260 (11)0.0955 (8)0.0044 (14)*
Ca30.2864 (7)0.1558 (12)0.0625 (8)0.004 (2)*
Ca40.3992 (5)0.1876 (9)0.1565 (8)0.0044 (14)*
P10.666670.333330.0976 (8)0.002 (4)*
P20.1577 (14)0.3495 (13)0.0288 (8)0.009 (3)*
P30.1366 (11)0.3111 (7)0.1306 (8)0.003 (3)*
O10.666670.333330.1387 (11)0.006 (11)*
O20.5229 (16)0.325 (2)0.0860 (9)0.006 (6)*
O30.082 (3)0.1873 (15)0.0421 (9)0.005 (3)*
O40.051 (2)0.394 (3)0.0420 (10)0.005 (3)*
O50.173 (2)0.3689 (18)0.0111 (9)0.005 (3)*
O60.316 (2)0.440 (2)0.0462 (10)0.005 (3)*
O70.0093 (19)0.267 (3)0.1100 (9)0.006 (4)*
O80.241 (3)0.4824 (13)0.1261 (9)0.006 (4)*
O90.221 (2)0.243 (3)0.1161 (10)0.006 (4)*
O100.0887 (18)0.267 (2)0.1700 (10)0.006 (4)*
Atomic displacement parameters (Å2) top
Geometric parameters (Å, º) top
Ag1—O102.476 (19)Ca3—O4ii2.62 (4)
Ag1—O10i2.476 (19)Ca3—O62.89 (3)
Ag1—O10ii2.476 (19)Ca4—O7ii2.40 (4)
Ca1—O82.17 (4)Ca4—O6ix2.45 (4)
Ca1—O8iii2.17 (4)Ca4—O4vi2.46 (4)
Ca1—O8iv2.17 (4)Ca4—O12.510 (14)
Ca1—O3v2.40 (4)Ca4—O5ix2.55 (3)
Ca1—O3vi2.40 (4)Ca4—O5vi2.59 (2)
Ca1—O3vii2.40 (4)Ca4—O92.67 (4)
Ca2—O62.28 (4)Ca4—O10ii2.692 (18)
Ca2—O5vi2.41 (4)Ca4—O22.97 (4)
Ca2—O82.43 (4)P1—O11.54 (5)
Ca2—O4iii2.45 (4)P1—O21.52 (2)
Ca2—O22.48 (3)P1—O2x1.52 (2)
Ca2—O8iii2.48 (3)P1—O2xi1.52 (2)
Ca2—O7iii2.57 (2)P2—O31.55 (2)
Ca2—O92.88 (3)P2—O41.49 (3)
Ca3—O7ii2.31 (4)P2—O51.50 (4)
Ca3—O3ii2.37 (2)P2—O61.58 (3)
Ca3—O22.37 (2)P3—O71.56 (3)
Ca3—O92.43 (4)P3—O81.570 (15)
Ca3—O32.44 (4)P3—O91.48 (3)
Ca3—O10viii2.46 (4)P3—O101.55 (5)
O10—Ag1—O10i118.5 (9)O1—P1—O2x106.6 (17)
O10—Ag1—O10ii118.6 (7)O2x—P1—O2xi112.3 (18)
O10i—Ag1—O10ii118.6 (8)O2—P1—O2x112.2 (16)
O7—P3—O8107.7 (19)O2—P1—O2xi112.2 (17)
O7—P3—O9114 (2)O3—P2—O4101 (2)
O7—P3—O10105.0 (17)O4—P2—O5109 (2)
O8—P3—O9105.5 (18)O3—P2—O5115.3 (19)
O8—P3—O10112 (2)O3—P2—O6109 (2)
O9—P3—O10112.7 (19)O4—P2—O6114 (2)
O1—P1—O2xi106.5 (16)O5—P2—O6108.6 (19)
O1—P1—O2106.5 (16)Ag1—O10—P3109.4 (15)
Symmetry codes: (i) y, xy, z; (ii) x+y, x, z; (iii) y+1, xy+1, z; (iv) x+y, x+1, z; (v) y+1/3, x+2/3, z+1/6; (vi) x+1/3, xy+2/3, z+1/6; (vii) x+y+1/3, y+2/3, z+1/6; (viii) y+2/3, x+1/3, z1/6; (ix) x+y+1/3, y1/3, z+1/6; (x) y+1, xy, z; (xi) x+y+1, x+1, z.

Experimental details

Crystal data
Chemical formulaAgCa10(PO4)7
Crystal system, space groupTrigonal, R3c
Temperature (K)293
a, c (Å)10.43723 (5), 37.3379 (7)
V3)3522.50 (7)
Radiation typeCu Kα, λ = 1.540560 Å
Specimen shape, size (mm)Flat sheet, 25 × 25
Data collection
DiffractometerShimadzu LabX XRD-6000
Specimen mountingGlass container
Data collection modeReflection
Scan methodStep
2θ values (°)2θmin = 9.045 2θmax = 100.045 2θstep = 0.020
R factors and goodness of fitRp = 0.094, Rwp = 0.125, Rexp = 0.042, RBragg = 0.051, R(F) = 0.038, χ2 = 8.821
No. of data points4551
No. of parameters150
No. of restraints3

Computer programs: PCXRD (Shimadzu, 2006), Dicvol (Boultif & Louër, 2004), FULLPROF (Rodriguez-Carvajal, 2006), DIAMOND (Brandenburg, 1999), PLATON (Spek, 2009) and enCIFer (Allen et al., 2004).

Acknowledgements top

no acknowledgements