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

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 β-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.

Key indicators

• Powder X-ray study
• T = 293 K
• Mean [sigma](P-O) = 0.050 Å
• R factor = 0.000
• wR factor = 0.000
• Data-to-parameter ratio = 0.0

checkCIF/PLATON results

No syntax errors found



Alert level C
REFI023_ALERT_1_C  _refine_diff_density_max is missing
            Maximum value of final difference map (e A-3).
            The following tests will not be performed
            DIFMN_01,DIFMX_01,DIFMX_02
REFI024_ALERT_1_C  _refine_diff_density_min is missing
            Minimum value of final difference map (e A-3).
            The following tests will not be performed
            DIFMN_01,DIFMN_02,DIFMN_03
PLAT242_ALERT_2_C Check Low       Ueq as Compared to Neighbors for        O10
PLAT799_ALERT_4_C Numeric Label on Displacement Par. Record ......          ?

Alert level G
PLAT152_ALERT_1_G The Supplied and Calc. Volume s.1. Differ by ...          2 Units
PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints .......          3
PLAT982_ALERT_1_G The Ag-f'=    -0.060 Deviates from the  IT-value      0.131
PLAT982_ALERT_1_G The Ca-f'=     0.341 Deviates from the  IT-value      0.364
PLAT982_ALERT_1_G The  O-f'=     0.047 Deviates from the  IT-value      0.049
PLAT982_ALERT_1_G The  P-f'=     0.283 Deviates from the  IT-value      0.296

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   4 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   6 ALERT level G = General information/check it is not something unexpected

   7 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

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 [CaO_x] 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 [CaO_x] 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

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).

S2. Experimental

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.

S3. Refinement

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

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

	Fig. 1. The asymmetric unit of the structure of compound (I).
	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.
	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].
	Fig. 4. Comparison of the coordination environment of M^I in the series MCa10(PO4)7; a) M = Ag; b) M = Na; c) M = Cs.
	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)

Crystal data

AgCa10(PO4)7	Dx = 3.319 Mg m^−3
Mr = 1173.46	Cu Kα radiation, λ = 1.540560 Å
Trigonal, R3c	T = 293 K
Hall symbol: R 3 -2"c	Particle morphology: isometric
a = 10.43723 (5) Å	white
c = 37.3379 (7) Å	flat sheet, 25 × 25 mm
V = 3522.50 (7) Å^3	Specimen preparation: Prepared at 293 K and 101.3 kPa
Z = 6

Data collection

Shimadzu LabX XRD-6000 diffractometer	Data collection mode: reflection
Radiation source: X-ray tube, X-ray	Scan method: step
Graphite monochromator	2θmin = 9.045°, 2θmax = 100.045°, 2θstep = 0.020°
Specimen mounting: glass container

Refinement

Rp = 0.094	150 parameters
Rwp = 0.125	3 restraints
Rexp = 0.042	3 constraints
RBragg = 0.051	Standard least squares refinement
R(F) = 0.038	(Δ/σ)max = 0.001
4551 data points	Background function: Linear Interpolation between a set background points with refinable heights
Profile function: Pearson VII	Preferred orientation correction: March-Dollase Numeric Multiaxial Function

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Ag1	0.00000	0.00000	0.1780 (8)	0.042 (2)*
Ca1	0.33333	0.66667	0.1632 (9)	0.002 (2)*
Ca2	0.4650 (10)	0.5260 (11)	0.0955 (8)	0.0044 (14)*
Ca3	0.2864 (7)	0.1558 (12)	0.0625 (8)	0.004 (2)*
Ca4	0.3992 (5)	0.1876 (9)	0.1565 (8)	0.0044 (14)*
P1	0.66667	0.33333	0.0976 (8)	0.002 (4)*
P2	0.1577 (14)	0.3495 (13)	0.0288 (8)	0.009 (3)*
P3	0.1366 (11)	0.3111 (7)	0.1306 (8)	0.003 (3)*
O1	0.66667	0.33333	0.1387 (11)	0.006 (11)*
O2	0.5229 (16)	0.325 (2)	0.0860 (9)	0.006 (6)*
O3	0.082 (3)	0.1873 (15)	0.0421 (9)	0.005 (3)*
O4	0.051 (2)	0.394 (3)	0.0420 (10)	0.005 (3)*
O5	0.173 (2)	0.3689 (18)	−0.0111 (9)	0.005 (3)*
O6	0.316 (2)	0.440 (2)	0.0462 (10)	0.005 (3)*
O7	−0.0093 (19)	0.267 (3)	0.1100 (9)	0.006 (4)*
O8	0.241 (3)	0.4824 (13)	0.1261 (9)	0.006 (4)*
O9	0.221 (2)	0.243 (3)	0.1161 (10)	0.006 (4)*
O10	0.0887 (18)	0.267 (2)	0.1700 (10)	0.006 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Ag1—O10	2.476 (19)	Ca3—O4^ii	2.62 (4)
Ag1—O10^i	2.476 (19)	Ca3—O6	2.89 (3)
Ag1—O10^ii	2.476 (19)	Ca4—O7^ii	2.40 (4)
Ca1—O8	2.17 (4)	Ca4—O6^ix	2.45 (4)
Ca1—O8^iii	2.17 (4)	Ca4—O4^vi	2.46 (4)
Ca1—O8^iv	2.17 (4)	Ca4—O1	2.510 (14)
Ca1—O3^v	2.40 (4)	Ca4—O5^ix	2.55 (3)
Ca1—O3^vi	2.40 (4)	Ca4—O5^vi	2.59 (2)
Ca1—O3^vii	2.40 (4)	Ca4—O9	2.67 (4)
Ca2—O6	2.28 (4)	Ca4—O10^ii	2.692 (18)
Ca2—O5^vi	2.41 (4)	Ca4—O2	2.97 (4)
Ca2—O8	2.43 (4)	P1—O1	1.54 (5)
Ca2—O4^iii	2.45 (4)	P1—O2	1.52 (2)
Ca2—O2	2.48 (3)	P1—O2^x	1.52 (2)
Ca2—O8^iii	2.48 (3)	P1—O2^xi	1.52 (2)
Ca2—O7^iii	2.57 (2)	P2—O3	1.55 (2)
Ca2—O9	2.88 (3)	P2—O4	1.49 (3)
Ca3—O7^ii	2.31 (4)	P2—O5	1.50 (4)
Ca3—O3^ii	2.37 (2)	P2—O6	1.58 (3)
Ca3—O2	2.37 (2)	P3—O7	1.56 (3)
Ca3—O9	2.43 (4)	P3—O8	1.570 (15)
Ca3—O3	2.44 (4)	P3—O9	1.48 (3)
Ca3—O10^viii	2.46 (4)	P3—O10	1.55 (5)
O10—Ag1—O10^i	118.5 (9)	O1—P1—O2^x	106.6 (17)
O10—Ag1—O10^ii	118.6 (7)	O2^x—P1—O2^xi	112.3 (18)
O10^i—Ag1—O10^ii	118.6 (8)	O2—P1—O2^x	112.2 (16)
O7—P3—O8	107.7 (19)	O2—P1—O2^xi	112.2 (17)
O7—P3—O9	114 (2)	O3—P2—O4	101 (2)
O7—P3—O10	105.0 (17)	O4—P2—O5	109 (2)
O8—P3—O9	105.5 (18)	O3—P2—O5	115.3 (19)
O8—P3—O10	112 (2)	O3—P2—O6	109 (2)
O9—P3—O10	112.7 (19)	O4—P2—O6	114 (2)
O1—P1—O2^xi	106.5 (16)	O5—P2—O6	108.6 (19)
O1—P1—O2	106.5 (16)	Ag1—O10—P3	109.4 (15)

Symmetry codes: (i) −y, x−y, z; (ii) −x+y, −x, z; (iii) −y+1, x−y+1, z; (iv) −x+y, −x+1, z; (v) −y+1/3, −x+2/3, z+1/6; (vi) x+1/3, x−y+2/3, z+1/6; (vii) −x+y+1/3, y+2/3, z+1/6; (viii) −y+2/3, −x+1/3, z−1/6; (ix) −x+y+1/3, y−1/3, z+1/6; (x) −y+1, x−y, z; (xi) −x+y+1, −x+1, z.