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Acta Crystallogr Sect E Struct Rep Online. 2010 May 1; 66(Pt 5): i41–i42.
Published online 2010 April 24. doi:  10.1107/S1600536810014327
PMCID: PMC2979107

Rietveld refinement of whitlockite-related K0.8Ca9.8Fe0.2(PO4)7

Abstract

The title compound, K0.8Ca9.8Fe0.2(PO4)7 (potassium deca­calcium iron hepta­phosphate), belongs to the whitlockite family. The structure is built up from several types of metal–oxygen polyhedra: two [CaO8], one [CaO7] and one [(Ca/Fe)O6] polyhedron with a mixed Ca/Fe occupancy in a 0.8:0.2 ratio, as well as three tetra­hedral [PO4] units. Of the 18 sites in the asymmetric unit, the site with the mixed Ca/Fe occupation, the K site, one P and one O site are on special positions 6a with 3 symmetry, whereas all other sites are on general positions 18b. The linkage of metal–oxygen polyhedra and [PO4] tetra­hedra via edges and corners results in formation of a three-dimensional framework with composition [Ca9.8Fe0.2(PO4)7]0.8−. The remaining K atoms (site-occupation factor = 0.8) are located in large closed cavities and are nine-coordinated by oxygen.

Related literature

For the structure of the mineral whitlockite with idealized composition Ca3(PO4)2 (β-polymorph), see: Calvo & Gopal (1975 [triangle]); Yashima et al. (2003 [triangle]). For KCa10(PO4)7, see: Sandström & Boström (2006 [triangle]). For powder diffraction investigations and Rietveld refinements of other phosphate-based whitlockites, see: Morozov et al. (2000 [triangle]) for M ICa10(PO4)7 (M I = Li, Na, K); Lazoryak et al. (1996 [triangle]) for Ca9Fe(PO4)7; Morozov et al. (2002 [triangle]) for Ca9In(PO4)7; Strunenkova et al. (1997 [triangle]) for Na1.5Ca9Fe0.5(PO4)7. For the profile function used in the Rietveld refinement, see: Thompson et al. (1987 [triangle]).

Experimental

Crystal data

  • K0.8Ca9.8Fe0.2(PO4)7
  • M r = 1100.02
  • Trigonal, An external file that holds a picture, illustration, etc.
Object name is e-66-00i41-efi1.jpg
  • a = 10.44282 (1) Å
  • c = 37.29443 (3) Å
  • V = 3522.17 (1) Å3
  • Z = 6
  • Cu Kα radiation, λ = 1.540598 Å
  • T = 293 K
  • Flat sheet, 25 × 25 mm

Data collection

  • Shimadzu LabX XRD-6000 diffractometer
  • Specimen mounting: glass container
  • Data collection mode: reflection
  • Scan method: step
  • min = 8.92°, 2θmax = 99.92°, increment in 2θ = 0.02°

Refinement

  • R p = 8.711
  • R wp = 11.243
  • R exp = 4.919
  • R Bragg = 3.849
  • R(F) = 2.48
  • 4551 data points with 839 reflections
  • 131 parameters
  • 4 restraints

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

Table 1
Selected bond lengths (Å)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536810014327/wm2324sup1.cif

Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536810014327/wm2324Isup2.rtv

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Comment

In the compound K0.8Ca9.8Fe0.2(PO4)7, (I), atoms Ca4/Fe4, K1, P1 and O11 are in special positions 6a that lie on a 3-fold rotation axis, whereas all other atoms are located in general positions 18b (Fig. 1).

Compound (I) might be represented as a result of an aliovalent substitution of calcium atoms in β-Ca3(PO4)2 (Calvo et al., 1975; Yashima et al., 2003) by a pair of K and Fe atoms.

[CaOx] polyhedra (two types of [CaO8], one of [CaO7] and one [(Ca/Fe)O6] with mixed Fe/Ca occupancy) and three different [PO4] tetrahedra are linked via edges and corners to built a three-dimensional framework with composition [Ca9.8Fe0.2(PO4)7]0.8- (Fig. 2). The K+ cations are located in large closed cavities inside the framework (K1 occupancy is equal to 0.8).

For (I), Ca—O distances of [CaO8]- and [CaO7]-polyhedra (2.295 (15)-2.78 (3) Å) are close to these in previously reported isotypic compounds s-Ca9Fe(PO4)7 (2.29 (3)-2.73 (3) Å), o-Ca9Fe(PO4)7 (2.29 (3)-2.70 (4) Å) (Lazoryak et al., 1996) and KCa10(PO4)7 (2.329 (3)- 2.76 (2) Å) (Sandström & Boström, 2006). The distances Ca/Fe—O (2.23 (4)-2.29 (3) Å) within the [(Ca/Fe)O6] polyhedron are close to these of the [CaO6] polyhedron in KCa10(PO4)7 (2.239 (4)-2.267 (4) Å), while they significantly differ from d(Fe—O) = 1.95 (3)-2.17 (3) Å in Ca9Fe(PO4)7.

Potassium atoms are nine-coordinated (three triples of K—O distances in the range of 2.508 (19)-3.24 (3) Å) (Fig. 3), while in KCa10(PO4)7 the K—O contacts vary in the range of 2.641 (3)-3.25 (4) Å .

In conclusion, compound (I) can be considered as a solid solution within the KCa10(PO4)7 / Ca9Fe(PO4)7 double system.

Experimental

The title compound was prepared by solid state reaction from a mixture of K2CO3, CaCO3, Fe2O3 and NH4H2PO4 in the molar ratio K/Ca/Fe/P = 0.8:9.8:0.2:7.0. The reagents were finely ground in an agate mortar and then placed in a porcelain crucible. The thermal treatment was carried out in three steps. The first included preheating to 873 K to decompose the ammonium salt and carbonates. After that, the mixture was heated at 1273 K for 12 h, cooled to room temperature, reground, and held at 1373 K for 6 h. The resulting product was a pale pink powder.

Refinement

The powder pattern was indexed in rhombohedral cell (hexagonal setting) by Dicvol 2004 (Boultif & Louër, 2004). The structure of KCa10(PO4)7 (Sandström & Boström, 2006) was selected as a starting model for Rietveld refinement. Profile matching refinement was performed firstly. Then scaling factor and background were added to the refined parameters. The background was approximated using linear interpolation between a set of background points with refineable heights. A modified pseudo-Voigt function (Thompson et al., 1987) was used for the profile refinement. As it was determined previously, only one position of calcium is suitable for heterovalent substitution by a three-valent 3d-metal. It is the octahedrally coordinated Ca4 site. Thus the iron site was placed into the Ca4 position. The occupancy of iron was fixed at 0.2 while the remaining calcium occupancy was set to 0.8. The potassium occupancy was set to 0.8 due to electroneutrality of the compound. The atomic coordinates and Biso of Ca and Fe were constrained to be equal. ADPs of all P atoms were constrained to be equal as well as the ADPs of all O atoms. The value of Biso for Ca4 was restrained in the range of 0.17-0.3. The value of Biso for O11 was also restrained in the range of 0.2-0.3. Two distance restraints for P2—O21 and P2—O23 bonds were applied. Experimental, calculated and difference patterns after the final refinement cycle are shown in Fig. 4.

Figures

Fig. 1.
A view of the unit cell content of compound (I).
Fig. 2.
Connectivity of the metal-oxygen polyhedra and PO4 groups in (I).
Fig. 3.
Coordination environment of the atoms in 6a position.
Fig. 4.
Rietveld refinement of K0.8Ca9.8Fe0.2(PO4)7. Experimental (dots), calculated (red curve) and difference (blue curve) data for 2θ range 9-72°.

Crystal data

K0.8Ca9.8Fe0.2(PO4)7Dx = 3.112 Mg m3
Mr = 1100.02Cu Kα radiation, λ = 1.540598 Å
Trigonal, R3cT = 293 K
Hall symbol: R 3 -2"cParticle morphology: isometric
a = 10.44282 (1) Ålight pink
c = 37.29443 (3) Åflat_sheet, 25 × 25 mm
V = 3522.17 (1) Å3Specimen preparation: Prepared at 293 K and 101.3 kPa
Z = 6

Data collection

Shimadzu LabX XRD-6000 diffractometerData collection mode: reflection
Radiation source: X-ray tube, X-rayScan method: step
graphitemin = 8.91°, 2θmax = 99.92°, 2θstep = 0.02°
Specimen mounting: glass container

Refinement

Rp = 8.711Profile function: Thompson–Cox–Hastings pseudo-Voigt * Axial divergence asymmetry
Rwp = 11.243131 parameters
Rexp = 4.9194 restraints
RBragg = 3.8494 constraints
R(F) = 2.48 Standard least squares refinement
χ2 = 5.368(Δ/σ)max = 0.001
4551 data pointsBackground function: Linear Interpolation between a set background points with refinable heights
Excluded region(s): undefPreferred orientation correction: Modified March's 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 esds are taken into account in the estimation of distances, angles and torsion angles

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

xyzUiso*/UeqOcc. (<1)
Ca10.3986 (5)0.1868 (7)0.0212 (4)0.0022 (18)*
Ca20.3922 (6)0.1887 (10)0.1265 (4)0.0022 (16)*
Ca30.1776 (11)0.3817 (6)0.0949 (5)0.003 (2)*
Ca40.333330.666670.0288 (5)0.002 (2)*0.80000
Fe40.333330.666670.0288 (5)0.002 (2)*0.20000
K10.000000.000000.0447 (5)0.004 (4)*0.80000
P10.000000.000000.1293 (5)0.0031 (11)*
P20.1351 (9)0.3124 (6)−0.0032 (4)0.0031 (11)*
P30.4897 (11)0.4749 (11)0.0609 (5)0.0031 (11)*
O110.000000.000000.1699 (8)0.0025 (11)*
O120.0071 (19)0.1449 (14)0.1115 (7)0.0025 (11)*
O210.0912 (15)0.2697 (15)0.0349 (4)0.0025 (11)*
O220.222 (2)0.233 (2)−0.0145 (6)0.0025 (11)*
O23−0.0066 (16)0.265 (2)−0.0248 (5)0.0025 (11)*
O240.229 (3)0.4728 (17)−0.0110 (6)0.0025 (11)*
O310.408 (3)0.567 (3)0.0709 (7)0.0025 (11)*
O320.5039 (17)0.4689 (16)0.0203 (5)0.0025 (11)*
O330.6427 (19)0.5475 (19)0.0808 (6)0.0025 (11)*
O340.3720 (19)0.3100 (19)0.0752 (7)0.0025 (11)*

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
???????

Geometric parameters (Å, °)

Ca1—O11i2.519 (10)Ca4—O31viii2.229 (35)
Ca1—O21ii2.702 (13)Fe4—O24viii2.299 (30)
Ca1—O222.509 (26)Fe4—O242.299 (30)
Ca1—O23ii2.397 (20)Fe4—O24vii2.299 (30)
Ca1—O322.579 (17)Fe4—O312.229 (35)
Ca1—O32iii2.573 (23)Fe4—O31vii2.229 (35)
Ca1—O33iii2.591 (27)Fe4—O31viii2.229 (35)
Ca1—O342.479 (28)K1—O122.896 (30)
Ca2—O12ii2.474 (16)K1—O12ii2.896 (30)
Ca2—O23iv2.629 (26)K1—O12ix2.896 (30)
Ca2—O24iv2.444 (19)K1—O212.508 (19)
Ca2—O24v2.484 (33)K1—O21ii2.508 (19)
Ca2—O32v2.410 (23)K1—O21ix2.508 (19)
Ca2—O33iii2.207 (27)K1—O223.245 (26)
Ca2—O342.362 (29)K1—O22ii3.245 (26)
Ca3—O122.295 (15)K1—O22ix3.245 (26)
Ca3—O212.477 (22)P1—O111.51 (4)
Ca3—O22vi2.485 (29)P1—O121.62 (2)
Ca3—O23iv2.301 (25)P1—O12ix1.62 (2)
Ca3—O312.383 (25)P1—O12ii1.62 (2)
Ca3—O31vii2.468 (36)P2—O211.49 (2)
Ca3—O33vii2.781 (25)P2—O221.56 (2)
Ca3—O342.597 (27)P2—O231.53 (2)
Ca4—O24viii2.299 (30)P2—O241.486 (17)
Ca4—O242.299 (30)P3—O311.62 (3)
Ca4—O24vii2.299 (30)P3—O321.53 (3)
Ca4—O312.229 (35)P3—O331.57 (3)
Ca4—O31vii2.229 (35)P3—O341.63 (2)
O24—Fe4—O24vii82.8 (11)O22—P2—O23114.2 (13)
O24—Fe4—O31vii101.7 (10)O22—P2—O24108.3 (15)
O24viii—Fe4—O31101.6 (10)O23—P2—O24104.3 (15)
O31—Fe4—O31viii75.9 (12)O31—P3—O32110.2 (15)
O24vii—Fe4—O31175.2 (13)O31—P3—O33108.4 (15)
O31—Fe4—O31vii75.9 (14)O31—P3—O34102.1 (15)
O24viii—Fe4—O31viii99.6 (11)O32—P3—O33113.1 (14)
O24viii—Fe4—O24vii82.8 (11)O32—P3—O34108.6 (13)
O24viii—Fe4—O31vii175.2 (12)O33—P3—O34113.8 (14)
O24vii—Fe4—O31viii101.7 (11)O12ix—P1—O12ii104.3 (12)
O31viii—Fe4—O31vii75.9 (13)O11—P1—O12ii114.2 (11)
O24vii—Fe4—O31vii99.6 (13)O11—P1—O12114.2 (11)
O24—Fe4—O3199.6 (9)O11—P1—O12ix114.2 (11)
O24—Fe4—O24viii82.8 (11)O12—P1—O12ix104.4 (12)
O24—Fe4—O31viii175.2 (12)O12—P1—O12ii104.4 (13)
O21—P2—O22105.8 (12)Fe4—O24—P2128.4 (15)
O21—P2—O23107.5 (11)Fe4—O31—P3121.8 (16)
O21—P2—O24117.0 (13)

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

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2324).

References

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