Rietveld refinement of whitlockite-related K_0.8Ca_9.8Fe_0.2(PO₄)₇

Abstract

The title compound, K_0.8Ca_9.8Fe_0.2(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 [Ca_9.8Fe_0.2(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 ▶); Yashima et al. (2003 ▶). For KCa₁₀(PO₄)₇, see: Sandström & Boström (2006 ▶). For powder diffraction investigations and Rietveld refinements of other phosphate-based whitlockites, see: Morozov et al. (2000 ▶) for M ^ICa₁₀(PO₄)₇ (M ^I = Li, Na, K); Lazoryak et al. (1996 ▶) for Ca₉Fe(PO₄)₇; Morozov et al. (2002 ▶) for Ca₉In(PO₄)₇; Strunenkova et al. (1997 ▶) for Na_1.5Ca₉Fe_0.5(PO₄)₇. For the profile function used in the Rietveld refinement, see: Thompson et al. (1987 ▶).

Experimental

Crystal data

• K_0.8Ca_9.8Fe_0.2(PO₄)₇

• M _r = 1100.02

• Trigonal,

• a = 10.44282 (1) Å

• c = 37.29443 (3) Å

• V = 3522.17 (1) ų

• 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

• 2θ_min = 8.92°, 2θ_max = 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 ▶); cell refinement: DICVOL 2004 (Boultif & Louër, 2004 ▶); data reduction: FULLPROF (Rodriguez-Carvajal, 2006 ▶); program(s) used to solve structure: FULLPROF; program(s) used to refine structure: FULLPROF; molecular graphics: DIAMOND (Brandenburg, 1999 ▶); software used to prepare material for publication: PLATON (Spek, 2009 ▶) and enCIFer (Allen et al., 2004 ▶).

Supplementary Material

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

Table 1. Selected bond lengths (Å).

Ca1—O11i	2.519 (10)
Ca1—O21ii	2.702 (13)
Ca1—O22	2.51 (3)
Ca1—O23ii	2.40 (2)
Ca1—O32	2.579 (17)
Ca1—O32iii	2.57 (2)
Ca1—O33iii	2.59 (3)
Ca1—O34	2.48 (3)
Ca2—O12ii	2.474 (16)
Ca2—O23iv	2.63 (3)
Ca2—O24iv	2.444 (19)
Ca2—O24v	2.48 (3)
Ca2—O32v	2.41 (2)
Ca2—O33iii	2.21 (3)
Ca2—O34	2.36 (3)
Ca3—O12	2.295 (15)
Ca3—O21	2.48 (2)
Ca3—O22vi	2.49 (3)
Ca3—O23iv	2.30 (3)
Ca3—O31	2.38 (3)
Ca3—O31vii	2.47 (4)
Ca3—O33vii	2.78 (3)
Ca3—O34	2.60 (3)
Ca4—O24	2.30 (3)
Ca4—O31	2.23 (4)
Fe4—O24	2.30 (3)
Fe4—O31	2.23 (4)
K1—O12	2.90 (3)
K1—O21	2.508 (19)
K1—O22	3.25 (3)
P1—O11	1.51 (4)
P1—O12	1.62 (2)
P2—O21	1.49 (2)
P2—O22	1.56 (2)
P2—O23	1.53 (2)
P2—O24	1.486 (17)
P3—O31	1.62 (3)
P3—O32	1.53 (3)
P3—O33	1.57 (3)
P3—O34	1.63 (2)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

supplementary crystallographic information

Comment

In the compound K_0.8Ca_9.8Fe_0.2(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.

[CaO_x] 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 [Ca_9.8Fe_0.2(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 B_iso 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 B_iso for Ca4 was restrained in the range of 0.17-0.3. The value of B_iso 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)7	Dx = 3.112 Mg m−3
Mr = 1100.02	Cu Kα radiation, λ = 1.540598 Å
Trigonal, R3c	T = 293 K
Hall symbol: R 3 -2"c	Particle morphology: isometric
a = 10.44282 (1) Å	light pink
c = 37.29443 (3) Å	flat_sheet, 25 × 25 mm
V = 3522.17 (1) Å3	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	2θmin = 8.91°, 2θmax = 99.92°, 2θstep = 0.02°
Specimen mounting: glass container

Refinement

Rp = 8.711	Profile function: Thompson–Cox–Hastings pseudo-Voigt * Axial divergence asymmetry
Rwp = 11.243	131 parameters
Rexp = 4.919	4 restraints
RBragg = 3.849	4 constraints
R(F) = 2.48	Standard least squares refinement
χ2 = 5.368	(Δ/σ)max = 0.001
4551 data points	Background function: Linear Interpolation between a set background points with refinable heights
Excluded region(s): undef	Preferred 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ca1	0.3986 (5)	0.1868 (7)	0.0212 (4)	0.0022 (18)*
Ca2	0.3922 (6)	0.1887 (10)	0.1265 (4)	0.0022 (16)*
Ca3	0.1776 (11)	0.3817 (6)	0.0949 (5)	0.003 (2)*
Ca4	0.33333	0.66667	0.0288 (5)	0.002 (2)*	0.80000
Fe4	0.33333	0.66667	0.0288 (5)	0.002 (2)*	0.20000
K1	0.00000	0.00000	0.0447 (5)	0.004 (4)*	0.80000
P1	0.00000	0.00000	0.1293 (5)	0.0031 (11)*
P2	0.1351 (9)	0.3124 (6)	−0.0032 (4)	0.0031 (11)*
P3	0.4897 (11)	0.4749 (11)	0.0609 (5)	0.0031 (11)*
O11	0.00000	0.00000	0.1699 (8)	0.0025 (11)*
O12	0.0071 (19)	0.1449 (14)	0.1115 (7)	0.0025 (11)*
O21	0.0912 (15)	0.2697 (15)	0.0349 (4)	0.0025 (11)*
O22	0.222 (2)	0.233 (2)	−0.0145 (6)	0.0025 (11)*
O23	−0.0066 (16)	0.265 (2)	−0.0248 (5)	0.0025 (11)*
O24	0.229 (3)	0.4728 (17)	−0.0110 (6)	0.0025 (11)*
O31	0.408 (3)	0.567 (3)	0.0709 (7)	0.0025 (11)*
O32	0.5039 (17)	0.4689 (16)	0.0203 (5)	0.0025 (11)*
O33	0.6427 (19)	0.5475 (19)	0.0808 (6)	0.0025 (11)*
O34	0.3720 (19)	0.3100 (19)	0.0752 (7)	0.0025 (11)*

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
?	?	?	?	?	?	?

Geometric parameters (Å, °)

Ca1—O11i	2.519 (10)	Ca4—O31viii	2.229 (35)
Ca1—O21ii	2.702 (13)	Fe4—O24viii	2.299 (30)
Ca1—O22	2.509 (26)	Fe4—O24	2.299 (30)
Ca1—O23ii	2.397 (20)	Fe4—O24vii	2.299 (30)
Ca1—O32	2.579 (17)	Fe4—O31	2.229 (35)
Ca1—O32iii	2.573 (23)	Fe4—O31vii	2.229 (35)
Ca1—O33iii	2.591 (27)	Fe4—O31viii	2.229 (35)
Ca1—O34	2.479 (28)	K1—O12	2.896 (30)
Ca2—O12ii	2.474 (16)	K1—O12ii	2.896 (30)
Ca2—O23iv	2.629 (26)	K1—O12ix	2.896 (30)
Ca2—O24iv	2.444 (19)	K1—O21	2.508 (19)
Ca2—O24v	2.484 (33)	K1—O21ii	2.508 (19)
Ca2—O32v	2.410 (23)	K1—O21ix	2.508 (19)
Ca2—O33iii	2.207 (27)	K1—O22	3.245 (26)
Ca2—O34	2.362 (29)	K1—O22ii	3.245 (26)
Ca3—O12	2.295 (15)	K1—O22ix	3.245 (26)
Ca3—O21	2.477 (22)	P1—O11	1.51 (4)
Ca3—O22vi	2.485 (29)	P1—O12	1.62 (2)
Ca3—O23iv	2.301 (25)	P1—O12ix	1.62 (2)
Ca3—O31	2.383 (25)	P1—O12ii	1.62 (2)
Ca3—O31vii	2.468 (36)	P2—O21	1.49 (2)
Ca3—O33vii	2.781 (25)	P2—O22	1.56 (2)
Ca3—O34	2.597 (27)	P2—O23	1.53 (2)
Ca4—O24viii	2.299 (30)	P2—O24	1.486 (17)
Ca4—O24	2.299 (30)	P3—O31	1.62 (3)
Ca4—O24vii	2.299 (30)	P3—O32	1.53 (3)
Ca4—O31	2.229 (35)	P3—O33	1.57 (3)
Ca4—O31vii	2.229 (35)	P3—O34	1.63 (2)
O24—Fe4—O24vii	82.8 (11)	O22—P2—O23	114.2 (13)
O24—Fe4—O31vii	101.7 (10)	O22—P2—O24	108.3 (15)
O24viii—Fe4—O31	101.6 (10)	O23—P2—O24	104.3 (15)
O31—Fe4—O31viii	75.9 (12)	O31—P3—O32	110.2 (15)
O24vii—Fe4—O31	175.2 (13)	O31—P3—O33	108.4 (15)
O31—Fe4—O31vii	75.9 (14)	O31—P3—O34	102.1 (15)
O24viii—Fe4—O31viii	99.6 (11)	O32—P3—O33	113.1 (14)
O24viii—Fe4—O24vii	82.8 (11)	O32—P3—O34	108.6 (13)
O24viii—Fe4—O31vii	175.2 (12)	O33—P3—O34	113.8 (14)
O24vii—Fe4—O31viii	101.7 (11)	O12ix—P1—O12ii	104.3 (12)
O31viii—Fe4—O31vii	75.9 (13)	O11—P1—O12ii	114.2 (11)
O24vii—Fe4—O31vii	99.6 (13)	O11—P1—O12	114.2 (11)
O24—Fe4—O31	99.6 (9)	O11—P1—O12ix	114.2 (11)
O24—Fe4—O24viii	82.8 (11)	O12—P1—O12ix	104.4 (12)
O24—Fe4—O31viii	175.2 (12)	O12—P1—O12ii	104.4 (13)
O21—P2—O22	105.8 (12)	Fe4—O24—P2	128.4 (15)
O21—P2—O23	107.5 (11)	Fe4—O31—P3	121.8 (16)
O21—P2—O24	117.0 (13)

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