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

Zatovsky, I.V.; Ogorodnyk, I.V.; Strutynska, N.Y.; Slobodyanik, N.S.; Sharkina, N.O.

doi:10.1107/S1600536810014327

inorganic compounds

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Volume 66| Part 5| May 2010| Pages i41-i42

https://doi.org/10.1107/S1600536810014327

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

Igor V. Zatovsky,^a ^* Ivan V. Ogorodnyk,^a Nataliya Yu. Strutynska,^a Nikolay S. Slobodyanik ^a and Nataliya O. Sharkina ^a

^aDepartment of Inorganic Chemistry, Taras Shevchenko National University, 64 Volodymyrska str., 01601 Kyiv, Ukraine
^*Correspondence e-mail: zvigo@yandex.ru

(Received 7 April 2010; accepted 19 April 2010; online 24 April 2010)

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, R 3c
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
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

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—O23^iv	2.63 (3)
Ca2—O24^iv	2.444 (19)
Ca2—O24^v	2.48 (3)
Ca2—O32^v	2.41 (2)
Ca2—O33ⁱⁱⁱ	2.21 (3)
Ca2—O34	2.36 (3)
Ca3—O12	2.295 (15)
Ca3—O21	2.48 (2)
Ca3—O22^vi	2.49 (3)
Ca3—O23^iv	2.30 (3)
Ca3—O31	2.38 (3)
Ca3—O31^vii	2.47 (4)
Ca3—O33^vii	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) $[-x+y+{\script{2\over 3}}, y+{\script{1\over 3}}, z-{\script{1\over 6}}]$ ; (ii) -x+y, -x, z; (iii) -y+1, x-y, z; (iv) $[x+{\script{1\over 3}}, x-y+{\script{2\over 3}}, z+{\script{1\over 6}}]$ ; (v) $[-x+y+{\script{1\over 3}}, y-{\script{1\over 3}}, z+{\script{1\over 6}}]$ ; (vi) $[-y+{\script{1\over 3}}, -x+{\script{2\over 3}}, z+{\script{1\over 6}}]$ ; (vii) -x+y, -x+1, z.

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 ).

Supporting information

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

Rietveld powder data: contains datablock I. DOI: https://doi.org/10.1107/S1600536810014327/wm2324Isup2.rtv

checkCIF report

Comment top

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.

Related literature top

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 top

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.

Structure description top

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.

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

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).

Computing details top

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 (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

	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 K_0.8Ca_9.8Fe_0.2(PO₄)₇. Experimental (dots), calculated (red curve) and difference (blue curve) data for 2θ range 9-72°.

potassium decacalcium iron heptaphosphate top

Crystal data top

K_0.8Ca_9.8Fe_0.2(PO₄)₇	D_x = 3.112 Mg m⁻³
M_r = 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 top

Shimadzu LabX XRD-6000 diffractometer	Data collection mode: reflection
Radiation source: X-ray tube, X-ray	Scan method: step
Graphite monochromator	2θ_min = 8.915°, 2θ_max = 99.915°, 2θ_step = 0.020°
Specimen mounting: glass container

Refinement top

R_p = 8.711	131 parameters
R_wp = 11.243	4 restraints
R_exp = 4.919	4 constraints
R_Bragg = 3.849	Standard least squares refinement
R(F) = 2.48	(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
Profile function: Thompson–Cox–Hastings pseudo-Voigt * Axial divergence asymmetry

Crystal data top

K_0.8Ca_9.8Fe_0.2(PO₄)₇	V = 3522.17 (1) Å³
M_r = 1100.02	Z = 6
Trigonal, R3c	Cu Kα radiation, λ = 1.540598 Å
a = 10.44282 (1) Å	T = 293 K
c = 37.29443 (3) Å	flat_sheet, 25 × 25 mm

Data collection top

Shimadzu LabX XRD-6000 diffractometer	Scan method: step
Specimen mounting: glass container	2θ_min = 8.915°, 2θ_max = 99.915°, 2θ_step = 0.020°
Data collection mode: reflection

Refinement top

R_p = 8.711	R(F) = 2.48
R_wp = 11.243	4551 data points
R_exp = 4.919	131 parameters
R_Bragg = 3.849	4 restraints

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 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) top

	x	y	z	U_iso*/U_eq	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 (Å²) top

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

Geometric parameters (Å, º) top

Ca1—O11ⁱ	2.519 (10)	Ca4—O31^viii	2.23 (4)
Ca1—O21ⁱⁱ	2.702 (13)	Fe4—O24^viii	2.30 (3)
Ca1—O22	2.51 (3)	Fe4—O24	2.30 (3)
Ca1—O23ⁱⁱ	2.40 (2)	Fe4—O24^vii	2.30 (3)
Ca1—O32	2.579 (17)	Fe4—O31	2.23 (4)
Ca1—O32ⁱⁱⁱ	2.57 (2)	Fe4—O31^vii	2.23 (4)
Ca1—O33ⁱⁱⁱ	2.59 (3)	Fe4—O31^viii	2.23 (4)
Ca1—O34	2.48 (3)	K1—O12	2.90 (3)
Ca2—O12ⁱⁱ	2.474 (16)	K1—O12ⁱⁱ	2.90 (3)
Ca2—O23^iv	2.63 (3)	K1—O12^ix	2.90 (3)
Ca2—O24^iv	2.444 (19)	K1—O21	2.508 (19)
Ca2—O24^v	2.48 (3)	K1—O21ⁱⁱ	2.508 (19)
Ca2—O32^v	2.41 (2)	K1—O21^ix	2.508 (19)
Ca2—O33ⁱⁱⁱ	2.21 (3)	K1—O22	3.25 (3)
Ca2—O34	2.36 (3)	K1—O22ⁱⁱ	3.25 (3)
Ca3—O12	2.295 (15)	K1—O22^ix	3.25 (3)
Ca3—O21	2.48 (2)	P1—O11	1.51 (4)
Ca3—O22^vi	2.49 (3)	P1—O12	1.62 (2)
Ca3—O23^iv	2.30 (3)	P1—O12^ix	1.62 (2)
Ca3—O31	2.38 (3)	P1—O12ⁱⁱ	1.62 (2)
Ca3—O31^vii	2.47 (4)	P2—O21	1.49 (2)
Ca3—O33^vii	2.78 (3)	P2—O22	1.56 (2)
Ca3—O34	2.60 (3)	P2—O23	1.53 (2)
Ca4—O24^viii	2.30 (3)	P2—O24	1.486 (17)
Ca4—O24	2.30 (3)	P3—O31	1.62 (3)
Ca4—O24^vii	2.30 (3)	P3—O32	1.53 (3)
Ca4—O31	2.23 (4)	P3—O33	1.57 (3)
Ca4—O31^vii	2.23 (4)	P3—O34	1.63 (2)

O24—Fe4—O24^vii	82.8 (11)	O22—P2—O23	114.2 (13)
O24—Fe4—O31^vii	101.7 (10)	O22—P2—O24	108.3 (15)
O24^viii—Fe4—O31	101.6 (10)	O23—P2—O24	104.3 (15)
O31—Fe4—O31^viii	75.9 (12)	O31—P3—O32	110.2 (15)
O24^vii—Fe4—O31	175.2 (13)	O31—P3—O33	108.4 (15)
O31—Fe4—O31^vii	75.9 (14)	O31—P3—O34	102.1 (15)
O24^viii—Fe4—O31^viii	99.6 (11)	O32—P3—O33	113.1 (14)
O24^viii—Fe4—O24^vii	82.8 (11)	O32—P3—O34	108.6 (13)
O24^viii—Fe4—O31^vii	175.2 (12)	O33—P3—O34	113.8 (14)
O24^vii—Fe4—O31^viii	101.7 (11)	O12^ix—P1—O12ⁱⁱ	104.3 (12)
O31^viii—Fe4—O31^vii	75.9 (13)	O11—P1—O12ⁱⁱ	114.2 (11)
O24^vii—Fe4—O31^vii	99.6 (13)	O11—P1—O12	114.2 (11)
O24—Fe4—O31	99.6 (9)	O11—P1—O12^ix	114.2 (11)
O24—Fe4—O24^viii	82.8 (11)	O12—P1—O12^ix	104.4 (12)
O24—Fe4—O31^viii	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.

Experimental details

Crystal data
Chemical formula	K_0.8Ca_9.8Fe_0.2(PO₄)₇
M_r	1100.02
Crystal system, space group	Trigonal, R3c
Temperature (K)	293
a, c (Å)	10.44282 (1), 37.29443 (3)
V (Å³)	3522.17 (1)
Z	6
Radiation type	Cu Kα, λ = 1.540598 Å
Specimen shape, size (mm)	Flat_sheet, 25 × 25

Data collection
Diffractometer	Shimadzu LabX XRD-6000
Specimen mounting	Glass container
Data collection mode	Reflection
Scan method	Step
2θ values (°)	2θ_min = 8.915 2θ_max = 99.915 2θ_step = 0.020

Refinement
R factors and goodness of fit	R_p = 8.711, R_wp = 11.243, R_exp = 4.919, R_Bragg = 3.849, R(F) = 2.48, χ² = 5.368
No. of parameters	131
No. of restraints	4

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

Selected bond lengths (Å) top

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

References

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