Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO4)2

Bouraima, A.; Ouaatta, S.; Khmiyas, J.; Anguilè, J. J.; Makani, T.; Assani, A.; Saadi, M.; El Ammari, L.

doi:10.1107/S2056989022006521

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ISSN: 2056-9890

Volume 78| Part 7| July 2022| Pages 746-749

https://doi.org/10.1107/S2056989022006521

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Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO₄)₂

Adam Bouraima,^a,^b ^* Said Ouaatta,^a Jamal Khmiyas,^a Jean Jacques Anguilè,^b Thomas Makani,^b Abderrazzak Assani,^a Mohamed Saadi ^a and Lahcen El Ammari ^a

^aLaboratoire de Chimie Appliquée des Matériaux, Centre des Sciences des Matériaux, Faculty of Science, Mohammed V University in Rabat, Avenue Ibn Batouta, BP 1014, Rabat, Morocco, and ^bLaboratoire de Chimie des Matériaux Inorganiques, Faculté des Sciences, Département de Chimie, Université des Sciences et Techniques de Masuku, BP 943, Franceville, Gabon
^*Correspondence e-mail: bouraima_adam@yahoo.com

Edited by M. Weil, Vienna University of Technology, Austria (Received 2 May 2022; accepted 23 June 2022; online 28 June 2022)

Single crystals of β-KCoFe(PO₄)₂, potassium cobalt(II) iron(III) bis(orthophosphate), were grown from the melt under atmospheric conditions. This phosphate crystallizes isotypically with KZnFe(PO₄)₂ in space group C2/c, adopting a zeolite-ABW type of structure. The structure of the present phosphate is distinguished by an occupational disorder of the two transition-metal sites with ratios Fe:Co of 0.5725:0.4275 for the first and 0.4275:0.5725 for the second site. In the crystal structure, PO₄ and (Co,Fe)O₄ tetrahedra are linked through vertices to form elliptical rings with the sequence DDDDUUUU of up (U) and down (D) pointing vertices. Each eight-membered ring is surrounded by four other rings of the same type, delimiting interstices with rectangular shape. This arrangement leads to the formation of [(Co/Fe)(PO₄)]⁻_∞ sheets parallel to (001). Stacking of the sheets into a three-dimensional framework results in the formation of two types of channels. The first one is occupied by potassium cations, whereas the second one remains vacant. Calculations of bond-valence sums and charge distribution were used to confirm the structure model.

Keywords: crystal structure; phosphate; zeolite-ABW; isotypism.

CCDC reference: 2181684

1. Chemical context

Transition-metal (TM) phosphates have been widely studied as potential candidates for various applications such as catalysis (Bautista et al., 2007 ), ion exchange (Szirtes et al., 2007 ), electrochemistry (Trad et al., 2010 ) or as magnetic materials (Ofer et al., 2012 ). In this context, zinc phosphates are of interest because the Zn²⁺ cation with its d¹⁰ electronic configuration is susceptible to strong polarization and thus can be used to design new non-linear optical (NLO) materials (Shen et al., 2016 ). In the family of transition-metal phosphate compounds, the anionic network is formed from PO₄ tetrahedra bonded to different types of coordination polyhedra of the form [TMO_n] (n = 4, 5 and 6), leading to a wide variety of crystal structure types such as NaZnAl(PO₄)₂ (Yakubovich et al., 2019 ). The structural diversity is mainly associated with the ability of TM cations to adopt different oxidation states with various types of coordination polyhedra (Moore & Ito, 1979 ; Hatert et al., 2004 ).

It is in this context that our research team was involved with investigations of new phosphates with A^I, M^II and M^III cations where A is an alkali metal, and M^II and M^III are bivalent and trivalent cations, respectively. For example, Na₂Co₂Fe(PO₄)₃ (Bouraima et al., 2015 ) and NaCuIn(PO₄)₂ (Benhsina et al., 2020 ) are among the recently studied compounds. The present work is devoted to synthesis and crystal structure analysis of β-KCoFe(PO₄)₂, a new compound in the family of transition-metal phosphates.

2. Structural commentary

The title compound crystallizes isotypically with KZnFe(PO₄)₂ (Badri et al., 2015 ). The principal building units of β-KCoFe(PO₄)₂ are shown in Fig. 1, revealing that three types of more or less distorted tetrahedra build up the framework structure. The two TM sites are characterized by partial disorder (see Refinement) with (Fe/Co)1—O distances varying between 1.877 (2) and 1.900 (2) Å and (Co/Fe)2—O distances between 1.881 (2) and 1.927 (2) Å. The two PO₄ tetrahedra are more regular with the P—O bonds lengths between 1.5172 (19) and 1.5306 (19) Å for P1O₄ and 1.509 (2) and 1.533 (2) Å for P2O₄.

Figure 1
The principal building units in the crystal structure of β-KCoFe(PO₄)₂. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) x, y, z + 1; (ii) x, −y, z + $[{1\over 2}]$ ; (iii) −x + 1, y, −z + $[{1\over 2}]$ ; (iv) −x, y, −z + $[{1\over 2}]$ ; (v) −x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z; (vi) x, −y, z − $[{1\over 2}]$ ; (vii) −x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z + 1; (viii) −x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , −z + $[{1\over 2}]$ .]

The three different types of tetrahedra are linked through vertices to form ellipse-shaped rings with the sequence DDDDUUUU of up (U) and down (D) pointing vertices, as shown in Fig. 2. Each eight-membered ring is surrounded by four other rings of the same type, delimiting two interstices with rectangular shape constituted by two PO₄ and two (Fe/Co)1O₄ tetrahedra or two PO₄ and two (Co/Fe)2O₄ tetrahedra. This assembly leads to the formation of [(Co/Fe)(PO₄)]⁻_∞ sheets extending parallel to (001) at z = 0, ½. Stacking of these sheets along [001] leads to the formation of a three-dimensional framework structure with two types of channels. The first one is occupied by potassium cations, whereas the second one remains vacant, as shown in Fig. 3. The K⁺ cation is surrounded by nine oxygen atoms with bond lengths between 2.694 (2) and 3.172 (2) Å.

Figure 2
(a) A (001) layer at z ≃ 0 and (b) at z ≃ 0.5, resulting from vertex-sharing between TMO₄ and PO₄ tetrahedra. Rings formed by eight corner-sharing tetrahedra according to the sequence DDDDUUUU are shown on the left.

Figure 3
Perspective view of the crystal structure of β-KCoFe(PO₄)₂ approximately along [010], showing the channels in which the K⁺ cations are located.

Bond-valence sum (BVS) calculations (Brown, 1977 ,1978 ; Brown & Altermatt, 1985 ) and charge distribution (CHARDI) (Hoppe et al., 1989 ) were used to confirm the structure model of β-KCoFe(PO₄)₂. BVS and CHARDI computations were carried out with EXPO2014 (Altomare et al., 2013 ) and CHARDI2015 (Nespolo & Guillot, 2016 ), respectively. Table 1 compiles the valences V_(i) of cations determined with the BVS approach, as well as their corresponding charges Q_(i) calculated with the CHARDI concept. The data reveal that the values Q_(i) and V_(i) are all very close to the corresponding charges q_(i)×sof_(i) (formal oxidation numbers q_(i) weighted by site occupation factors (sof_(i)). For all cations, the internal criterion q_(i)/Q_(i) is very close to 1, and the mean absolute percentage deviation (MAPD) that evaluates the agreement between the q_(i) and Q_(i) charges is 0.3%, confirming the validity of the structural model (Eon & Nespolo, 2015 ). The global instability index (GII) was also used to check the plausibility of the crystal-structure model (Salinas-Sanchez et al., 1992 ). The GII index evaluates the deviation of BVS parameters from the theoretical valence V_(i) averaged across all the constitutive atoms of the asymmetric unit. In an unstrained structure, GII is less than 0.1 and reaches 0.2 for those with lattice-induced deformations (Adams et al., 2004 ). For the current crystal structure GII amounts to 0.1, indicating its stability.

Table 1
CHARDI and BVS analysis for the cations in the title compound

q_(i) = formal oxidation number; sof_(i) = site occupation factor; CN_(i) = classical coordination number; Q_(i) = calculated charge; V_(i) = calculated valence; ECoN_(i) = effective coordination number.

Cation	q_(i)×sof_(i)	CN_(i)	ECoN_(i)	V_(i)	Q_(i)	q_(i)/Q_(i)
(Fe/Co)1	2.57	4	4.00	2.48	2.57	1.00
(Fe/Co)2	2.43	4	3.99	2.27	2.43	1.00
K1	1.00	9	8.71	0.94	0.99	1.00
P1	5.00	4	4.00	5.14	5.00	1.00
P2	5.00	4	3.99	5.15	5.01	1.01

3. Database survey

The phosphate KCoFe(PO₄)₂ crystallizes in two polymorphs in the same crystal system but with different unit-cell parameters and space groups. The α-form of KCoFe(PO₄)₂ reported by Badri et al. (2019 ) crystallizes in space group P2₁/c with unit-cell parameters a = 5.148 (1), b = 14.403 (2), c = 9.256 (1) Å, β = 104.87 (2)°. The title compound crystallizes in space group C2/c. Whereas the environments around the two TM sites are tetrahedral in the title compound, an octahedral coordination is found for one site (Co) in the α-form. The crystal structure of β-KCoFe(PO₄)₂ is isotypic with that of KZnFePO₄)₂ (Badri et al., 2014 ), while that of α-KCoFe(PO₄)₂ is isotypic with those of KNiFe(PO₄)₂ and KMgFe(PO₄)₂ (Badri et al., 2015).

4. Synthesis and crystallization

The phosphate β-KCoFe(PO₄)₂ was synthesized by mixing cobalt nitrate (Co(NO₃)₂·6H₂O), iron nitrate [Fe(NO₃)₃·9H₂O] orthophosphoric acid (H₃PO₄) and potassium nitrate (KNO₃) in molar ratios of 1:1:1:2. The mixture was placed in a small beaker containing distilled water and homogenized for 24 h. After evaporation to dryness, the reaction mixture underwent heat treatments at 573 and 773 K before being brought to fusion for crystal growth at 1223 K, followed by slow cooling. Crystals of purple color and of sufficient size for the analysis by X-ray diffraction were obtained from the final product.

A Quattro ESEM scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), operating under 20 kV accelerating voltage, was used for chemical analysis and photographs of the obtained crystals (Fig. 4). Determined mass percentage (+/-3%), calculated mass percentage: K (10.7, 11.4) Fe (12.4, 16.2), Co (13.4, 17.1), P (20.2, 18.0), O (43.3, 37.3)

Figure 4
(a) EDS spectrum and (b) SEM micrographs of the title compound.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. During the refinement, several models were tested, with the best result for a model with occupational disorder of the two TM sites. Since the Co:Fe ratio determined from EDS measurements is almost 1:1, this ratio was constrained for the refinement of the individual site occupation, also taking into account full occupancy of both TM sites. For the TM1 site a ratio of Fe:Co = 0.5725:0.4275 was obtained, for the TM2 site a ratio of Co:Fe = 0.5725/0.4275. The maximum and minimum remaining electron density are located at 0.69 Å and 0.31 Å, respectively, from O8.

Table 2
Experimental details

Crystal data
Chemical formula	KCoFe(PO₄)₂
M_r	343.82
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	296
a, b, c (Å)	13.5860 (6), 13.2320 (6), 8.7316 (4)
β (°)	100.335 (2)
V (Å³)	1544.21 (12)
Z	8
Radiation type	Mo Kα
μ (mm⁻¹)	4.99
Crystal size (mm)	0.36 × 0.27 × 0.15

Data collection
Diffractometer	Bruker D8 VENTURE Super DUO
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.391, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	30042, 3574, 2633
R_int	0.068
(sin θ/λ)_max (Å⁻¹)	0.820

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.036, 0.088, 1.04
No. of reflections	3574
No. of parameters	118
Δρ_max, Δρ_min (e Å⁻³)	0.98, −0.91
Computer programs: APEX3 and SAINT (Bruker, 2016 ), SHELXT2014/7 (Sheldrick, 2015a ), SHELXL2018/3 (Sheldrick, 2015b ), ORTEP-3 for Windows (Farrugia, 2012 ), DIAMOND (Brandenburg, 2006 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2181684

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989022006521/wm5647sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989022006521/wm5647Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT2014/7 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Potassium cobalt(II) iron(III) bis(orthophosphate) top

Crystal data top

KCoFe(PO₄)₂	F(000) = 1328
M_r = 343.82	D_x = 2.958 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.5860 (6) Å	Cell parameters from 3574 reflections
b = 13.2320 (6) Å	θ = 2.2–35.6°
c = 8.7316 (4) Å	µ = 4.99 mm⁻¹
β = 100.335 (2)°	T = 296 K
V = 1544.21 (12) Å³	Parallelepiped, purple
Z = 8	0.36 × 0.27 × 0.15 mm

Data collection top

Bruker D8 VENTURE Super DUO diffractometer	3574 independent reflections
Radiation source: INCOATEC IµS micro-focus source	2633 reflections with I > 2σ(I)
HELIOS mirror optics monochromator	R_int = 0.068
Detector resolution: 10.4167 pixels mm^-1	θ_max = 35.6°, θ_min = 2.2°
φ and ω scans	h = −13→22
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −21→21
T_min = 0.391, T_max = 0.747	l = −14→14
30042 measured reflections

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F² > 2σ(F²)] = 0.036	Secondary atom site location: difference Fourier map
wR(F²) = 0.088	w = 1/[σ²(F_o²) + (0.0363P)² + 2.2954P] where P = (F_o² + 2F_c²)/3
S = 1.04	(Δ/σ)_max = 0.001
3574 reflections	Δρ_max = 0.98 e Å⁻³
118 parameters	Δρ_min = −0.91 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	U_iso*/U_eq	Occ. (<1)
Fe1	0.37263 (3)	0.06558 (3)	0.61452 (4)	0.01728 (8)	0.5725
Co1	0.37263 (3)	0.06558 (3)	0.61452 (4)	0.01728 (8)	0.4275
Co2	0.07555 (3)	0.11785 (3)	0.04344 (4)	0.01854 (8)	0.5725
Fe2	0.07555 (3)	0.11785 (3)	0.04344 (4)	0.01854 (8)	0.4275
P1	0.42702 (5)	0.14198 (5)	−0.01872 (7)	0.01656 (12)
P2	0.14880 (5)	0.06783 (5)	0.41434 (7)	0.01769 (12)
K1	0.31255 (6)	0.25345 (6)	0.27514 (8)	0.03896 (17)
O1	0.39529 (17)	0.07417 (15)	0.1059 (2)	0.0288 (4)
O2	0.54020 (16)	0.13970 (16)	−0.0087 (2)	0.0313 (4)
O3	0.39339 (19)	0.24769 (14)	0.0152 (3)	0.0338 (5)
O4	0.37488 (19)	0.1089 (2)	−0.1801 (2)	0.0411 (6)
O5	0.14870 (17)	−0.04718 (15)	0.4068 (2)	0.0312 (4)
O6	0.1372 (2)	0.11411 (18)	0.2542 (2)	0.0431 (6)
O7	0.24615 (14)	0.11037 (15)	0.5076 (2)	0.0262 (4)
O8	0.06510 (17)	0.1025 (2)	0.4989 (3)	0.0475 (7)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Fe1	0.01435 (16)	0.02100 (16)	0.01659 (14)	0.00191 (12)	0.00307 (11)	−0.00249 (11)
Co1	0.01435 (16)	0.02100 (16)	0.01659 (14)	0.00191 (12)	0.00307 (11)	−0.00249 (11)
Co2	0.01525 (16)	0.02218 (16)	0.01902 (15)	0.00088 (12)	0.00533 (11)	0.00266 (11)
Fe2	0.01525 (16)	0.02218 (16)	0.01902 (15)	0.00088 (12)	0.00533 (11)	0.00266 (11)
P1	0.0185 (3)	0.0157 (2)	0.0169 (2)	0.0005 (2)	0.0071 (2)	−0.00144 (19)
P2	0.0128 (3)	0.0229 (3)	0.0171 (2)	−0.0018 (2)	0.0021 (2)	0.0034 (2)
K1	0.0399 (4)	0.0476 (4)	0.0342 (3)	−0.0028 (3)	0.0197 (3)	−0.0083 (3)
O1	0.0370 (12)	0.0246 (9)	0.0273 (9)	−0.0062 (8)	0.0130 (8)	0.0033 (7)
O2	0.0196 (10)	0.0354 (11)	0.0416 (11)	0.0015 (8)	0.0129 (8)	−0.0039 (9)
O3	0.0478 (14)	0.0212 (9)	0.0388 (11)	0.0109 (8)	0.0254 (10)	0.0033 (8)
O4	0.0407 (14)	0.0625 (16)	0.0201 (9)	−0.0067 (11)	0.0057 (9)	−0.0133 (9)
O5	0.0352 (12)	0.0231 (9)	0.0373 (11)	−0.0083 (8)	0.0116 (9)	0.0011 (8)
O6	0.0560 (16)	0.0449 (13)	0.0247 (10)	−0.0090 (11)	−0.0027 (10)	0.0135 (9)
O7	0.0160 (9)	0.0289 (10)	0.0314 (10)	−0.0003 (7)	−0.0019 (7)	−0.0038 (7)
O8	0.0173 (11)	0.0795 (19)	0.0482 (14)	0.0017 (11)	0.0124 (10)	−0.0163 (13)

Geometric parameters (Å, º) top

Fe1/Co1—O4ⁱ	1.877 (2)	P2—O5	1.523 (2)
Fe1/Co1—O1ⁱⁱ	1.8783 (19)	P2—O7	1.5308 (19)
Fe1/Co1—O7	1.8972 (19)	P2—O8	1.533 (2)
Fe1/Co1—O2ⁱⁱⁱ	1.900 (2)	K1—O3	2.694 (2)
Co2/Fe2—O6	1.881 (2)	K1—O7^vii	2.832 (2)
Co2/Fe2—O8^iv	1.891 (2)	K1—O6	2.991 (3)
Co2/Fe2—O3^v	1.9191 (19)	K1—O2ⁱⁱⁱ	2.994 (2)
Co2/Fe2—O5^vi	1.927 (2)	K1—O8^vii	3.019 (3)
P1—O3	1.5172 (19)	K1—O7	3.029 (2)
P1—O4	1.524 (2)	K1—O1	3.110 (2)
P1—O2	1.525 (2)	K1—O4^v	3.120 (3)
P1—O1	1.5306 (19)	K1—O5^viii	3.172 (2)
P2—O6	1.509 (2)

O4ⁱ—Fe1/Co1—O1ⁱⁱ	111.34 (10)	O7^vii—K1—O7	78.19 (6)
O4ⁱ—Fe1/Co1—O7	103.45 (10)	O6—K1—O7	47.65 (5)
O1ⁱⁱ—Fe1/Co1—O7	115.36 (9)	O2ⁱⁱⁱ—K1—O7	58.15 (5)
O4ⁱ—Fe1/Co1—O2ⁱⁱⁱ	113.73 (10)	O8^vii—K1—O7	98.73 (7)
O1ⁱⁱ—Fe1/Co1—O2ⁱⁱⁱ	111.57 (9)	O3—K1—O1	48.80 (5)
O7—Fe1/Co1—O2ⁱⁱⁱ	100.83 (9)	O7^vii—K1—O1	166.58 (6)
O6—Co2/Fe2—O8^iv	116.47 (11)	O6—K1—O1	81.51 (7)
O6—Co2/Fe2—O3^v	101.81 (11)	O2ⁱⁱⁱ—K1—O1	71.69 (6)
O8^iv—Co2/Fe2—O3^v	108.09 (12)	O8^vii—K1—O1	126.06 (7)
O6—Co2/Fe2—O5^vi	113.85 (11)	O7—K1—O1	91.04 (5)
O8^iv—Co2/Fe2—O5^vi	116.25 (11)	O3—K1—O4^v	103.24 (7)
O3^v—Co2/Fe2—O5^vi	97.02 (9)	O7^vii—K1—O4^v	59.48 (5)
O3—P1—O4	109.84 (14)	O6—K1—O4^v	74.98 (7)
O3—P1—O2	110.05 (13)	O2ⁱⁱⁱ—K1—O4^v	152.69 (6)
O4—P1—O2	110.14 (13)	O8^vii—K1—O4^v	97.61 (7)
O3—P1—O1	105.59 (12)	O7—K1—O4^v	102.49 (6)
O4—P1—O1	110.20 (13)	O1—K1—O4^v	131.79 (6)
O2—P1—O1	110.93 (12)	O3—K1—O5^viii	58.15 (5)
O6—P2—O5	111.47 (13)	O7^vii—K1—O5^viii	84.15 (6)
O6—P2—O7	106.28 (13)	O6—K1—O5^viii	133.14 (6)
O5—P2—O7	112.60 (12)	O2ⁱⁱⁱ—K1—O5^viii	127.49 (6)
O6—P2—O8	111.23 (16)	O8^vii—K1—O5^viii	71.38 (7)
O5—P2—O8	108.96 (14)	O7—K1—O5^viii	162.10 (6)
O7—P2—O8	106.18 (13)	O1—K1—O5^viii	106.85 (5)
O3—K1—O7^vii	142.04 (6)	O4^v—K1—O5^viii	65.29 (6)
O3—K1—O6	111.80 (7)	O3—K1—O3^v	77.20 (8)
O7^vii—K1—O6	96.69 (7)	O7^vii—K1—O3^v	102.00 (5)
O3—K1—O2ⁱⁱⁱ	103.65 (7)	O6—K1—O3^v	54.25 (5)
O7^vii—K1—O2ⁱⁱⁱ	95.61 (6)	O2ⁱⁱⁱ—K1—O3^v	149.32 (6)
O6—K1—O2ⁱⁱⁱ	99.16 (6)	O8^vii—K1—O3^v	140.19 (7)
O3—K1—O8^vii	107.98 (8)	O7—K1—O3^v	101.04 (5)
O7^vii—K1—O8^vii	49.37 (6)	O1—K1—O3^v	87.81 (5)
O6—K1—O8^vii	140.19 (7)	O4^v—K1—O3^v	44.42 (5)
O2ⁱⁱⁱ—K1—O8^vii	69.51 (7)	O5^viii—K1—O3^v	79.62 (6)
O3—K1—O7	139.67 (6)

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

CHARDI and BVS analysis for the cations in the title compound top

q₍_i₎ = formal oxidation number; sof₍_i₎ = site occupation factor; CN₍_i₎ = classical coordination number; Q₍_i₎ = calculated charge; V₍_i₎ = calculated valence; ECoN₍_i₎ = effective coordination number.

Cation	q₍_i₎×sof₍_i₎	CN₍_i₎	ECoN₍_i₎	V₍_i₎	Q₍_i₎	q₍_i₎/Q₍_i₎
(Fe/Co)1	2.57	4	4.00	2.48	2.57	1.00
(Fe/Co)2	2.43	4	3.99	2.27	2.43	1.00
K1	1.00	9	8.71	0.94	0.99	1.00
P1	5.00	4	4.00	5.14	5.00	1.00
P2	5.00	4	3.99	5.15	5.01	1.01

Acknowledgements

The authors thank the Faculty of Science, Mohammed V University in Rabat, Morocco for the X-ray measurements and the Unit of Support for Technical and Scientific Research (UATRS, CNRST) for the SEM and EDX analysis.

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