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

Single crystals of β-KCoFe(PO4)2, potassium cobalt(II) iron(III) bis-(ortho-phosphate), 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 tetra-hedra 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 inter-stices 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. © Bouraima et al. 2022.

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 d 10 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 = 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.

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 +  ; (iii) −x + 1, y, −z +  ; (iv) −x, y, −z +  ; (v) −x +  , −y +  , −z; (vi) x, −y, z −  ; (vii) −x +  , −y +  , −z + 1; (viii) −x +  , y +  , −z +  .]

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 ( of cations determined with the BVS approach, as well as their corres­ponding charges Q ( calculated with the CHARDI concept. The data reveal that the values Q ( and V ( are all very close to the corresponding charges q (×sof( (formal oxidation numbers q ( weighted by site occupation factors (sof(). For all cations, the inter­nal criterion q (/Q ( is very close to 1, and the mean absolute percentage deviation (MAPD) that evaluates the agreement between the q ( and Q ( 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 ( 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 ( = formal oxidation number; sof( = site occupation factor; CN( = classical coordination number; Q ( = calculated charge; V ( = calculated valence; ECoN( = 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

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 ▸).

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.

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[F 2 > 2σ(F 2)], wR(F 2), S	0.036, 0.088, 1.04
No. of reflections	3574
No. of parameters	118
Δρmax, Δρmin (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 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989022006521/wm5647sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989022006521/wm5647Isup2.hkl

CCDC reference: 2181684

Additional supporting information: crystallographic information; 3D view; checkCIF report

KCoFe(PO4)2	F(000) = 1328
Mr = 343.82	Dx = 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

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	Rint = 0.068
Detector resolution: 10.4167 pixels mm-1	θmax = 35.6°, θmin = 2.2°
φ and ω scans	h = −13→22
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −21→21
Tmin = 0.391, Tmax = 0.747	l = −14→14
30042 measured reflections

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(Fo2) + (0.0363P)2 + 2.2954P] where P = (Fo2 + 2Fc2)/3
S = 1.04	(Δ/σ)max = 0.001
3574 reflections	Δρmax = 0.98 e Å−3
118 parameters	Δρmin = −0.91 e Å−3

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.

	x	y	z	Uiso*/Ueq	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)

	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)

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—O7vii	2.832 (2)
Co2/Fe2—O8iv	1.891 (2)	K1—O6	2.991 (3)
Co2/Fe2—O3v	1.9191 (19)	K1—O2iii	2.994 (2)
Co2/Fe2—O5vi	1.927 (2)	K1—O8vii	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—O4v	3.120 (3)
P1—O1	1.5306 (19)	K1—O5viii	3.172 (2)
P2—O6	1.509 (2)
O4i—Fe1/Co1—O1ii	111.34 (10)	O7vii—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)	O8vii—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)	O7vii—K1—O1	166.58 (6)
O6—Co2/Fe2—O8iv	116.47 (11)	O6—K1—O1	81.51 (7)
O6—Co2/Fe2—O3v	101.81 (11)	O2iii—K1—O1	71.69 (6)
O8iv—Co2/Fe2—O3v	108.09 (12)	O8vii—K1—O1	126.06 (7)
O6—Co2/Fe2—O5vi	113.85 (11)	O7—K1—O1	91.04 (5)
O8iv—Co2/Fe2—O5vi	116.25 (11)	O3—K1—O4v	103.24 (7)
O3v—Co2/Fe2—O5vi	97.02 (9)	O7vii—K1—O4v	59.48 (5)
O3—P1—O4	109.84 (14)	O6—K1—O4v	74.98 (7)
O3—P1—O2	110.05 (13)	O2iii—K1—O4v	152.69 (6)
O4—P1—O2	110.14 (13)	O8vii—K1—O4v	97.61 (7)
O3—P1—O1	105.59 (12)	O7—K1—O4v	102.49 (6)
O4—P1—O1	110.20 (13)	O1—K1—O4v	131.79 (6)
O2—P1—O1	110.93 (12)	O3—K1—O5viii	58.15 (5)
O6—P2—O5	111.47 (13)	O7vii—K1—O5viii	84.15 (6)
O6—P2—O7	106.28 (13)	O6—K1—O5viii	133.14 (6)
O5—P2—O7	112.60 (12)	O2iii—K1—O5viii	127.49 (6)
O6—P2—O8	111.23 (16)	O8vii—K1—O5viii	71.38 (7)
O5—P2—O8	108.96 (14)	O7—K1—O5viii	162.10 (6)
O7—P2—O8	106.18 (13)	O1—K1—O5viii	106.85 (5)
O3—K1—O7vii	142.04 (6)	O4v—K1—O5viii	65.29 (6)
O3—K1—O6	111.80 (7)	O3—K1—O3v	77.20 (8)
O7vii—K1—O6	96.69 (7)	O7vii—K1—O3v	102.00 (5)
O3—K1—O2iii	103.65 (7)	O6—K1—O3v	54.25 (5)
O7vii—K1—O2iii	95.61 (6)	O2iii—K1—O3v	149.32 (6)
O6—K1—O2iii	99.16 (6)	O8vii—K1—O3v	140.19 (7)
O3—K1—O8vii	107.98 (8)	O7—K1—O3v	101.04 (5)
O7vii—K1—O8vii	49.37 (6)	O1—K1—O3v	87.81 (5)
O6—K1—O8vii	140.19 (7)	O4v—K1—O3v	44.42 (5)
O2iii—K1—O8vii	69.51 (7)	O5viii—K1—O3v	79.62 (6)
O3—K1—O7	139.67 (6)