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

The crystal structure of β-KCoFe(PO4)2 is isotypic with that of KZnFe(PO4)2.


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 Zn 2+ 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 PO 4 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(PO 4 ) 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, Na 2 Co 2 Fe(PO 4 ) 3 (Bouraima et al., 2015) and NaCuIn(PO 4 ) 2 (Benhsina et al., 2020) are among the recently studied compounds. The present work is devoted to synthesis and crystal structure analysis of -KCoFe(PO 4 ) 2 , a new compound in the family of transitionmetal phosphates.
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 PO 4 and two (Fe/ Co)1O 4 tetrahedra or two PO 4 and two (Co/Fe)2O 4 tetrahedra. This assembly leads to the formation of [(Co/ Fe)(PO 4 )] À 1 sheets extending parallel to (001) at z = 0, 1 2 . 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 .
Cation 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) Âsof (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.

Database survey
The phosphate KCoFe(PO 4 ) 2 crystallizes in two polymorphs in the same crystal system but with different unit-cell parameters and space groups. The

Synthesis and crystallization
The phosphate -KCoFe(PO 4 ) 2 was synthesized by mixing cobalt nitrate (Co(NO 3 ) 2 Á6H 2 O), iron nitrate [Fe(NO 3 ) 3 Á-9H 2 O] orthophosphoric acid (H 3 PO 4 ) and potassium nitrate (KNO 3 ) 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)

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.  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 data
KCoFe ( Special details 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 (Å 2 )