Crystal structure of SrCo4(OH)(PO4)3, a new hydroxyphosphate

Three [CoO6] octahedra, one [CoO4] tetrahedron and three PO4 tetrahedra are linked into a three-dimensional framework structure exhibiting channels parallel to [100] in which the eleven-coordinate strontium cations are located.


Chemical context
The search for new inorganic materials with open-frame structures comprising transition-metal polyhedra, [MO x ], with tetrahedral phosphate or vanadate units by sharing corners or edges is still ongoing (Rghioui et al., 2019;Ouaatta et al., 2019;Khmiyas et al., 2020). Generally, these interconnections can lead to structures with cages, interlayer spaces or channels, and the corresponding compounds are explored extensively for their excellent physical properties and various applications in electrical, electrochemical, magnetic or catalytic processes (Goodenough et al., 1976;Borel et al., 1991;La Parola et al., 2018;Hadouchi et al., 2019). The introduction of borate groups (BO 3 or BO 4 ) to phosphate (PO 4 ) units leads to a group of borophosphates with specific structural characteristics. Compounds of this family likewise exhibit remarkable physicochemical properties that allow them to be applied in different fields (Kniep et al., 1998;Ewald et al., 2007;Lin et al., 2008;Menezes et al., 2008). About a decade ago, we managed to synthesize two borophosphate phases, viz.  (Zouihri et al., 2011a,b). In this context, we attempted to synthesize a strontium-and cobalt-based borophosphate, namely SrCo 2 BPO 7 , by means of the hydrothermal process. Instead, we have isolated a new hydroxyphosphate, SrCo 4 (OH)(PO 4 ) 3 , and report here its crystal structure and its infrared spectrum.

Structural commentary
In the three-dimensional framework structure of SrCo 4 (OH)(PO 4 ) 3 , an octahedral coordination of three cobalt ISSN 2056-9890 atoms (Co1, Co2, Co3) and a tetrahedral coordination of the fourth cobalt (Co4) is observed. Atom O13 bears a hydrogen atom and bridges two of the six-coordinate Co atoms (Co1, Co2) and the four-coordinate Co4 atom. The hydroxide group also forms a weak bifurcated hydrogen bond (Table 1) to two phosphate tetrahedra (Fig. 1).
The [CoO 6 ] octahedra share edges to form infinite undulating chains extending parallel to [001]. Adjacent chains are cross-linked via common vertex atoms (O3) to build up (010) layers ( Fig. 2) with the formation of oval voids surrounded by eight octahedra. Two PO 4 tetrahedra occupy the void space, whereby P1O 4 shares three of its vertices with five [CoO 6 ] octahedra and P2O 4 shares an edge with an octahedron and a vertex with two opposite octahedra (Fig. 3

Figure 2
[CoO 6 ] octahedra sharing edges to form chains that are linked together via a common corner every three octahedra.

Figure 5
The crystal structure of SrCo 4 (OH)(PO 4 ) 3 in a projection along [100], showing channels in which the strontium cations are located.

Figure 4
Ribbons formed by [CoO 4 ] and P2O 4 and P3O 4 tetrahedra, and strontium atoms and P1O 4 tetrahedra at the same height forming the second layer parallel to (010).
The crystal structure can be described by the stacking of the two types of layers along [010], which leads to the formation of channels extending parallel to [100] in which the strontium cations are located (Fig. 5). Each Sr II atom is surrounded by eleven oxygen atoms, forming a distorted polyhedron. Comparison of the metal-oxygen polyhedra in the title structure with the same type of polyhedra in comparable structures shows a similar behaviour. All [CoO 6 ] octahedra in SrCo 4 (OH)(PO 4 ) 3 are distorted, with the Co-O distance varying between 2.022 (2) and 2.284 (2) (Feng et al., 1997). The PO 4 tetrahedra in the title structure have averaged distances of 1.542 Å for P1, 1.539 Å for P2 and 1.539Å for P3, and are compatible with the P-O distances in the orthophosphate SrCo 2 Fe(PO 4 ) 3 (Bouraima et al., 2016).
The structure model of SrCo 4 (OH)(PO 4 ) 3 is in good agreement with calculations of the bond-valence sums (Brown & Altermatt, 1985

Infrared spectroscopy
An infrared spectrum of SrCo 4 (OH)(PO 4 ) 3 was recorded in order to verify the existence of the hydroxyl and PO 4 groups in the title compound (Fig. 6). The FT-IR spectrum shows characteristic vibration bands of isolated PO 4 groups. The bands observed at around 420 and 463 cm À1 can be assigned to the 2 asymmetric stretching mode while the vibration at 573 cm À1 is attributed to 4 asymmetric O-P-O deformation. The weak band observed at 750 cm À1 most likely originates from [4] Co-O vibrations, as observed in many other phosphates (Rusakov et al., 2006;Antony et al., 2011;Bushiri et al., 2002;De Pedro et al., 2010). The vibration at 1014 cm À1 corresponds to the 3 asymmetric stretching mode of the phosphate tetrahedra. The remaining vibrations centred at 3566, 3433 and 1632 cm À1 are commonly assigned to the stretching vibration of the bridging -OH group, as in Co 2 PO 4 OH (Wang et al., 2014), in addition to the OH À librational mode, which is observed at 637 cm À1 . We also note the presence of bands at 1384 and 875 cm À1 , indicating C-O bonds (Ribeiro et al., 2006). This observation suggests that the powdered sample contained impurities of a carbonate. The assignments of all vibration bands are summarized in Table 2.
was collected, filtered, rinsed with distilled water and dried in air. Optical microscopy revealed two types of crystals, viz. dark-purple and dark-red rectangular crystals. X-ray diffraction analysis showed the red crystals to be Co 2 (OH)PO 4 (Harrison et al., 1995). The purple parallelepipeds correspond to the title compound. Infrared spectroscopic measurements were performed on a VERTEX 70 FT-IR spectrometer, using the MRI transmission technique using KBr pellets. An adequate quantity of the studied phosphate powder, obtained by grinding the SrCo 4 (OH)(PO 4 ) 3 crystals, was diluted in KBr before being pressed into a pellet. The analysis was performed at room temperature, and the spectrum was recorded in the range 4000-400 cm À1 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atom of the OH group was located in a difference-Fourier map and was refined was a fixed O-H bond length of 0.82 Å and U iso (H) = 1.5U eq (O). The maximum and minimum remaining electron density was located at 0.69 Å from Sr1 and 0.56 Å from Co4, respectively. The reflection (011) was affected by the beamstop (F o 2 = 0) while reflections (052) and (053), having F o 2 > F c 2 , were probably affected by the Renninger effect. All three reflections were omitted from the refinement.

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.