Crystal structure of Ba2Co(BO3)2

Ba2Co(BO3)2 crystallizes in a novel structure type, with [CoO5] square pyramids linked to BO3 triangles through corner- or edge-sharing, and the Ba2+ cations in the voids of this arrangement.


Chemical context
The crystal chemistry of borates differs from those of silicates, phosphates, sulfates, carbonates or nitrates due to the possibility of forming borate anions with trigonal-planar and tetrahedral configurations (Zhang et al., 2011a;Filatov & Bubnova, 2000;Chen et al., 2005;Reshak, 2016). In general, borate compounds are applied in different fields, such as nonlinear optical (NLO) materials (Becker, 1998), for photoluminescence (Mergen & Pekgö zlü , 2013), for their optical properties (Zhang et al., 2011b;Lv et al., 2018), or as ferroelectrics (Dhanasekaran, 2009;Murugan et al., 2001). The borate systems A 2 M(BO 3 ) 2 , where A = Ba, Sr, Pb and M = Cu, Mg, Cd, Ca, Zn, have been studied previously. For these compounds, several structures types have been reported that depend on the size and nature of the A and M atoms, as shown in Table 1.
In this investigation we have isolated single crystals of Ba 2 Co(BO 3 ) 2 from the melt. The new compound crystallizes in the monoclinic system in the same space-group type as some other A 2 M(BO 3 ) 2 compounds, but with different cell parameters (Table 1).

Structural commentary
In the crystal structure of the title compound, except for the two oxygen atoms O1 and O3 that lie in general positions of the C2/m space group, all atoms are located on a mirror plane (Wyckoff position 4i).
The principal building units in the crystal structure are two trigonal-planar borate anions, one five-coordinate Co 2+ cation and two nine-coordinate Ba 2+ cations (Fig. 1). Relevant bond lengths and angles are collated in

Figure 2
Edge-and corner-sharing [

Comparison with related structures
Comparison of the crystal structure of the title compound with those of other orthoborates with formula type A 2 M(BO 3 ) 2 listed in Table 1 reveals that the first three compounds crystallize in the monoclinic system with the same space group (C2/m) but a halved unit-cell volume. -Sr 2 Cu(BO 3 ) 2 and Pb 2 Cu(BO 3 ) 2 also crystallize in the monoclinic system but in space group P2 1 /c. The remaining compounds adopt an orthorhombic structure, except for the last, Ba 2 Mg(BO 3 ) 2 , which is hexagonal. In the crystal structures of all these borates, the small metal M has either a coordination number of four (CuO 4 , ZnO 4 ) or six (CuO 6 , MgO 6 , CaO 6 , CdO 6 ). Accordingly, it is important to note the originality of the title structure with its five-coordination of the cobalt cation instead of four-or six-coordination for M in the other

Figure 5
Projection of the crystal structure of Ba 2 Co(BO 3 ) 2 approximately along [010], emphasizing the voids between the cobalt borate rows in which the barium cations are located.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The maximum and minimum remaining electron density peaks are at 0.47 Å from Ba1 and 1.08 Å from Co1, respectively.

Crystal data
Ba 2  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.