Re-investigation and correct symmetry of Ca3CoAl4O10

The crystal structure of Ca3CoAl4O10 was redetermined from single-crystal X-ray data and is isotypic with Ca3MgAl4O10.


Chemical context
In a recent paper on the phase relationships in the system CaO-MgO-Al 2 O 3 , we reported the existence and the crystal structure of Ca 3 MgAl 4 O 10 (Kahlenberg et al., 2018), a phase of interest for slags occurring in secondary refining processes in metallurgy or refractories, for example. In the course of this study it became obvious that the compound is closely related to the corresponding Zn and Co analogues that have already been reported in the literature (Barbanyagre et al., 1997;Vazquez et al., 2002). In fact Vazquez et al. (2002) used the coordinates from the Zn compound as a starting model for their Rietveld refinement of Ca 3 CoAl 4 O 10 . The major difference from our investigation on Ca 3 MgAl 4 O 10 results from the fact that the previous study attributed Ca 3 CoAl 4 O 10 to the acentric space group Pbc2 1 , while Ca 3 MgAl 4 O 10 crystallizes in the centrosymmetric space group Pbcm. However, for the former compound the description in an acentric space group has to be scrutinized. A detailed analysis of the atomic coordinates using the program PSEUDO (Kroumova et al., 2001) indicated that the published model fulfills the symmetry requirements of Pbcm. Notably, Vazquez et al. (2002) reported problems during their structure analysis of Ca 3 CoAl 4 O 10 , including unstable refinements and unrealistically short cation-oxygen distances. Both observations are typical features when a structure is refined in an unnecessarily low space-group symmetry (Baur & Tillmanns, 1986). Therefore, it was deemed appropriate to re-investigate the crystal structure of Ca 3 CoAl 4 O 10 using single-crystal X-ray diffraction data obtained from melt-grown crystals.

Structural commentary
The crystal structure of Ca 3 CoAl 4 O 10 can be described as a three-dimensional network with four symmetrically different corner-sharing [(Al,Co)O 4 ] tetrahedra around the central atoms T1-T4 (Fig. 1). The basic building units of the structure are chains of tetrahedra running parallel to [001]. Using the crystal chemical classification developed by Liebau (1985), these linear elements can be described as mixed-branched vierer single chains (Fig. 2). Condensation of adjacent chains along [010] results in the formation of stepped layers parallel to (100) (Fig. 3). Within these layers, channels can be identified which host the additional calcium ions.
Site-occupancy refinements indicated that cobalt incorporation is limited to two of the four T sites within the asymmetric unit (T1 and T2  (Shannon, 1976), and can be used as an indication that T1 and T2 have higher Co contents. This observation compares well with the site-population refinements. Quadratic elongations as defined by Robinson et al. (1971), which can be used as numerical descriptors for the distortions, take the following values for the individual [(Al,Co)O 4 ]-groups: T1: 1.015, T2: 1.006, T3: 1.016, T4: 1.001.
Among the extra-framework cations, two crystallographically independent calcium sites (Ca1, Ca2) can be distinguished. They are coordinated by six and eight nearest oxygen neighbours. Their coordination polyhedra can be described as distorted octahedra and square antiprisms, respectively. Each two [Ca1O 6 ] octahedra and a single [Ca2O 8 ] square antiprism form a polyhedral unit by sharing edges.
A detailed analysis of the topological features of the tetrahedral network including coordination sequences and extended point symbols has been already presented for isotypic Ca 3 MgAl 4 O 10 (Kahlenberg et al., 2018) and will not be duplicated here. However, it is interesting to note that the framework consists of three (T3), four (T4) and five (T1, T2)connected tetrahedra. Notably, the net contains an O [3] -type bridging oxygen (O3), simultaneously linking three tetrahedra      ) oxygen atoms have been observed so far. In the present structure, the oxygen atoms O1 and O2, O4, O5, O6, O7 belong to these two groups. Notably, O3 is solely involved in O-T bonds with the two tetrahedra showing an Al/Co substitution.

Database survey
As mentioned above, the title compound is isotypic with The distribution of the cobalt and aluminium ions on the different T sites is another difference between the new centrosymmetric model in Pbcm (this work) and the previous acentric model in Pbc2 1 (Vazquez et al., 2002). Actually, in the latter case five different tetrahedral positions have to be distinguished. The authors considered four of them to be exclusively occupied with Al while the remaining fifth position was attributed to be a pure cobalt site. This distribution, however, was derived from the crystal-structure refinement of the zinc analog (Barbanyagre et al., 1997) and not determined by site-occupancy refinements.
Furthermore, the new model in Pbcm results in considerably less distorted tetrahedra. Although soft constraints on the Al-O and Co-O bond lengths had been applied, individual T-O distances and O-T-O angles in the Pbc2 1 structure model showed a pronounced variation between 1.68 and 2.05 Å and 92.9 and 124.6 , respectively. The corresponding values in the present model are in the ranges from 1.719 (4) to 1.847 (2) Å and from 98.95 (16) to 120.38 (18) , respectively. Finally, the displacement parameters in Pbcm are all well behaved, while the overall isotropic temperature factor for the oxygen atoms reported in the study of Vazquez et al. (2002) takes a physically unrealistic value of U iso = 0.001 (2) Å 2 .

Synthesis and crystallization
Single crystals of Ca 3 CoAl 4 O 10 were obtained during a series of synthesis experiments in the system CaO-CoO-Al 2 O 3 .
1.35 g of the educts consisting of CaCO 3 , CoO and Al 2 O 3 in the molar ratio 14:6:5 were homogenized in an agate mortar, transferred into a platinum crucible and covered with a lid. The container was fired in a resistance-heated furnace from 590 to 1623 K with a ramp of 100 K h À1 . The target temperature was held for 1 h. Subsequently, the sample was cooled down to 1273 K at a rate of 7.5 K h À1 and, finally, the temperature was reduced to 473 K at a rate of 100 K h À1 . After removal of the crucible, the solidified melt cake was immediately crushed in an agate mortar and transferred to a glass slide under a polarizing binocular. A first inspection revealed the presence of two crystalline phases: larger colourless optically isotropic crystals of Ca 3 Al 2 O 6 (up to 500 mm in size) and considerably smaller, intensively blue birefringent crystals of Ca 3 CoAl 4 O 10 . A platy fragment of the latter compound showing sharp extinction under crossed polarizers was selected for further structural studies and mounted on the tip of a glass fibre using fingernail hardener as glue.
coordinates were taken from the crystal structure of Ca 3 Al 4 MgO 10 (Kahlenberg et al., 2018). Initially, mixed cobalt-aluminium populations were considered for all four T sites. However, the resulting values of the site occupancies for T3 and T4 indicated pure Al populations (within two standard uncertainties each). In the final cycles a restraint was introduced, fixing the total amount of cobalt distributed among the remaining T1 and T2 sites to four atoms per unit cell.

Crystal data
Ca 3

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.