Acta Crystallographica Section B

Structural Science

Volume 55, Part 6 (December 1999)

research papers

Acta Cryst. (1999). B55, 841-848 [ doi:10.1107/S0108768199006485 ]

Determination of the modulated structure of Sr_14/11CoO₃ through a (3 + 1)-dimensional space description and using non-harmonic ADPs

O. Gourdon, V. Petricek, M. Dusek, P. Bezdicka, S. Durovic, D. Gyepesova and M. Evain

Abstract: Sr_14/11CoO₃ (i.e. Sr₁₄Co₁₁O₃₃, tetradecastrontium undecacobalt tritriacontaoxide), a new phase in the hexagonal perovskite Sr_xCoO₃ system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional formalism. Sr_14/11CoO₃ crystallizes in the trigonal symmetry, R3m(00 [gamma] )0s superspace group with the following lattice parameters: a_s = 9.508 (2), c_s = 2.5343 (7) Å, q = 0.63646 (11)c^* and V_s = 198.40 (13) Å³. With the commensurate versus incommensurate test not being conclusive, the structure was considered as commensurate (P32 three-dimensional space group), but refined within the (3 + 1)-dimensional formalism to a residual factor R = 0.0351 for 47 parameters and 1169 independent reflections. Crenel functions were used for the oxygen and cobalt description and a Gram-Charlier expansion up to the third order of the atomic displacement parameter was employed for one Co atom. The structure is similar to that of Sr_6/5CoO₃, but with a different sequence of the octahedra and trigonal prism polyhedra along the [CoO₃] chains. An interesting feature evidenced by the non-harmonic expansion is the displacement of the prismatic Co atoms from the site center, towards the prism rectangular faces.

Keywords: Hexagonal perovskite-type structure; X-ray structure determination; Incommensurate composite compound; Crenel functions; Non-harmonic ADP.