Determination of the modulated structure of Sr14/11CoO3 through a (3+1)-dimensional space description and using non-harmonic ADPs

Sr14/11CoO3 (i.e. Sr14Co11O33, tetradecastrontium undecacobalt tritriaconta oxide), a new phase in the hexagonal perovskite SrxCoO3 system, has been pr epared and its structure solved from single-crystal X-ray data within the ( 3 + 1)-dimensional formalism. Sr14/11CoO3 crystallizes in the trigonal symm etry, R (3) over bar m(00 gamma)0s superspace group with the following latt ice parameters: a(s) = 9.508(2), c(s) = 2.5343(7) Angstrom, q = 0.63646(11) c* and V-s = 198.40(13) Angstrom(3). With the commensurate versus incommens urate test not being conclusive, the structure was considered as commensura te (P32 three-dimensional space group), but refined within the (3 + 1)-dime nsional formalism to a residual factor R = 0.0351 for 47 parameters and 116 9 independent reflections. Crenel functions were used for the oxygen and co balt 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 Sr6/5CoO3, but With a different sequence of the octah edra and trigonal prism polyhedra along the [CoO3] chains. An interesting f eature evidenced by the non-harmonic expansion is the displacement of the p rismatic Co atoms from the site center, towards the prism rectangular faces .