Synthesis, structure determination, and twinning of two new composite compounds in the hexagonal perovskite-like sulfide family: Eu8/7TiS3 and Sr8/7TiS3

Eu8/7TiS3 and Sr8,7TiS3, two members in the hexagonal perovskite-like A(x)T iS(3) family, have been obtained as single crystals and their structure sol ved from twinned crystal X-ray data within the (3 + 1)-dimensional [(3 + 1) D] formalism. Eu8/7TiS3 and Sr8/7TiS3 crystallizes in the trigonal symmetr y, R (3) over capm(OOy) Os superspace group with the following lattice para meters: Eu8/7TiS3. a(s) = 11.3892(13) A. c(s) = 2.989(2) A, q = 0.5741(4) a nd V-s = 335.8(3) A(3); Sr8/7TiS3, a(s) = 11.480(5) A, c(s) 2.9940(13) A, q = 0.5716(2) c* and V-s = 341.7(5) A(3). The structures were considered as commensurate (P (3) over cap three-dimensional (3D) spacegroup) but refined within the (3 + 1) D formalism (t = 0 section) to a residual factor: Eu8/7 TiS3, R = 3.84% for 128 parameters and 4500 independent reflections: Sr8/7T iS3, R = 4.92% for 125 parameters and 2719 independent reflections. The two structures are isotypic and resemble that of Sr9/8TiS3. They differ from t he structures of the oxide counterparts by the presence of original PO [TiS 6] units, intermediate between octahedra (Oh) and trigonal prisms (TP), and replacing the TP units found in the oxides. In addition, they are differen t from the Sr9/8TiS3 structure by the occurrence of a particular twinning a nd the presence of two different [TiS3] chain types. with two consecutive P O units in one of them.