Polar titanium polychalcogenides: Syntheses and characterization of Cs₆Ti₆S₂₇, Cs₄BaTi₆Se₂₇, Rb₅AgTi₆Se₂₇, and Cs₅AgTi₆Se₂₇

The compounds Cs₆Ti₆S₂₇ and Cs₄BaTi₆Se₂₇ have been synthesized by means of the reactive flux method. Cs₆Ti₆S₂₇ and Cs₄BaTi₆Se₂₇ crystallize in a new structure type in the trigonal space group C₃⁴-R3 with three formula units in cells with hexagonal axes at T = 153(2) K of a = 13.5203(8) Å, c = 19.784(1) Å and a = 13.804(2) Å, c = 20.850(2) Å, respectively. The compounds Rb₅AgTi₆Se₂₇ and Cs₅AgTi₆Se₂₇ have also been synthesized by the reactive flux method. Rb₅AgTi₆Se₂₇ and Cs₅AgTi₆Se₂₇ crystallize in the trigonal space group C_3v⁴-P31c with two formula units in cells with hexagonal axes at T = 153(2) K of a= 13.266(1) Å, c = 13.908(2) Å and a = 13.5713(4) Å, c = 13.9367(6) Å, respectively. The polar structures are closely related and consist of similar infinite one-dimensional _∞¹[Ti₂(Q₂)₄(Q) ^2-] (Q = S, Se) chains that run along [001] and are separated by cations. If these chains are considered to be cylinders, then these structures adopt hexagonal closest-packing along [001], a general feature of the packing for known A/group IV/Q compounds (A = alkali metal). These chains are built from seven-coordinate TiQ₇ pentagonal bipyramids that are bonded together by edge- and face-sharing in turn. Four Q₂^2- groups are in the translational unit of the chain. An unusual feature of the Ag chalcogenides is octahedral coordination about Ag.