Syntheses and structures of the infinite chain compounds Cs₄Ti₃Se₁₃, Rb₄Ti₃S₁₄, Cs₄Ti₃S₁₄, Rb₄Hf₃S₁₄, Rb₄Zr₃Se₁₄, Cs₄Zr₃Se₁₄, and Cs₄Hf₃Se₁₄

The alkali metal/group 4 metal/polychalcogenides Cs₄Ti₃Se₁₃, Rb₄Ti₃S₁₄, Cs₄Ti₃S₁₄, Rb₄Hf₃S₁₄, Rb₄Zr₃Se₁₄, Cs₄Zr₃Se₁₄, and Cs₄Hf₃Se₁₄ have been synthesized by means of the reactive flux method at 823 or 873 K. Cs₄Ti₃Se₁₃ crystallizes in a new structure type in space group C₂²-P2₁ with eight formula units in a monoclinic cell at T = 153 K of dimensions a = 10.2524(6) Å, b = 32.468(2) Å, c = 14.6747(8) Å, β = 100.008(1)°. Cs₄Ti₃Se₁₃ is composed of four independent one-dimensional _∞¹[Ti₃Se₁₃^4-] chains separated by Cs⁺ cations. These chains adopt hexagonal closest packing along the [100] direction. The _∞¹[Ti₃Se₁₃^4-] chains are built from the face- and edge-sharing of pentagonal pyramids and pentagonal bipyramids. Formal oxidation states cannot be assigned in Cs₄-Ti₃Se₁₃. The compounds Rb₄Ti₃S₁₄, Cs₄Ti₃S₁₄, Rb₄Hf₃S₁₄, Rb₄Zr₃Se₁₄, Cs₄Zr₃Se₁₄, and Cs₄Hf₃Se₁₄ crystallize in the K₄Ti₃S₁₄ structure type with four formula units in space group C_2h⁶-C2/c of the monoclinic system at T = 153 K in cells of dimensions a = 21.085(1) Å, b = 8.1169(5) Å, c = 13.1992(8) Å, β = 112.835(1)° for Rb₄Ti₃S₁₄; a = 21.329(3) Å, b = 8.415(1) Å, c = 13.678(2) Å, β = 113.801(2)° for Cs₄Ti₃S₁₄; a = 21.643(2) Å, b = 8.1848(8) Å, c = 13.331(1) Å, β = 111.762(2)° for Rb₄Hf₃S₁₄; a = 22.605(7) Å, b = 8.552(3) Å, c = 13.880(4) Å, β = 110.919(9)° for Rb₄Zr₃Se₁₄; a = 22.826(5) Å, b = 8.841(2) Å, c = 14.278(3) Å, β = 111.456(4)° for Cs₄Zr₃Se₁₄; and a = 22.758(5) Å, b = 8.844(2) Å, c = 14.276(3) Å, β = 111.88(3)° for Cs₄Hf₃Se₁₄. These A₄M₃Q₁₄ compounds (A = alkali metal; M = group 4 metal; Q = chalcogen) contain hexagonally closest-packed _∞¹[M₃Q₁₄^4-] chains that run in the [101] direction and are separated by A⁺ cations. Each _∞¹[M₃Q₁₄^4-] chain is built from a [M₃Q₁₄] unit that consists of two MQ₇ pentagonal bipyramids or one distorted MQ₈ bicapped octahedron bonded together by edge- or face-sharing. Each [M₃Q₁₄] unit contains six Q₂^2- dimers, with Q-Q distances in the normal single-bond range 2.0616(9)-2.095(2) Å for S-S and 2.367(1)-2.391(2) Å for Se-Se. The A₄M₃Q₁₄ compounds can be formulated as (A⁺)₄(M⁴⁺)₃ (Q₂^2-)₆(Q^2-)₂.