New quaternary chalcogenides, Tl18Pb2M7Q25 (M = Ti, Zr, and Hf; Q = S and Se): crystal structure, electronic structure, and electrical transport properties.

We have synthesized new quaternary chalcogenides of the general formula Tl(18)Pb(2)M(7)Q(25) (M = Ti, Zr and Hf, Q = S, Se), and studied their crystal and electronic structures. They are all isostructural, with a large cubic unit cell of space group Pa3, and a = 17.0952(6) Å in case of Tl(18)Pb(2)Ti(7)S(25) (with four formula units per cell). The structure is composed of several interesting subunits such as isolated M(7)Q(24) entities, weakly connected Tl(9)Pb supertetrahedra (or 4-capped distorted octahedra) and STl(6) distorted octahedra. The finite unit M(7)Q(24) is formed by seven edge-shared MQ(6) octahedra wherein all except the central one are distorted because of the neighborhood of Tl(+) ions that carry a lone pair of electrons. These materials are semiconductors with all elements in their common oxidation states, for example, (Tl(+))(18)(Pb(2+))(2)(Ti(4+))(7)(S(2-))(25). The calculations yielded band gaps of 0.64 eV for the sulfides Tl(18)Pb(2)Ti(7)S(25) and 1.0 eV for Tl(18)Pb(2)Zr(7)S(25). The selenide Tl(18)Pb(2)Ti(7)Se(25) was calculated to have a band gap of 0.44 eV. Electrical conductivity measurements and reflectance spectroscopy also revealed the semiconducting nature of these samples, with experimentally determined gaps between 0.10 and 0.50 eV.