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 Tl18Pb2M7Q25 (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 Pa3̅, and a = 17.0952(6) Å in case of Tl18Pb2Ti7S25 (with four formula units per cell). The structure is composed of several interesting subunits such as isolated M7Q24 entities, weakly connected Tl9Pb supertetrahedra (or 4-capped distorted octahedra) and STl6 distorted octahedra. The finite unit M7Q24 is formed by seven edge-shared MQ6 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(Pb2+)2(Ti4+)7(S2–)25. The calculations yielded band gaps of 0.64 eV for the sulfides Tl18Pb2Ti7S25 and 1.0 eV for Tl18Pb2Zr7S25. The selenide Tl18Pb2Ti7Se25 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.