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.
Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
Cheriyedath Raj
CR
Sankar
Anna
A
Becker
Abdeljalil
A
Assoud
Holger
H
Kleinke