Semiconducting layered technetium dichalcogenides: insights from first-principles.

The structures and properties of layered technetium dichalcogenides TcX2 (X = S, Se, Te) have been investigated using density functional theory. The equilibrium structures of TcSe2 and TcTe2, adopting distorted Cd(OH)2-type unit cells similar to TcS2, are reported for the first time at the atomic level, along with their electronic properties. In contrast to previous X-ray diffraction analyses, calculations reveal that stoichiometric TcTe2 is not isomorphous to the high-temperature monoclinic phase β-MoTe2. All three compounds are found to be semiconductors with calculated band gaps of 0.9 eV for TcS2, 0.8 eV for TcSe2, and 0.3 eV for TcTe2. The thermal properties of these TcX2 compounds have also been predicted using phonon frequencies calculated with density functional perturbation theory.