β-Technetium dichloride: solid-state modulated structure, electronic structure, and physical properties.

A second polymorph of technetium dichloride, β-TcCl2, has been synthesized from the reaction of Tc metal and chlorine in a sealed tube at 450 °C. The crystallographic structure and physical properties of β-TcCl2 have been investigated. The structure of β-TcCl2 consists of infinite chains of face sharing [Tc2Cl8] units; within a chain, the Tc≡Tc vectors of two adjacent [Tc2Cl8] units are ordered in the long-range where perpendicular and/or parallel arrangement of Tc≡Tc vectors yields a modulated structure. Resistivity and Seebeck measurements performed on a β-TcCl2 single crystal indicate the compound to be a p-type semiconductor while a magnetic susceptibility measurement shows technetium dichloride to be diamagnetic. A band gap of 0.12(2) eV was determined by reflectance spectroscopy measurements. Theoretical calculations at the density functional level were utilized for the investigation of other possible stable forms of TcCl2.