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 [Tc ₂Cl₈] 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.