Synthesis and Structure of Ta4SI11: Disorder and Mixed Valency in the First Tantalum Sulfide Iodide

The new compound Ta4SI11 has been prepared by direct reaction of the elements at 430 °C for 2 weeks in evacuated Pyrex ampules and characterized by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, and semiempirical electronic structure calculations. Ta4SI11 crystallizes with orthorhombic symmetry in space group Pmmn; a = 16.135(3) (angstrom), b = 3.813(1) (angstrom), c = 8.131(2) (angstrom), and Z = 1. The disordered structure involves two crystallographically distinct sites for Ta atoms, both of which are 50% occupied as well as a bridging anion site that is 50% S and 50% I. Magnetic susceptibility above 100 K gives (mu)eff = 1.53 (mu)B to suggest one unpaired electron per formula unit. X-ray photoelectron spectroscopy and extended Hückel calculations suggest that the structure consists of Ta3 triangles and "isolated" Ta atoms, leading to the formulation (Ta3)^9+(Ta^4+)(S^2-)(I-)11 and we hypothesize that each Ta3 is capped by a sulfur atom.