Synthesis and Structure of Ta<sub>4</sub>SI<sub>11</sub>:  Disorder and Mixed Valency in the First Tantalum Sulfide Iodide

2003-05-29T00:00:00Z (GMT) by Mark D. Smith Gordon J. Miller
The new compound Ta<sub>4</sub>SI<sub>11</sub> 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. Ta<sub>4</sub>SI<sub>11</sub> crystallizes with orthorhombic symmetry in space group <i>Pmmn</i>; <i>a</i> = 16.135(3) Å, <i>b</i> = 3.813(1) Å, <i>c</i> = 8.131(2) Å, and <i>Z</i> = 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 <i>μ</i><sub>eff</sub> = 1.53 μ<sub>B</sub> to suggest one unpaired electron per formula unit. X-ray photoelectron spectroscopy and extended Hückel calculations suggest that the structure consists of Ta<sub>3</sub> triangles and “isolated” Ta atoms, leading to the formulation (Ta<sub>3</sub>)<sup>9+</sup>(Ta<sup>4+</sup>)(S<sup>2-</sup>)(I<sup>-</sup>)<sub>11</sub> and we hypothesize that each Ta<sub>3</sub> is capped by a sulfur atom.