Ta<sub>4</sub>SBr<sub>11</sub>: A Cluster Mott Insulator\nwith a Corrugated, Van der Waals Layered Structure

The compound Ta<sub>4</sub>SBr<sub>11</sub> was prepared\nby a comproportionation\nreaction of tantalum bromide with tantalum and elemental sulfur. The\ncrystal structure, as refined by single-crystal X-ray diffraction,\nis composed of clusters with Ta<sub>4</sub>S cores, arranged in corrugated\nvan der Waals layers. Individual layers appear to be displaced relative\nto each other along one direction. Successful crystal growth in a\nmelt of CsBr yielded black platelets of Ta<sub>4</sub>SBr<sub>11</sub>, which were used to investigate the electrical properties of the\ncompound. The electronic structure was studied by diffuse reflectance\ninfrared Fourier transform (DRIFT) spectroscopy and by density functional\ntheory (DFT) band structure calculations, revealing this material\nto be a small-gap semiconductor. DFT results, in combination with\nmagnetic susceptibility measurements, suggest that metallicity originating\nfrom the one unpaired Ta d electron per cluster is most likely suppressed\nby electronic correlations, forming a cluster Mott insulator.