AEuAsS3 (A = Li, K, Rb, and Cs): New As3+ species from an arsenic-rich polysulfide flux

Four europium compounds, LiEuAsS 3 ( I), KEuAsS 3 ( IIa), RbEuAsS 3 ( IIb), and CsEuAsS 3 ( IIc), containing As (3+) were synthesized in molten alkali-metal polysulfide salts. An As-rich flux was found to be necessary to stabilize the pyramidal building unit AsS 3] (3-). All crystallize in the monoclinic space group P2 1/ c with two new structure types. Compound I features a new coordination mode for the AsS 3] (3-) unit and three-dimensional Eu(AsS 3)] (-) framework. Compounds IIa- c are isostructural and feature a layer of Eu(AsS 3)] (-) with an unusual mu 5-sulfide anion in a umbrella-like geometry. The compounds are paramagnetic semiconductors with an energy gap of approximately 2.0 eV.