AEuAsS₃ (A = Li, K, Rb, and Cs): New As³⁺ Species from an Arsenic-Rich Polysulfide Flux

Four europium compounds, LiEuAsS3 (I), KEuAsS3 (IIa), RbEuAsS3 (IIb), and CsEuAsS3 (IIc), containing As3+ were synthesized in molten alkali-metal polysulfide salts. An As-rich flux was found to be necessary to stabilize the pyramidal building unit [AsS3]3−. All crystallize in the monoclinic space group P21/c with two new structure types. Compound I features a new coordination mode for the [AsS3]3− unit and three-dimensional [Eu(AsS3)]− framework. Compounds IIa−c are isostructural and feature a layer of [Eu(AsS3)]− with an unusual μ5-sulfide anion in a umbrella-like geometry. The compounds are paramagnetic semiconductors with an energy gap of ∼2.0 eV.