Na2EuAs2S5, NaEuAsS4, and Na4Eu(AsS4)2: Controlling the Valency of Arsenic in Polysulfide Fluxes

The reactivity of europium with As species in Lewis basic alkali-metal polysulfide fluxes was investigated along with compound formation and the As3+/As5+ interplay vis-à-vis changes in the flux basicity. The compound Na2EuAs2S5 containing trivalent As3+ is stabilized from an arsenic-rich polysulfide flux. It crystallizes in the monoclinic centrosymmetric space group P21/c. Na2EuAs2S5 has [As2S5]4– units, built of corner sharing AsS3 pyramids, which are coordinated to Eu2+ ions to give a two-dimensional (2D) layered structure. A sodium polysulfide flux with comparatively less arsenic led to the As5+ containing compounds NaEuAsS4 (orthorhombic, Ama2) and Na4Eu­(AsS4)2 (triclinic, P1) depending on Na2S/S ratio. The NaEuAsS4 and Na4Eu­(AsS4)2 have a three-dimensional (3D) structure built of [AsS4]3– tetrahedra coordinated to Eu2+ ions. All compounds are semiconductors with optical energy gaps of ∼2 eV.