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.