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 As(3+)/As(5+) interplay vis-à-vis changes in the flux basicity. The compound Na(2)EuAs(2)S(5) containing trivalent As(3+) is stabilized from an arsenic-rich polysulfide flux. It crystallizes in the monoclinic centrosymmetric space group P2(1)/c. Na(2)EuAs(2)S(5) has As(2)S(5)](4-) units, built of corner sharing AsS(3) pyramids, which are coordinated to Eu(2+) ions to give a two-dimensional (2D) layered structure. A sodium polysulfide flux with comparatively less arsenic led to the As(5+) containing compounds NaEuAsS(4) (orthorhombic, Ama2) and Na(4)Eu(AsS(4))(2) (triclinic, P1) depending on Na(2)S/S ratio. The NaEuAsS(4) and Na(4)Eu(AsS(4))(2) have a three-dimensional (3D) structure built of AsS(4)](3-) tetrahedra coordinated to Eu(2+) ions. All compounds are semiconductors with optical energy gaps of ～2 eV.