Na<sub>2</sub>EuAs<sub>2</sub>S<sub>5</sub>, NaEuAsS<sub>4</sub>, and Na<sub>4</sub>Eu(AsS<sub>4</sub>)<sub>2</sub>: Controlling the Valency of Arsenic in Polysulfide
Fluxes
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Abstract
The reactivity of europium with As species in Lewis basic
alkali-metal polysulfide fluxes was investigated along with compound
formation and the As<sup>3+</sup>/As<sup>5+</sup> interplay vis-à-vis
changes in the flux basicity. The compound Na<sub>2</sub>EuAs<sub>2</sub>S<sub>5</sub> containing trivalent As<sup>3+</sup> is stabilized
from an arsenic-rich polysulfide flux. It crystallizes in the monoclinic
centrosymmetric space group <i>P</i>2<sub>1</sub>/<i>c</i>. Na<sub>2</sub>EuAs<sub>2</sub>S<sub>5</sub> has [As<sub>2</sub>S<sub>5</sub>]<sup>4–</sup> units, built of corner
sharing AsS<sub>3</sub> pyramids, which are coordinated to Eu<sup>2+</sup> ions to give a two-dimensional (2D) layered structure. A
sodium polysulfide flux with comparatively less arsenic led to the
As<sup>5+</sup> containing compounds NaEuAsS<sub>4</sub> (orthorhombic, <i>Ama</i>2) and Na<sub>4</sub>Eu(AsS<sub>4</sub>)<sub>2</sub> (triclinic, <i>P</i>1) depending on Na<sub>2</sub>S/S ratio. The NaEuAsS<sub>4</sub> and Na<sub>4</sub>Eu(AsS<sub>4</sub>)<sub>2</sub> have a
three-dimensional (3D) structure built of [AsS<sub>4</sub>]<sup>3–</sup> tetrahedra coordinated to Eu<sup>2+</sup> ions. All compounds are
semiconductors with optical energy gaps of ∼2 eV
