Crystalline and Glassy Phases in the Cs/Bi/As/S System

The
quaternary Cs2S/Bi/As/S system was studied in an attempt
to introduce two different asymmetric but isoelectronic building units,
namely, [BiIIIxSy] and [AsIIIxSy], in a single structure. Reactions
with a comparatively lower equivalent of arsenic in the Cs2S/Bi/As/S mixture led to the crystalline compound Cs3Bi­(AsS4)2. The structure features tetrahedral [AsVS4]3– connected to BiIII centers to give infinite 1/∞[Bi­(AsS4)23–] chains. When the basicity
was raised in these low arsenic fluxes by increasing the Cs2S fraction, the crystalline compound Cs9Bi­(AsS4)4, also featuring [AsVS4]3– anions, was formed. On the other hand, arsenic-rich mixtures of
Cs2S/Bi/As/S led to the formation of the glassy phase Cs2BiAs3S7, which contains AsIII species. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy,
and pair distribution function (PDF) analysis indicate the presence
of AsIII-containing [AsnS2n+1] fragments in the glass structure. Several
glasses in the series Csn–1BiAsnS2n+1 were also
prepared using solid-state fusion reactions. The band gaps of the
Csn–1BiAsnS2n+1 glasses are in the range
of 1.51–1.81 eV, while that of the crystalline compound Cs3Bi­(AsS4)2 is ∼2.33 eV. The thermal
and optical behaviors of these compounds are correlated with their
structures and building units.