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, [Bi(III)(x)S(y)] and [As(III)(x)S(y)], 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 [As(V)S4](3-) connected to Bi(III) centers to give infinite (1/∞)[Bi(AsS4)2(3-)] chains. When the basicity was raised in these low arsenic fluxes by increasing the Cs2S fraction, the crystalline compound Cs9Bi(AsS4)4, also featuring [As(V)S4](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 As(III) species. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and pair distribution function (PDF) analysis indicate the presence of As(III)-containing [AsnS2n+1] fragments in the glass structure. Several glasses in the series Cs(n-1)BiAs(n)S(2n+1) were also prepared using solid-state fusion reactions. The band gaps of the Cs(n-1)BiAs(n)S(2n+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.
Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
Tarun K
TK
Bera
Ratnasabapathy G
RG
Iyer
Christos D
CD
Malliakas
Mercouri G
MG
Kanatzidis