Vacancy Ordering as the Cause for the Electrical Resistivity Anomalies and Superlattice Modulations inACu_7-xS₄(A=Tl, K, Rb)

The electronic structure of KCu₇S₄was calculated using the extended Hückel tight binding method, and the origin of the resistivity anomalies and the superlattice reflections ofACu_7-xS₄(A=Tl, K, Rb) was probed. The present work shows thatACu_7-xS₄(A=Tl, K, Rb) can be metallic only forx>0 and that the superlattice modulations ofACu_7-xS₄(A=Tl, K, Rb) are caused by the vacancy ordering in the Cu(2) atom sites. The resistivity anomalies ofACu_7-xS₄are explained in terms of the vacancy ordering in the Cu(2)-Cu(2) chains.