VACANCY ORDERING AS THE CAUSE FOR THE ELECTRICAL-RESISTIVITY ANOMALIES AND SUPERLATTICE MODULATIONS IN ACU(7-X)S(4) (A = TL, K, RB)

The electronic structure of KCu7S4 was calculated using the extended H uckel tight binding method, and the origin of the resistivity anomalie s and the superlattice reflections of ACu(7-x)S(4) (A=Tl, K, Rb) was p robed, The present work shows that ACu(7-x)S(4) (A = Tl, K, Rb) can be metallic only for x > 0 and that the superlattice modulations of ACu( 7-x)S(4) (A = Tl, K, Rb) are caused by the vacancy ordering in the Cu( 2) atom sites, The resistivity anomalies of ACu(7-x)S(4) are explained in terms of the vacancy ordering in the Cu(2)-Cu(2) chains, (C) 1997 Academic Press.