PHYSICAL-PROPERTIES AND SUCCESSIVE PHASE-TRANSITIONS IN QUASI-ONE-DIMENSIONAL SULFIDES ACU(7)S(4) (A=TL, K, RB)

Quasi-one-dimensional sulfides ACu(7)S(4) (A = Tl, K) with (NH4)Cu7S4-type structure and a new isotypic sulfide RbCu7S4 were prepared. Thermal analysis, electrical resistivity, thermoelectric power, and magnetic susceptibility measurements showed a large variety of successive phase transitions. The transitions have characteristics common to all three compounds. They are labeled as T-1(A) congruent to 395, 445, and 400 K, respectively, for A = Tl, K, and Rb; T-2(A) congruent to 245, 250, and 260 K, respectively, for A = Tl, K, and Rb; T-3(A) congruent to 220 K for all the compounds; T-4(A) congruent to 190, 180, and 190 K, respectively for A = Tl, K, and Rb; T-5(A) = 160-170 K for both A = Tl and Rb; T-6(A) congruent to 60 K for all the compounds; and T-7(Rb) congruent to 30 K. Electron diffraction measurements of KCu7S4 showed two kinds of superstructures below T-2(K) and T-4(K), which are characterized by the corresponding commensurate wave vectors q = 1/2c* and q = +/-(1/3a* + 1/3b*) + 2/3c*, respectively. TlCu7S4 showed the superstructure with q = 1/2c* below T-2(TI) and one with q = 1/2 (a* + b*) + 1/3c* on further cooling. RbCu7S4 showed only q = 1/2c* below T-2(Rb). An order-disordering mechanism was proposed to be the possible origin for the transition T-2(A). The origins of the other transitions are not clear. Through studies on nonstoichiometric samples, it was revealed that the relatively high conductivity of these compounds originates from the atomic vacancies of both Cu and Tl. (C) 1995 Academic Press, Inc.