Synthesis and structures of ternary chalcogenides of aluminum and gallium with stacking faults:: KMQ2 (M = Al, Ga; Q = Se, Te)

The synthesis and crystal structures of new ternary chalcogenides of aluminum and gallium, KMQ(2) (M = Al, Ga; Q = Se, Te) are reported. These compounds were synthesized by solid-state reaction at 800 degrees C in Nb containers, The single-crystal structures of KMTe2 (M = Al, Ga) have been determined; KAlTe2, a = 11.808(2) Angstrom, b = 11.812(2) Angstrom, c = 16.456(3) Angstrom, beta = 100.32(3)degrees, C2/c (No. 15, Z = 16); KGaTe2, a = 11.768(3) Angstrom, b = 11.775(3) Angstrom, c = 16.503(4) Angstrom, beta = 100.36(2)degrees, C2/c (No. 15, Z = 16). The fundamental building blocks of the title compounds are adamantane-like M(4)Q(10) (M(4)Q(10) = M(4)Q(4)Q(12/2)) supertetrahedra consisting of four MQ(4) (M = Al, Ga; Q = Se, Te) tetrahedra. These supertetrahedra form two-dimensional (2)(infinity)[M(4)Q(6)Q(4/2) = MQ(2)](-) layers by sharing terminal chalcogen atoms with adjacent supertetrahedra. These layers form a stack up to the c axis with each layer rotated by 90 degrees relative to its neighbors. Potassium cations are located inside trigonal prisms formed by six chalcogen atoms. Stacking faults up the c* axis are evidenced by streaks in oscillation photographs. Only half of the data set is affected by this type of faulting. We demonstrate both analytically and by use of the DIFFaX program that our structural model for the faulting is consistent with the observed Streaking patterns. (C) 2000 Academic Press.