STRUCTURE OF THE METASTABLE BINARY BETA-MO15SE19

Mo15Se19, M(r) = 2939.34, trigonal, R3cBAR, a(rh) = 20.360 (1) angstrom, alpha(rh) = 26.981 (1)-degrees, V(rh) = 1532.2 (1) angstrom 3, Z = 2, D(x) = 6.371 g cm-3, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 281.28 cm-1, F(000) = 2552, T = 295 K, R = 0.029 for 781 observed reflections. The title compound constitutes the host structure of the ternary compounds M2Mo15Se19 (M = K, Rb, In, Tl, Ba). Its crystal structure is built up from an equal mixture of Mo6Se8 and Mo9Se11 cluster units linked together through Mo-Se interunit bonds. The absence of cationic charge transfer towards the Mo clusters results in an expansion of both types of clusters along the threefold axis and in a decrease of the intercluster distances.