CHARACTERIZATION OF THE BINARY URANIUM AND THORIUM TELLURIDES U7TE12 AND TH7TE12

Single crystals of U7Te12 and Th7Te12 were obtained by chemical vapor transport reactions in evacuated and sealed quartz tubes, starring fro m a slightly deficient tellurium stoichiometry and using iodine as the transporting agent in a temperature gradient of 1030-1000 degrees C. Their crystal structures were solved in the noncentrosymmetric hexagon al space group P (6) over bar (No. 174), Z = 1. The cell constants are a = 12.312(1) Angstrom and c = 4.260(1) Angstrom for U7Te12 and a = 1 2.300(2) Angstrom and c = 4.566(1) Angstrom for Th7Te12. The three non equivalent actinoid (A = U, Th) atoms per unit cell, A(1), A(2), and A (3), occupy 1a. 3k, and 3j Wyckoff positions and the four nonequivalen t tellurium atoms occupy two 3k and two 3j positions. All the actinoid atoms are centered in tricapped (A(1)) or bicapped (A(2), A(3)) tellu rium trigonal prisms, which form infinite columns running along the [0 01] direction. The bicapped trigonal prisms around A(2) and around A(3 ) are linked also in both cases by edge-sharing, to form large distort ed hexacapped trigonal prisms, U7Te12 exhibits ferromagnetic ordering below 54 K and a semimetallic type conductivity.