Roger Naslain

The crystal structure of the ф phase in the boron-sodium system

Abstract The crystal structure of a complex boride, NaB 15 , has been determined by X-ray diffraction methods using single crystals. The structure consists principally of B 12 icosahedra arranged in a novel way but with additional boron atoms linking icosahedra together. The sodium atoms occur in large cages that result from the boron arrangement.

Structure cristalline de la phase du systeme bore-thorium. Un nouveau borure a structure type YB66

Abstract A single crystal of the δ phase has been obtained in the boron-thorium system by slow cooling of a liquid corresponding to an atomic ratio B Th = 70 . The structure has been established, from two sets of 650 independent reflexions. Absorption correction was made necessary by the thorium atoms. From least-squares refinement, Fourier syntheses and electron density maps it appears that the δ phase is isotypic with the yttrium boride YB66. The crystal is cubic with a = 23.53 A ; the space group is Fm3c, Oh6. There are 1584 boron atoms and 24 thorium atoms in the unit cell. 1248 boron atoms are contained in 13 icosahedron units corresponding to the formula B12(B12)12. B-B bonds within icosahedra range from 1.749 to 1.911 A. The shortest intericosahedral bond is 1.598 A, the longest 1.837 A. The remaining boron atoms are statistically distributed in channels resulting from packing of the B12(B12)12 units and form nonicosahedral cages as in YB66. The thorium atoms are inserted in the channels; the thorium-boron bonds range from 2.71–2.84 A.