New Nb6Cl15-xFx chlorofluoride: stabilization of the Ta6Cl15 structure-type for binary Nb-6 halide

The replacement of a small amount of chlorine by fluorine has made it possi ble to synthesize Nb6Cl15-xFx chlorofluoride with the Ta6Cl15 structure-typ e. The crystal structures of two compositions in the solid solution (x = 2. 2 (I) and x = 4.4 (II)) have been determined by single crystal X-ray diffra ction and by isotypy with Ta6Cl15 (cubic, Ia-3d n(degrees) 230, a = 20.099( 1) Angstrom, V = 8119.2(1) Angstrom(3), Z = 16, R = 0.047, R-w = 0.054 for (I) and a = 19.370(2) Angstrom, V= 7964.3(8) Angstrom(3), Z = 16, R = 0.067 , R-w = 0.067 for (II). The structure is based on (Nb6L12L6/2a-a)-L-j (L = ligand) units in which F is randomly distributed on the inner positions for (I) and (II), and additionally on the apical-apical ones for (II). The uni t-cell contains 16 units connected in the 3 directions by bent Nb-La-a-Nb b ridges. This structure is different from the one encountered for Nb6F15 whi ch exhibits 2 independent interpenetrating networks with linear Nb-Fa-a-Nb bridges. The interatomic distances obtained from structural:data are discus sed by comparison with the binary Ta6Cl15 and isotypic NaNb6Cl15 ternary ch loride, taking into account electronic effects and the size of the fluorine . The small size of fluorine allows the reduction of the Nb-Nb intracluster distances by steric effects and thus the stabilization of the Ta6Cl15 stru cture-type with Nb-6 binary halides. (C) 1999 Academie des sciences/Edition s scientifiques et medicales Elsevier SAS.