The crystal structures of molybdenum trifluoride, MoF 3 , and tantalum trifluoride, TaF 3

Abstract

Molybdemun trifluoride and tantalum triftuoride are isostructural with rhenium trioxide, ReOs. The cubic unit cell, with space group Pm3m, contains one metal atom at 0, 0, 0 and three fluorine atoms at ½, 0, 0; 0, ½, 0; and 0, 0, ½. The structure consists of regular oetahedra of fluorine af~ms, each with a metal atom at its centre, joined by sharing corners only. The structure is alternatively described as a cubic close-packing of fluorine atoms in which one-quarter of the octahedral inter- stices are occupied by metal atoms and in which one-quarter of the fluorine atom sites remain vacant. For MoF 8, a= 3.8985 _+ 0.0005, Mo-F = 1-949 and F-F = 2.757 A. For TaF 3, a = 3.9012 + 0.0002, Ta-F= 1.951 and F-F= 2.759 A.