NMR study of molecular motions and disorder in K₃ZrF₇and K₂TaF₇

Abstract

Fluorine second moments and spin-lattice relaxation times in the laboratory and rotating frames were measured as a function of temperature. The results show that the low-temperature orthorhombic structure of K3ZrF7 is disordered. Each of the four non-equivalent ZrF73- molecules has four possible orientations which can be described as twofold reorientations about the crystallographic axes. The activation energies associated with these motions range from 6.3 to 10.0 kcal mol-1. In the high-temperature cubic phase the ZrF73- molecules are equivalent. The molecule has sixteen possible orientations but the results show that an additional reorientational mechanism results in an isotropic reorientation in which each F-F vector occupies all other F-F positions in the course of time. Due to the cross-relaxation between 91Zr and 19F nuclei, the quadrupolar relaxation time of 91Zr could be obtained from the T1 results. Although the TaF72- molecules has no symmetry axis, the relaxation results show clearly that it is involved in a two-stage reorientation mechanism. These reorientations are described as 'floppy' reorientations in which the shape of the molecule changes during each jump.