19F and 11B spin-lattice relaxation times were measured in [Zn(ptz)6](BF4)2 (ptz = 1-n-propyl-1H-tetrazole) and in the spin-crossover compound [Fe(ptz)6](BF4)2. For both compounds BF-4 anion reorientation is active above 50 K. For [Zn(ptz)6](BF4)2, the anion-reorientation dynamics is different in the temperature regions of 50-90 K, 90-120 K, and above 150 K; between 120 and 150 K it changes rapidly reflecting a structural change. In [Fe(ptz)6](BF4)2 the mechanism for the paramagnetic relaxation involving the 19F nuclei is found to be of the diffusion-limited type according to the theory of Lowe and Tse. The present results prove that the spin-crossover takes place in a dynamic surrounding and not in a static crystal lattice.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics