Ionic Motion and Phase Transition in Potassium Hydrogendifluoride KHF2 Studied by Pulsed NMR

Abstract

Abstract Nuclear spin-lattice relaxation times, T1 and T1ρ, have been measured above room temperature for 1H and 19F nuclei in powdered KHF2 and its deuterated analogue. Below 400 K and in the high-temperature β-phase (Tt=469.8 K), the magnetization recovery for both nuclei in KHF2 is strongly non-exponential owing to the heteronuclear H–F interaction. In the low-temperature α-phase the T1 vs. 1/T curves show broad minima ascribable to the 180°-flip motion of linear [FHF]− anions. The activation energy Ea of this motion was determined to be 50.5±2 kJ/mol. The phase transition, which is defined by discontinuous changes in both T1 and T1ρ, is associated with diffusional disorder superimposed on rapid isotropic reorientation of the HF2− ion. The relaxational process in the β-phase is governed by self-diffusion of the anions with an Ea of 80±6 kJ/mol deduced from the T1ρ data. In contrast to the earlier suggestion from electrical measurements, the NMR results indicate that the dominantly mobile species in the conducting high-temperature β-phase are not single F− ions but complex [FHF]− anions.