13C and 39K High-Resolution Solid-State NMR Study of the Nonferroic Phase Transition of Potassium Hydrogen Carbonate. Complementarity between NMR and Incoherent Neutron Scattering

Abstract

Potassium hydrogen carbonate KHCO 3 is formed of centrosymmetric dimers (HCO 3 ) 2 linked by hydrogen bonds, the protons being disordered in an asymmetric double-well potential at ambient temperature. This compound is a model for 0-dimensional hydrogen bonds. It undergoes a nonferroic nonferroelectric phase transition at T c = 318 K, and the double-well potential becomes symmetric in the high-temperature phase. This phase transition is studied for the first time by variable-temperature 1 3 C and 3 9 K high-resolution solid-state NMR experiments. We present an original method to correlate NMR data to other local probes such as incoherent neutron scattering experiments to extract structural information from powder NMR experiments. Using this method, the eigenvalues and jump angle of the chemical shift tensor of the carbon 1 3 C and the quadrupolar interaction of the potassium 3 9 K could be extracted in the completely ordered phase, and their evolution with temperature is correlated to the proton-dynamic disorder within the hydrogen bond. This study also illustrates the complementarity between NMR and incoherent neutron-scattering experiments in the study of hydrogen bonding.