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 13 C and 39 K high-resolution solidstate 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 13 C and the quadrupolar interaction of the potassium 39 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.

Critical behavior of KDCO3 from 2H and 39K single crystal NMR

“…A simple study of the difference of the two expressions indicates that, except very close to the transition temperature, both expressions predict the same ρ value with an error less than 10%. Owing to the larger experimental errors in previous studies, [18,19] our finding for KDCO 3 would also be consistent with a single expression of the order parameter, say ρ ≈ 1.15 (1 − T/T c ) 0.27 , that can be adapted to the compound of interest in this family of hydrogen carbonates AHCO 3 or ADCO 3 by using the transition temperature of the sample under study. Within this approximation, full ordering is predicted for temperatures typically lower than ≈0.4 T c .…”

“…This is consistent with the value of the quadrupolar coupling constant C q ≈ 1.5 MHz estimated for 39 K in KHCO 3 . [18] Since the structure of KHCO 3 and KDCO 3 are isomorphic with a slight modification of the lattice parameters, this values is also expected to give a good estimation of the quadrupolar interaction in KDCO 3 . The temperature regulation was performed by a nitrogen flux with a precision of 1 K with the Bruker VT2000 unit.…”

“…In the following, we interpret our NMR data within the framework of such a two-site-jump model, which has been successful in interpreting similar experiments in carboxylic acids, [27 -29] KHCO 3 and RbHCO 3 , [18,19] or other hydrogen-bond compounds. For KDCO 3 , the time scale of the deuterium jump correlation time can be estimated from the spin-lattice relaxation experiments reported in Ref.…”

“…It was found from 13 C chemical shift eigenvalues temperature evolution that ρ H ≈ 1.14 (1 − T/T c ) 0.25 , [18] where ρ H is the order parameter of the hydrogenated sample. This expression was also found to represent well the data obtained with RbHCO 3 [19] by simply shifting the transition temperature T c to the value observed for the compound under study.…”

“…[1] In contrast, our approach is strictly in the framework of critical phenomena. From 13 C and 39 K powder NMR on KHCO 3 , [18] we interpreted the evolution of the NMR parameters using a two-site-jump model coupled to an order/disorder phase transition order parameter. The same model was also fruitful to draw conclusions about the phase transition of RbHCO 3 by comparison of the 13 C and 87 Rb NMR results to those obtained from KHCO 3 .…”