Evidence of a phase transition of RbHCO₃ from high‐resolution solid‐state ¹³C and ⁸⁷Rb NMR by comparison with KHCO₃

Abstract

A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar.

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 .…”

“…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. 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%.…”

“…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.…”

“…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 . [19] This work aims to characterize the critical behavior of KDCO 3 near its phase transition by 2 H and 39 K single crystal, variabletemperature NMR experiments. In the first part, we measure the evolution of the order parameter as a function of temperature and quantify it.…”

The phase transition of rubidium hydrogen carbonate, RbHCO3

“…On cooling, it transforms into an ordered P2 1 /a phase (Thomas et al, 1974). Rubidium hydrogen carbonate RbHCO 3 shows an analogous phase transition at T C = 245 K, which has been thoroughly studied by NMR spectroscopy (Odin, 2004). The published structural data, on the other hand, leave much to be desired.…”