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

Abstract

Potassium hydrogenocarbonate KDCO3 presents an order/disorder phase transition at Tc approximately 353 K. The critical behavior of this phase transition was studied by single crystal 2H and 39K NMR. The evolution of the order parameter as a function of temperature is quantified, and the critical exponent was determined, indicating a transition close to a tricritical point. The 2H Zeeman relaxation rate is strongly increased near the transition temperature. By calculating the noncritical contribution to the Zeeman relaxation rate, we show that the observed relaxation rate clearly presents a pseudo-divergent behavior near Tc, with a logarithmic singularity. The nature of the phase transition is discussed in the light of these results.

Single-Crystal NMR Spectroscopy

“…The study of phase transitions by SC NMR is not so obvious, at least one should require that the crystal undergoes a single crystal to single crystal transformation, in which case SC NMR may provide a higher level of detail than obtainable from powdered samples. Odin [114] studied a phase transition in KHCO 3 , where the carbonates form dimers that are hydrogen-bonded in two different ways. At low temperature, the two configurations result in distinct resonances in 2 H and 39 K SC NMR (see typical 2 H SC NMR spectra in Fig.…”

“…A final measurement in the work of Odin [114] was the measurement of the 2 H spin-lattice relaxation time T1 as a function of temperature as reproduced in Fig. 33c.…”

“…This way, we obtained valuable information about the crystal morphology and the orientation of the crystallographic axes directly from the SC NMR drupolar parameters for 2 H ( • ) and 39 K (⊠) and (c) inverse 2 H spin-lattice relaxation time (1/T1) in KDCO 3 as a function of temperature. Adapted from [114] with permission. data.…”

Physical methods and techniques : NMR spectroscopy

“…The evolution of the order parameter with temperature was quantified, and the critical exponent determined; the 2 H Zeeman relaxation rate increases near the transition temperature. 163 Inhomogeneous and lower-field NMR applications outside the chemistry lab, such as in oil wells, endoscopes, food processing, polymer analysis, concrete and rheology are covered in wide ranging review. 164…”