The polymorphism of alkali metal formates, part 2. A raman spectroscopic study of the kinetics of the I′–II phase transition in NaHCOO

Abstract

When the room temperature phase of NaHCOO (NaHCOO-II) is heated, it undergoes a first-order transition to a high temperature phase (NaHCOO-I) at 500 K. Upon cooling this phase, an intermediate phase (NaHCOO-I′) is formed and after an induction period of approximately 10 hours it slowly retransforms into the original phase NaHCOO-II at room temperature. The kinetics of this I′–II phase transition has been studied using Raman spectroscopy at various temperatures. Applying the log-log method, derived from the Avrami-Erofeev equation, to the experimental data, an average value of n = 1.70 has been obtained at 298 K. This indicates a diffusion-controlled process with initial growth of particles nucleated largely at the start of the transformation, but also including some nucleation throughout the growth process. At higher temperatures n decreases, possibly because the particles become more cylindrical in shape. An activation energy of E A = 45±1 KJ.mole − has been obtained for the phase transformation.