Neutron scattering study of the nucleation and growth process at the pressure-induced first-order phase transformation of RbI

Abstract

We have studied the kinetics of the NaCl-CsCl phase transformation in RbI at room temperature (${P}_{c}$apeq23.5 kbar) and at temperatures down to 200 K by monitoring the time-dependent changes in neutron powder diffraction peaks. Within the interval of temperature studied, the rate of growth of the stable phase increases as \ensuremath{\Delta}P=\ensuremath{\Vert}P-${P}_{c}$\ensuremath{\Vert} is increased. The characteristic time of completion of transformation ${t}_{0}$(P) at room temperature diverges as the pressure approaches ${P}_{c}$ consistent with the form \ensuremath{\Delta}${P}^{\mathrm{\ensuremath{-}}3/4}$exp[B(\ensuremath{\Delta}P${)}^{\mathrm{\ensuremath{-}}2}$], predicted by classical theory. At room temperature the observed growth curves for various pressures display a nearly universal shape, consistent with the Kolmogorov model of nucleation and growth, when plotted against a scaled time parameter \ensuremath{\tau}=t/${t}_{0}$(P). At 240 K the growth curves deviate from the universal shape increasingly with increasing \ensuremath{\Delta}P. Moreover, within a single growth curve the deviation becomes more evident in the later stage of growth. These observations can be explained qualitatively by taking into account the effect of stress produced by the volume change associated with the transformation.