Investigations of the NaCl-to-CsCl phase transition in RbI by elastic diffuse and inelastic neutron scattering

Abstract

Two groups of measurements were performed: In the first group elastic diffuse neutron scattering measurements were done in two pressure cycles, up to 4.2 and 4.6 kbar, respectively. Above 4 kbar Debye-Scherrer intensities indicate CsCl phase inclusions in the NaCl phase matrix. In the first cycle a very pronounced orientation relationship, together with a more broad one, is observed; in the second cycle the sharp relationship becomes broadened. Although already 75% of the NaCl phase matrix have been transformed the original single-crystal orientation is recovered upon pressure release ("memory effect"). An elastic diffuse intensity discovered in a previous measurement and ascribed to an "intermediate structure" has been confirmed. In the second group of measurements phonons of the $\mathrm{TA}(00\ensuremath{\zeta})$ branch show anomalies in frequency and intensity between 3 and 4 kbar for the smaller $q$ values. These anomalies indicate that changes in the imperfection structure start already at 3 kbar, i.e., below thermodynamic equilibrium. Based on these findings a sequence of steps for the NaCl-to-CsCl phase transition is put forward.