Phonon anomalies in RbI transforming from the NaCl to the CsCl phase and a model for the nucleation of the CsCl phase

A search for phonon frequency anomalies in RbI during the early stages of the NaCl-to-CsCl phase transition has been undertaken for the TA ($\zeta \zeta 0$) branch. The phonon frequencies behave linearly with pressure, but the data suggest possible deviations from linearity at small $q$ values similar to those found for the TA ($00\zeta$) in a previous work. In an aging experiment at constant pressure, at 3.6 kbars an astonishing time dependence of the intensity of all phonons investigated has been observed: In the first 60 h the intensities decreased to a minimum of 50% of the initial values and then in the next 40 h an intensity regain to nearly 100% was found. After a further pressure step at 3.7 kbars a similar time behavior was found, but the whole effect was somewhat attenuated. Moreover, a model for the initial stages of the NaCl-to-CsCl transition in RbI, mainly based on results of previous investigations, is put forward. The main idea of the model is centered around the fact that the NaCl structure can transform to the CsCl structure by creating a stacking fault in the NaCl(001) planes.