THE KINETICS OF THE PRESSURE-INDUCED B1-B2 PHASE TRANSFORMATION IN RUBIDIUM IODIDE. A. R. Rezai-Fard and J. Anwar, King's College London, Manresa Road, London SW3 6LX, UK; S. M. Clark, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

The kinetics of the pressure-induced B1(Fm3m)-B2(Pm3m) phase transition of rubidium iodide have been investigated in a large-volume high-pressure cell using energy dispersive X-ray powder diffraction. The kinetics were followed at various excess pressures under isobaric conditions. The transformation did not always go to completion, with the fraction transformed as a function of time tending to plateau. Both the rate and extent of transformation were generally higher at higher excess pressures. This behaviour, which has been observed before (Hamaya and Akimoto 1981; Dassler 1990), can be explained by considering that the powder sample is characterised by nucleation sites with a distribution of activation energies. Thus crystallites containing sites with lower activation energy transform at low excess pressures whilst those with higher activation energies remain unreacted. The kinetics were analysed considering only the fraction of material that had transformed. Various kinetic models were fitted and the Avrami equation with n=1 was found to be the best, though in absolute terms just acceptable.

Finally, the fraction transformed with time in terms of the new ([[alpha]]) phase and the parent ([[beta]]) phase did not always add up to one as expected. After consideration and exclusion of all other possibilities this problem was attributed to the existence of an amorphous phase. This amorphous phase is not considered to be an intermediate but thought to coexist. It interconverts with the crystalline phases during phase transition.

References:

Dassler R, High Temp.-High Press., 22, 599-612, (1990)

Hamaya N and Akimoto S, High Temp.-High Press., 13, 347-358, (1981)