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https://hdl.handle.net/2142/30764
Description
Title
The phase transition of the rubidium halides
Author(s)
Wallat, Richard John
Issue Date
1975
Director of Research (if dissertation) or Advisor (if thesis)
Holder, J.T.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Rubidium Halides
phase transitions
Language
en
Abstract
It has been shown that the relatively simple Born model for the interatomic potential energy is sufficient to successfully predict the cohesive energies, the transition pressure, the effects of non-hydrostatic stress, energy differences, and the volume changes at the transition in the Rubidium Halides. Although we agree with an earlier statement by Tosi and Fumi that it is essential to use crystal data over a wide range of pressures in determining the parameters in the model, it was shown that the same set of parameters can be used in both phases provided that care is taken to 1) carry the lattice sums over virtually the entire lattice; 2) choose the lattice parameters very carefully; and 3) respect the sensitivity of the model to the choice of van der Waal parameters. A measurement of the longitudinal elastic constant in the (111) direction as a function of pressure was carried out and gave a completely linear result all the way to transition pressure. Including the
calculations, no second-order elastic constant is found to disappear at the transition indicating that a simple elastic criterion for this transformation does not exist.
Thermal effects in this transition system can be adequately treated using the elastic continuum approximation. This approximation was able to approximately account for the variation in Debye temperature across the transition, the thermal energy and the entropy change at the transition. The procedure is also useful in predicting the change in transition pressure with temperature when elastic constants are known for both phases. The uniaxial stress dependence of the transition pressure was found experimentally to be dPt/dO= 3.30, which is in excellent agreement lll with the calculated value. The accuracy of the predicted value further supports the conclusion that the Born model is adequate in describing this phase transition.
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