The Born-model and the effects of hydrostatic and non-hydrostatic stresses on rubidium halide structural phase transitions

An analytic Born-model, with the same set of repulsive parameters for both phases in each salt, has been used to calculate the properties of the NaCl-CaCl structural phase transformation in three rubidium halides. The treatment required a careful evaluation of the three repulsive parameters by comparison with equilibrium conditions in both phases and measured bulk moduli, and involved a self consistent analysis which takes into account the experimental uncertainties in reported values of CsCl-phase lattice parameters. Calculated values for the equilibrium transition pressure, lattice parameters and lattice energies are in satisfactory agreement with reported experimental results. The model has also been used to calculate the lattice energy continuously from the NaCl to the CsCl phases, as a function of both hydrostatic and non-hydrostatic stresses. These calculations give a semiquantitative estimate of an energy barrier between the two stable structures, which is consistent with reported measurements of elastic constant and hysteresis effects near the transition pressure. The calculated effects of a uniaxial stress are found to be as much as three times larger than those of a hydrostatic stress, and the effects of the uniaxial stress on the barrier height are found to be approximately the same as the effects on the equilibrium energy differences. Measurements of the effect of this uniaxial stress on the forward transition pressure of RbI were carried out and the measured variations were found to be in excellent agreement with the calculated change in equilibrium transition pressure—as expected from the energy barrier calculations.