Theory of Phase Transformations in Alkali Metals

A theory is suggested to account for the phase changes which have been observed by Barrett in lithium and sodium. The elastic constants for the face-centred and body-centred cubic structures have been calculated and from them the free energies determined. An interesting conclusion is that the electrostatic and ionic terms alone are not sufficient to account for the phase change. The Fermi energy of the conduction electrons is affected by shear, and hence this part of the total energy gives rise to a contribution to the elastic shear constants. The difference in this contribution between the two cubic structures was found to be adequate to account for the observed phase change. An estimation of the transition temperature indicated that it lies in the range 40°-100°K.