ROLE OF NON-SYMMETRY-BREAKING ORDER PARAMETERS IN DETERMINING THE MARTENSITIC ENERGY BARRIER - THE BCC-TO-9R TRANSFORMATION

Distortions that do not affect the symmetry of the final product phase obtained in a martensitic transition, so-called non-symmetry-breaking order parameters, but that do affect the energy barrier encountered during homogeneous nucleation, are considered. For the bcc-to-9R transition undergone by Na, first-principles total-energy calculations verify the importance of these distortions in that a 40% reduction is obtained when the non-symmetry-breaking order parameters are allowed to alter the transformation path defined only by the symmetry-breaking order parameters.