MODULATED LATTICE-RELAXATION AND INCOMMENSURABILITY OF LATTICE WAVES IN BETA-BASED PREMARTENSITIC PHASE

A microscopic model has been proposed which reproduces the extra-ordinary characteristics of the diffraction pattern observed in the premartensitic phase of TiNi. The modulated lattice relaxation (MLR) model is constructed on the two basic assumptions: (i) existence of sharp dip in phonon dispersion surface of the host lattice (ii) existence of a particular type of defect. In this system, the host lattice distorts giving rise to modulated lattice relaxation extending around the defect. The diffraction effect due to such MLR produces incommensurate superlattice reflections and a uniform shift of the peak positions from the commensurate value towards (or away from) the origin of the reciprocal space has been obtained, which is consistent with the observations. A microscopic mechanism of non-diffusional phase transitions is discussed on the basis of the proposed model.