Martensitic Transformation: Nonlinear Order Parameter
The second order structural phase transformation observed in a perovskite type crystal as SrTiO3 or BaTiO3 has been successfully described in terms of an order parameter which corresponds to a certain specific normal mode of lattice vibrations. However, in the martensitic transformation, which is defined as the first order structural transformation observed in metals and alloys, the presence of an order parameter has not yet been established. In der second order structural phase transformation of SrTiO3, freezing of the Γ25phonon at R point has been established by neutron inelastic scattering experiment . In the martensitic transformation, although some tendency towards softening of phonons in some area of the k-space has been observed. Such complete freezing as observed in SrTiO3 has never been observed.
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