Soft heat diffusion mode at first order displacive phase transitions

We investigate a first order displacive phase transition driven by an optical phonon instability. We find that in the low temperature phase the frequency of the optical phonon does not vanish at the stability limit T = Tu, defined by χT-1 = 0 where χT is the isothermal susceptibility. Instead, the heat diffusion constant, renormalized by the interaction with the soft optical phonon goes to zero as Tu is approached from below. © 1972.