Structural phase transition and anharmonic effects in crystals - art. no. 104107

The anharmonic oscillator is used as a trial Hamiltonian in the variational method at a finite temperature, and a Morse-type potential is adopted as a potential between the atoms in the crystal. When instability takes place i n high-temperature regions, it is found that the variational parameter of t he second-order term, obtained by renormalizing that of the fourth-order te rm in the anharmonic oscillator; corresponding to the square of angular fre quency, varies in proportion as T-c - T below T-c. We obtain, from the micr oscopic point of view, the result that the softening in crystal is raised b y the "breaking of the theorem of equipartition of energy,'' as the result of the anharmonic motion.