Order-disorder, tunnelling and phonons in structural phase transitions

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Abstract
Starting from a general Hamiltonian of pair-coupled anharmonic oscillators and using the concept of local normal cordinates, a unified dynamical theory of structural phase transitions is proposed. By making use of the variational procedure, the model pseudospin - phonon Hamiltonian, describing the statistical order-disorder, the tunnelling and the phonon vibrations of active atoms is formulated. The qualitative and the detailed numerical analysis show that the structural phase transition can be either of the order-disorder, or of the displacive or of the mixed type, depending on both the reduced lattice coupling strength and the zero-point vibrational energy.