Self-consistent theory of lattice distortion in solid p-H-2, o-D-2 and HD

A mean-field theory of the lattice distortion in J-even species of solid hy drogens and HD is developed. The Hamiltonian of the system includes contrib utions from two-body and three-body static energy, zero-point phonons, kine tic and potential Energies of quantum rotors and the lattice-rotation coupl ing. The Hamiltonian is described by two Order parameters: the deviation of the axial ratio, c/a, from its ideal value of root8/3 and the orientationa l order parameter. The coefficients of the Landau expansion of the free ene rgy of the system in powers of these order parameters are calculated as a f unction of pressure and temperature. It is shown that the lattice distortio n has a threshold behavior. As follows from preliminary numerical estimates , the lattice remains ideal up to the I-II phase transition.