Theory of Phase Transition in Solid Ortho-Hydrogen. I

Abstract
The orientiational phase transition in solid ortho-hydrogen of a face-centred cubic Pa3 structure is theoretically investigated. The method of symmetry-breaking potential is applied with the use of the inverse temperature expansion. The truncated electric quadrupole-quadrupole interaction is taken in the Hamiltonian and the sum of the pair interaction is extended to the fifth neighbour pairs. The expression of the free energy is derived up to third order of the inverse temperature. The rotation-phonon coupling is also taken into account. It is found that a first order phase transition occurs so long as the rotation-phonon couplling constant exceeds a certain critical value.

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