Der Einfluß des elastischen Gitters auf den Orientierungsübergang von festem Ortho-Wasserstoff in Molekularfeldnäherung |
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Authors: | S Diederich |
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Institution: | 1. Institut für Theoretische Physik III, Justus-Liebig-Universit?t Gie?en, Leihgesterner Weg 108, D-6300, Gie?en, Bundesrepublik Deutschland
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Abstract: | We calculate the influence of the elastic lattice on the orientational order-disorder transition in fcc solid ortho-hydrogen. The angular momentaJ = 1 of the molecules are coupled by quadrupole-quadrupole interaction which we reduce within the space group Pa 3 to the Ising model approximation. Local mechanical equilibrium requires the lattice to adjust itself to the forces caused by the inhomogeneous fluctuations in the angular momentum configuration. Following Wagner 6] this results in a 4-angular-momentum interaction with long range and short range contributions depending on the one-phonon Green function of the lattice. We use a Debye phonon spectrum and the molecular field approximation in order to obtain numerical results. According to the first order nature of the phase transition we get a pressure drop at the critical pointT c of 9.7 bar. The experimental value is estimated to be 8 bar. Compared to the rigid lattice the phase transition on the elastic lattice is smoothed out by the short range contribution. This corresponds to a reduction of the discontinuity of the order parameter at the critical pointT c by about 13%. The phase transition will vanish completely if we increase the magnitude of the short range terms by a factor of three. |
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