Solid o-p-Hydrogen: A Comparison of Theories of Statistically Disordered Crystals
The solid hydrogens (p-H2, o-H2; o-D2, p-D2) are the simplest molecular crystals with well known intermolcular forces and they are quantum crystals. The anisotropic forces between the molecules are of purely quadrupolar origin apart from very small additional terms (overlap and van der Waals). In these solids p-H2 and o-D2 molecules occupy their lattice sites with spherical rotational states (J = 0) and the anisotropic forces do not influence them. But o-H2 and p-D2 molecules are in J = 1 rotational states at low temperatures. Pure crystals of these modifications are known to exhibit a phase transition into an ordered phase when cooled down to 3 K. The structure of this phase is the cubic Pa3. The crystal can be compared with an antiferromagnet. While in antiferromagnets magnons are elementary excitations of the magnetic ordering, in o-H2 resp. p-D2 the comparable excitations of the rotational ordering are called librons. Their existence is well established by experiment and theory including anharmonic effects.1
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