The development of the technique of preferential adsorption1 has made possible the preparation of enriched ortho-H2 and para-D2 samples and the growing of crystals with concentrations X of the J = 1 species close to 100%. In such crystals a phase transition occurs at a transition temperature T λ which, in zero-pressure H2 and D2 at X = 1, is equal to about 3.0 and 4.0 K, respectively, and which decreases with decreasing X. 2, 3 In the ordered phase the rotational angular momenta of the J = 1 molecules are aligned in a way analogous to the spins in antiferromagnetic systems, and a long-range order exists. The stable form of the disordered phase above T λ has the same hcp structure as the J = 0 solids, but the structure of the ordered phase is fcc.4, 5 There is an interplay between the order-disorder and the structural phase changes, and hysteresis effects occur upon repeated cooling and warming of a sample through the transition region.4–6 Below a critical concentration X c of about 50–55%, the long-range order disappears, and the J = 1 molecules form a quadrupole glass characterized by fixed but randomly distributed orientations.7, 8
KeywordsOrientational Order Ultrahigh Pressure Solid Hydrogen Pair Frame Exact Ground State
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