Orientational ordering in solid ortho–para H2 mixtures and analogy with a spin glass

A review is presented of our understanding of the orientational ordering in solid hydrogen, where ortho-H2 molecules are subject to anisotropic electric quadrupole–quadrupole interactions. This review pertains to solid H2 at low pressures and is concerned mostly with NMR experiments. After a discussion of the phase transition from the disordered hexagonal close-packed (hep) to the long-range ordered cubic phase, a description is given of experimental investigations of the ordering in the hep phase for ortho concentrations X < 0.53. This ordering appears to take place continuously as the temperature decreases. The question as to whether or not this phase is to be called a quadrupolar glass phase is addressed, and comparison is made with the orientational freezing in other compounds. A theoretical analysis is given to show that the ortho–para mixture is the analog of a spin glass in a random field for which no phase transition is expected.