Orientational ordering in a dilute system of classical interacting quadrupoles : (N2-Ar) solid mixtures

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Abstract
We present an NMR study of a (N2-Ar) solid mixture (X N2 = 0.67) in which the N2 molecules can be considered as almost classical rotators randomly distributed on an h.c.p. lattice. The orientational ordering of the molecules due to their short range highly anisotropic quadrupolequadrupole interactions presents an original case study of the combined effects of frustration and disorder. At low temperatures the NMR results show that the N2 molecules adopt random frozen-in orientations for which any residual motion occurs at frequencies less than 10-2 Hz. This regime, called an orientational glass, grows continuously on cooling without any detectable transition but it is nevertheless believed to result from a highly cooperative phenomenon