MICROSCOPIC APPROACH TO THE QUADRUPOLAR-GLASS PROBLEM

We report results of the microscopic approach to the quadrupolar-glass problem in mixed molecular crystals formed by linear and spherical mo lecules. The temperature-concentration analysis of the orientational o rder parameter derived from NMR spectroscopy data is given on the two- component vector model of site-diluted ortho-para-hydrogen-type mixtur es. Two features of the short-range orientational ordering, characteri stically different from the spin-glass case, namely, incomplete freezi ng and residual ordering, respectively, at low and high temperatures a re discussed. It is established that the first effect is due to spatia l correlated local molecular and reaction fields and the second caused by quadrupolar intrinsic field. A zero-field freezing temperature is estimated taking into account the threshold concentration for quadrupo lar glass formation.