COMPUTER-SIMULATIONS OF A LENNARD-JONES MODEL FOR AR1-X(N-2)(X) - A PROTOTYPE SYSTEM FOR QUADRUPOLAR GLASSES

Recent theoretical studies of orientational ordering in pure and dilut ed nitrogen crystals are summarized. While pure N-2 has a first order phase transition from a plastic crystal to a phase with long-range ori entational order, dilution with argon atoms leads to a quadrupolar gla ss phase. Monte Carlo simulations are used to study these phases, cons idering also the behavior of isolated N-2 impurities in Ar crystals. I t is shown that a simple model that neglects electrostatic interaction s and takes only Lennard-Jones interactions into account can describe already many properties in qualitative agreement with experiment. Even the slow dynamics of the quadrupole moments can be modeled by the Mon te Carlo process. These simulations also give derailed information on the role of translation-rotation coupling in these systems. A brief co mparison with various theoretical concepts about quadrupolar glasses i s also made.