Computer simulations of a Lennard-Jones model for Ar_1—x(N₂)_x: A prototype system for quadrupolar glasses

Abstract

Recent theoretical studies of orientational ordering in pure and diluted nitrogen crystals are summarized. While pure N₂ has a first order phase transition from a plastic crystal to a phase with long-range orientational order, dilution with argon atoms leads to a quadrupolar glass phase. Monte Carlo simulations are used to study these phases, considering also the behavior of isolated N₂ impurities in Ar crystals. It is shown that a simple model that neglects electrostatic interactions 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 Monte Carlo process. These simulations also give detailed information on the role of translation-rotation coupling in these systems. A brief comparison with various theoretical concepts about quadrupolar glasses is also made.

Keywords:

• Nitrogen crystals
• Monte Carlo process
• phase transition