The α–β phase transition in solid N2 has been investigated using the constant pressure Monte Carlo method. This is accomplished by examining both phases in the temperature range 25≤T≤50 K, where they are everywhere at least metastable. It is found that the cubic α phase undergoes an orientational order–disorder transition into a disordered cubic phase as the temperature is increased to T=41 K, and remains in this state until melting. Similarly the orientationally disordered hexagonal phase persists from melting down to 33 K, where it undergoes a transition into a hexagonal structure with short-range orientational order.

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Articles you may be interested inInfluence of anthracene doping on the order-disorder phase transition in phenanthrene AIP Conf. Proc. 1512, 44 (2013); 10.1063/1.4790902 Anisotropic displacive to order-disorder crossover of the orientational phase transition in anthracene tetracyanobenzene crystal J. Chem. Phys. 90, 7346 (1989); 10.1063/1.456214Static disorder in crystals of the anthracene-tetracyanobenzene charge transfer complex Phase transition effects: A crystallographic characterization of the temperature dependency of the crystal structure of the 1:1 charge transfer complex between anthracene and tetracyanobenzene in the temperature range 297 to 119 K The structure of ternary, charge-transfer crystals of anthracene, phenanthrene, and tetracyanobenzene, A x Ph 1Ϫx TCNB, in a wide range of concentrations has been studied. We have focused on the problem of substitutional and orientational disorder of anthracene and phenanthrene molecules and its influence on the orientational instability. The crystal structure determination has been performed at room temperature and 120 K, and the refinement procedures allowed us to determine the orientational disorder for different concentrations. The phase diagram for the orientational phase transition with symmetry change, C2/m to P2 1 /a, has been determined and a critical concentration (x c ϭ0.74) was found. The disorder and the phase diagram has been successfully modeled by a minimization of the crystal energy using the ''split molecule'' concept and the virtual crystal approximation to mimic both orientational and substitutional disorders. The ternary crystals, their structure, disorder, and phase transitions are well understood in terms of a competition between orientational disorder of anthracene molecules and local stresses created by the substitutional disorder ͑with statistical orientations͒ of phenanthrene molecules. A possibility for an existence of molecular orientational glassy state is suggested for concentrations below the critical one.

The paper presents a simple model to mimic orientational disorder in molecular solids. The model is based on the concept of split molecule, an object which is a superposition of two molecules in orientations which are supposed to mimic orientational distribution. The split molecule is characterized by a disorder angle, an extra parameter which labels disorder states and allows one to calculate free energy of the crystal. The model is simple for applications illustrated by numerical calculations for charge-transfer crystals A-TCNB and N-TCNB, the family of solids where the onset of orientational disorder is responsible for structural phase transitions.