Orientational disorder modeling in mixed-stack molecular crystals

Abstract

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.

Structure, disorder, and phase transition in ternary crystals of anthracene, phenanthrene, and tetracyanobenzene, AxPh1−xTCNB

“…10,17,22 The equation represents a surface, spanned in the configurational space of lattice parameters ͑a, b, c, and ␤͒, molecular orientations ͑three i angles͒ and two parameters which characterize the disorder ͑⌽ 0 and x͒. The energy function has been minimized with respect to the lattice parameters and molecular orientations and such optimized surface is presented in Fig. 14, in coordinates ⌽ 0 and x.…”

“…The systems are almost ideal to study the relationships between orientational disorder and phase transitions and this has been well documented in our recent studies on the orientational disorder modeling. 17 The crystal of Ph-TCNB, although isomorphic with A-TCNB and N-TCNB, does not exhibit a phase transition, and this can be related to the statistical disorder of Ph molecules. As the Ph molecule has no inversion-center symmetry, it has to occupy, statistically, a special position in C2/m structure.…”