ORIENTATIONAL DISORDER MODELING IN MIXED STACK MOLECULAR-CRYSTALS

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.