INCOMMENSURATE-COMMENSURATE PHASE-TRANSITION OF INTRAMOLECULAR HYDROGEN-BONDED SYSTEM - 5-BROMO-9-DEUTEROXYPHENALENONE (D-BRHPLN)

Low-temperature X-ray Weissenberg photographs of deuterated 5-bromo-9-hydroxyphenalenone (d-BrHPLN) were taken at 14.4 K to investigate the phase transition of an intramolecular and isolated hydrogen-bonded system composed of a tautomeric molecule. The system has one hydrogen bond in a rigid molecular skeleton, with the hydrogen atom being replaced by deuterium. The superlattice reflection is found at (h k+0.5 0) at 14.4 K, and it starts to deviate from the commensurate position above T-C=22.8 K on heating. The transition is of the first kind and is characterized by a commensurate-incommensurate transition. The peak shift defined as 0.5+delta was also measured. Above T-I=34.5 K, the profile becomes very broad and shows diffuse scattering behavior. The transition is of the second kind and is characterized by a so-called incommensurate-normal phase transition.