Isotope Effect on the Dielectric Phase Transitions of 5-Bromo-9-Hydroxyphenalenone

Abstract

Dielectric properties have been investigated for proton-deuteron mixed crystals of 5-bromo-9-hydroxyphenalenone with strong intramolecular hydrogen bonds. The fully deuterated compound shows two successive structural transitions, i.e. , incommensurate (IC) transition ( T IC ∼ 37 K) and lock-in transition ( T C ∼20 K). With increasing proton concentration ( x ), both of the critical temperatures decrease and the transitions disappear at x IC ∼ 0.65 and x C ∼ 0.25, respectively. This observation suggests that intramolecular tunneling motion of the protons significantly influences the structural phase transitions via the molecular tautomerization process.