ISOTOPE EFFECT ON THE DIELECTRIC PHASE-TRANSITIONS OF 5-BROMO-9-HYDROXYPHENALENONE

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 similar to 37 K) and lock-in transition (T-C similar to 20 K). With increasing proton concentration (x), both of the critical temperatures decrease and the transitions disappear at x(IC) similar to 0.65 and x(C) similar to 0.25, respectively. This observation suggests that intramolecular tunneling motion of the protons significantly influences the structural phase transitions via the molecular tautomerization process.