Phase transition and temperature dependent electronic state of an organic ferroelectric, phenazine–chloranilic acid (1:1)

The isotope effect of hydrogen motion in an organic ferroelectric, phenazine (Phz)–chloranilic acid (H2ca and D2ca for normal and deuterated compounds, respectively) co-crystal, was studied by 35Cl nuclear quadrupole resonance (NQR). Besides a ferroelectric transition at Tc = 253 K (303 K), a neutral-to-ionic transition was found below 170 K (200 K) for Phz–H2ca (Phz–D2ca). 1H–14N nuclear quadrupole double resonance measurements were also made in order to study the temperature dependent electronic state of Phz–(H/D)2ca. 14N NQR parameters suggested that donor orbital populations of the two nitrogen atoms in a phenazine molecule become nonequivalent (1.78 and 1.97) in the ferroelectric phase, while they are both equal to 1.89 in the paraelectric phase. In the ionic phase of Phz–D2ca, which was obtained by cooling below 188 K, they became 1.50 and 1.95, suggesting a proton transfer from D2ca to Phz.