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 (H(2)ca and D(2)ca for normal and deuterated compounds, respectively) co- crystal, was studied by Cl-35 nuclear quadrupole resonance (NQR). Besides a ferroelectric transition at T-c = 253 K (303 K), a neutral-to-ionic transition was found below 170 K ( 200 K) for Phz -H(2)ca ( Phz-D(2)ca). H-1-N-14 nuclear quadrupole double resonance measurements were also made in order to study the temperature dependent electronic state of Phz-(H/D)(2)ca. N-14 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-D(2)ca, which was obtained by cooling below 188 K, they became 1.50 and 1.95, suggesting a proton transfer from D(2)ca to Phz.