³⁵Cl NQR of an organic ferroelectric phenazine chloranilic acid co-crystal

A ferroelectric transition of phenazine (Phz) chloranilic acid (H₂ca) co-crystal was studied by ³⁵Cl nuclear quadrupole resonance (NQR). The single NQR line observed at 36.689 MHz at 295 K, split into a doublet below the ferroelectric transition temperature T_c=253 K in accord with the inversion-symmetry breaking at the H₂ca molecular center in the crystal. The continuous variation of the splitting-width through T_c suggests a second-order nature of this phase transition. The NQR frequency is consistent with the crystal structure in which the two acid hydrogen atoms of H₂ca remain as attached to the quinone molecules. The spin-lattice relaxation time T₁ showed a decrease with decreasing temperature in the ferroelectric phase. This suggests the existence of a fast motion of nearby atomic group, which is a cause of electric field gradient fluctuation.