Structural study of Organic Complexs where Neutral ionic transition occurs

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Using X-ray, we directly observe structural changes associated with temperature and pressure induced neutral-ionic transition (NIT) in tetrathiafulvalene-p-chloranil (TTF-CA) and 2,6-dimethyltetrathiafulvalene-p-chloranil (DMTTF-CA). For TTF-CA,below Tc of 84K,changes in intramolecular bond lengths verified a increment in molecular ionicity from 0.4 (neutral phase) to 0.7 (ionic phase). In ionic (I) phase below 84K,TTF and CA molecules, which were originally located at the inversion center in neutral (N) phase, shift in contrary directions owing to the spin-Peierls instability and form a magnetic singlet dimer with a 3.3% distortion. Three dimensional order of the molecular dimerization changes space group for TTF-CA from P2_1/n (neutral) to Pn (ionic). Owing to absence of the inversion symmetry, Ionic TTF-CA become a organic ferroelectrics.In DMTTF-CA,changes in unit cell parameters, anisotropic temperature factors and bond lengths were observed below the neutral-ionic transition temperature of 65K.These changes indicate, in low temperature phase 1) coexistence of neutral and ionic portions in crystal 2) a increase in molecular ionicity and 3) a molecular dimerization in ionic portion whose amplitude is similar to TTF-CA.The predicted staging superlattice in the N-I coexisting phase was not observed. Taking account of interchain electrostatic interaction together with the lattice parameter variations at low temperature and under pressure, this coexistence can be understood.