Structural study of Organic Complexs where Neutral ionic transition occurs

利用X射线衍射技术，我们直接观察了四硫富瓦烯-对-四氯苯醌（TTF-CA）和2，6-二甲基四硫富瓦烯-对-四氯苯醌（DMTTF-CA）在温度和压力诱导下的中性离子转变（NIT）的结构变化。对于TTF-CA，低于84 K的Tc，分子内键长的变化验证了分子离子性从0.4（中性相）到0.7（离子相）的增量。在低于84 K的离子（I）相中，TTF和CA分子由于自旋Peierls不稳定性而向相反的方向移动，形成畸变率为3.3%的磁性单重态二聚体。分子二聚化的三维有序性使TTF-CA的空间群从P2_1/n（中性）变为Pn（离子）。在中性-离子转变温度（65 K）以下，观察到了DMTTF-CA的晶胞参数、各向异性温度因子和键长的变化，这些变化表明：在低温阶段，1）晶体中中性部分和离子部分共存，2）分子离子性增加，以及3）离子部分的分子二聚化，其幅度与TTF-CA相似，没有观察到预测的N-I共存相的分级超晶格。考虑到链间静电相互作用以及低温和高压下晶格参数的变化，这种共存是可以理解的。

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.