Abstract
Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π -electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by this class of crystals, Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer along the stack. The analysis of such a complex yet fascinating phenomenon has required many years of investigation, and still presents some open questions and challenges. We present an updated and extensive summary of the phenomenology of the temperature induced NIT, with emphasis on the spectroscopic signatures of the transition. A much shorter summary is given for the NIT induced by pressure. Finally, we report on the exploration, by chemical substitution, of the phase space of ...DADAD... CT crystals, aimed at finding materials with important semiconducting or ferroelectric properties, and at understanding the subtle factors determining the crystal packing.
Highlights
Organic charge-transfer (CT) crystals occupy a special place among functional molecular crystals.They are among the first examples of low-dimensional systems, since the strongly directional CT interaction between π-electron Donor (D) and Acceptor (A) molecules dominates over the other intermolecular interactions and yields the formation of one-dimensional stacks
T, one reaches a multistability region [51], with a competition between a first order valence instability driven by the 3D Madelung energy and a second order one-dimensional Peierls instability
The initial idea of the Madelung energy as the only driving-force of the phase transition soon had to be integrated with the role of the e-ph coupling, which yields the stack dimerization through the Peierls mechanism
Summary
Organic charge-transfer (CT) crystals occupy a special place among functional molecular crystals. The presence of low-lying CT excitation(s) gives rise to many different transport, optical and magnetic properties These properties strongly depend on the molecular stacking/packing and on degree of charge-transfer or ionicity ρ from D to A. We remark that the N and I phases can be distinguished on a more fundamental basis than the ρ value: As it was early recognized, mixed stack CT crystals are subject to the Peierls instability, yielding stack dimerization · · ·DA DA DA· · · [10] Peierls dimerization, implying both electron and spin degrees of freedom (generalized Peierls) may occur at the same time as the ionicity change [10], or independently [11]. The competition between two instabilities, a first-order one driven by the 3D Madelung energy (valence instability, order parameter: ρ) and a second order one, driven by 1D electron-lattice phonon (e-ph) coupling (Peierls instability, order parameter: stack dimerization) is what makes NIT such a complex and intriguing phenomenon
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
