Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

• DOI: 10.3390/cryst7040108
• Publication Date: 2017-04-11T15:41:42Z
• Authors: Matteo Masino; Nicola Castagnetti; Alberto Girlando
• Keywords: Inorganic Chemistry; charge-transfer crystals; Neutral-Ionic phase transition; electron-phonon coupling; optical spectroscopy; valence and Peierls instability; General Chemical Engineering; 0103 physical sciences; General Materials Science; Condensed Matter Physics; 01 natural sciences

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.