crystals Review Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions Marylise Buron-Le Cointe 1, Eric Collet 1 ID , Bertrand Toudic 1, Piotr Czarnecki 2 and Hervé Cailleau 1,* 1 Institut de Physique de Rennes, Université de Rennes 1-CNRS, UMR 6251, 263 Avenue du Général Leclerc, 35042 Rennes CEDEX, France; [email protected] (M.B.-L.C.); [email protected] (E.C.); [email protected] (B.T.) 2 Faculty of Physics, A. Mickiewicz University, ul.Umultowska 85, 61-614 Poznan, Poland; [email protected] * Correspondence: [email protected]; Tel.: +33-2-2323-6056 Academic Editors: Anna Painelli and Alberto Girlando Received: 1 August 2017; Accepted: 13 September 2017; Published: 23 September 2017 Abstract: Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is the most spectacular manifestation of electronic ferroelectricity, in connection with a high degree of covalency between alternating donor and acceptor molecules along stacks. In addition, a charge-transfer instability from a quasi-neutral to a quasi-ionic state takes place concomitantly with the stack dimerization, which breaks the inversion symmetry. Moreover, these systems exhibit exceptional one-dimensional fluctuations, with an enhancement of the effects of electron-lattice interaction. This may lead to original physical pictures for the dynamics of pre-transitional phenomena, as the possibility of a pronounced Peierls-type instability and/or the generation of unconventional non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linear responsiveness. These different aspects will be discussed through the structural and dynamical features of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and conclude by their interplay with the photo-induced dynamics. Keywords: neutral-ionic phase transition; structural changes; symmetry breaking; Landau phenomenology; dynamical properties; quantum vs. thermal effects; electronic ferroelectricity; P-T phase diagram; photo-induced phase transitions; ultrafast dynamics 1. Introduction Despite the Neutral-Ionic (N-I) phase transition was discovered a long time ago, and that a large number of progresses have been carried out since through numerous experimental and theoretical works, many key physical features are hand in hand with many today’s science topics: Mott physics, electronic ferroelectricity, excitations in one-dimension (1D), light or electric field pulse control, ... The story has begun with the observation, by Torrance et al. in 1981, of a marked color change under pressure of several mixed-stack organic charge-transfer (CT) materials [1], as illustrated by optical microscopy in Figure1. In contrast with segregated-stack CT crystals giving rise to the well-known family of 1D organic conductors, in the mixed-stack CT crystals electron donor (D) and acceptor (A) molecules alternate along the same stack [2]. A partial degree of charge transfer q, in other words Crystals 2017, 7, 285; doi:10.3390/cryst7100285 www.mdpi.com/journal/crystals Crystals 2017, 7, 285 2 of 37 a partial ionicity, results from the hybridization of the D and A molecular orbitals along stacks with +q −q +q −q +q −q +q −q a highCrystals degree 2017 of, 7 covalency:, 285 ... D A D A D A D A ... [3]. The color change2 of observed 37 throughCrystals the 2017 N-I, 7 phase, 285 transition (Figure1) has been demonstrated to originate from an unusual2 of 37 large increasehigh of degree the degree of covalency: of charge … D transfer,+q A−q D+q A i.e.,−q D the+q A− ionicityq D+q A−q … [4 ,[3].5], The from color a quasi-neutral change observed molecular through state high degree of covalency: … D+q A−q D+q A−q D+q A−q D+q A−q … [3]. The color change observed through the N-I phase transition (Figure 1) has been demonstrated to originate from an unusual large increase with athe low N-I q, phase stable transition at low pressure,(Figure 1) has to abeen quasi-ionic demonstr stateated to with originate a high from q, stablean unusual at high large pressure. increase This of the degree of charge transfer, i.e., the ionicity [4,5], from a quasi-neutral molecular state with a low is in agreementof the degree with of charge the gain transfer, under i.e., pressure the ionicity in [4,5], the Madelungfrom a quasi-neutral electrostatic molecular energy state which with a competeslow q, stable at low pressure, to a quasi-ionic state with a high q, stable at high pressure. This is in with theq, stable ionization at low cost pressure, of a DAto a pair. quasi-ionic In a simple state with physical a high picture, q, stable a at N-I high boundary pressure. may This beis definedin agreement with the gain under pressure in the Madelung electrostatic energy which competes with agreement with the gain under pressure in the Madelung electrostatic energy which competes with whenthe ionization ionicity qcost crosses of a DA a criticalpair. In valuea simple of phys 1/2.ical Actually, picture, a the N-I N bounda phasery is may considered be defined to when be a band the ionization cost of a DA pair. In a simple physical picture, a N-I boundary may be defined when insulatorthe ionicity (nonmagnetic) q crosses anda critical the I value phase of a 1/2. Mott Actually, insulator the (magnetic),N phase is considered which originally to be a band meets the the ionicity q crosses a critical value of 1/2. Actually, the N phase is considered to be a band physicsinsulator of correlated (nonmagnetic) electrons. and Inthe addition, I phase aa Mott dimerization insulator (magnetic), process may which take originally place along meets the the mixed insulator (nonmagnetic) and the I phase a Mott insulator (magnetic), which originally meets the stack givingphysics riseof correlated to polar electrons. DA chains, In addition, mainly ina di themerization I Mott phaseprocess and may generally take place discussed along the mixed in terms of physics of correlated electrons. In addition, a dimerization process may take place along the mixed spin-Peierlsstack giving instability. rise to Consequently,polar DA chains, cooperative mainly in the inter-stack I Mott phase interactions and generally may discussed drive (anti)ferroelectric in terms of stack giving rise to polar DA chains, mainly in the I Mott phase and generally discussed in terms of spin-Peierls instability. Consequently, cooperative inter-stack interactions may drive orderingspin-Peierls phenomena. instability. Consequently, cooperative inter-stack interactions may drive (anti)ferroelectric ordering phenomena. (anti)ferroelectric ordering phenomena. FigureFigure 1. Snapshot 1. Snapshot of the of coexistence the coexistence of N of and N Iand state I atstate TNI atmanifested TNI manifested in tetrathiafulvalene- in tetrathiafulvalene-p-chloranilp- Figure 1. Snapshot of the coexistence of N and I state at TNI manifested in tetrathiafulvalene-p- (TTF-CA)chloranil by color (TTF-CA) changes. by color Adapted changes. with Adapted permission with permission from Reference from Reference [6]. Copyright [6]. Copyright 2003 2003 American chloranil (TTF-CA) by color changes. Adapted with permission from Reference [6]. Copyright 2003 American Physical Society. PhysicalAmerican Society. Physical Society. Figure 2. Chemical forms of the donor and the acceptor molecules for the two DA compounds discussed Figure 2. Chemical forms of the donor and the acceptor molecules for the two DA compounds discussed Figurein2. detailChemical in this review: forms TTF of tetrathiafulvalene, the donor and DMTTF the acceptor dimethyltetrathiafulvalene molecules for the and two CA DAp-chloranil. compounds in detail in this review: TTF tetrathiafulvalene, DMTTF dimethyltetrathiafulvalene and CA p-chloranil. discussed in detail in this review: TTF tetrathiafulvalene, DMTTF dimethyltetrathiafulvalene and CA Since the discovery of this intriguing phase transition, the main experimental investigations p-chloranil.Since the discovery of this intriguing phase transition, the main experimental investigations have been focused on the prototypical compound, the tetrathiafulvalene-p-chloranil (TTF-CA), where have been focused on the prototypical compound, the tetrathiafulvalene-p-chloranil (TTF-CA), where TTF are D and CA are A molecules (Figure 2). At atmospheric pressure, the crystal of TTF-CA TTF are D and CA are A molecules (Figure 2). At atmospheric pressure, the crystal of TTF-CA Sincetransforms the discovery on cooling of from this the intriguing N phase to phase the I one transition, at TNI ≅ 81 the K. main In addition, experimental the crystalline investigations structure have transforms on cooling from the N phase to the I one at TNI ≅ 81 K. In addition, the crystalline structure been focusedof I phase on exhibits the prototypical a ferroelectric compound, array of polar the dimerized tetrathiafulvalene- stacks [7]. Otherp-chloranil mixed stacks (TTF-CA), CT crystals where TTF of I phase exhibits a ferroelectric array of polar dimerized stacks [7]. Other mixed stacks CT crystals are D andexhibiting CA are N-I A phase molecules transition (Figure have2 ).been At discover atmosphericed, with pressure, other structural the crystal organization, of TTF-CA but transforms their exhibiting N-I phase transition have been discovered, with other structural organization, but their on coolingnumber from remains the Nquite phase limited, to the in Iparticular one at Tfor =∼those81 presenting K. In addition, this transition the crystalline at atmospheric structure of number remains quite limited, in particular forNI those presenting this transition at atmospheric pressure.