Special Issue "The Neutral–Ionic Phase Transition"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystal Engineering".

Deadline for manuscript submissions: closed (1 August 2017)

Special Issue Editors

Guest Editor
Prof. Dr. Anna Painelli

Dipartimento di Chimica, Parma University, Italy
Website | E-Mail
Interests: molecular functional materials; organic dyes and aggregates; linear and non-linear optical spectroscopy; charge-transfer crystals; electron and energy transfer; multistability and phase transitions
Co-Guest Editor
Prof. Dr. Alberto Girlando

Dipartimento di Chimica, Parma University, Italy
Website | E-Mail
Interests: molecular functional materials; organic semiconductors; organic superconductors; charge-transfer crystals; neutral–ionic phase transition; optical spectroscopy; chemical physics of organic solid state

Special Issue Information

Dear Colleagues,

The neutral–ionic phase transition was discovered in mixed-stack charge-transfer crystals almost 40 years ago. The transition, induced by temperature, pressure or light, involves two interrelated instabilities: (a) the crossover between a band-insulator (the neutral phase) and a Mott-insulator (the ionic phase), through a marginally metallic state; and (b) the lattice dimerization induced by a pure-Peierls mechanism close to the neutral–ionic interface and by a spin-Peierls mechanism far in the ionic regime. The subtle interplay between strongly correlated electrons and phonons in reduced dimensions drives the appearance of multiple competing phases, divergent responses, collective phenomena, ferroelectricity, multiferroicity, anomalous metallic states, etc., making the neutral–ionic phase transition and related materials an interesting playground for theoretical and experimental investigations.

The Special Issue will provide an international forum to cover a broad description of theoretical and experimental studies on the phenomenology of the transition, its mechanism and the properties of related materials. Scientists working in a wide range of disciplines are invited to contribute with original papers or short reviews on their activity in the field. The topics summarized under the keywords broadly cover the greater number of relevant sub-topics.

Prof. Dr. Anna Painelli
Guest Editor

Prof. Dr. Alberto Girlando
Co-Guest Editor

Manuscript Submission Information

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Keywords

  • Valence and structural instabilities in charge-transfer crystals
  • Correlated electrons and electron–phonon coupling
  • Photoinduced phase transitions, multistability and domain-walls
  • Organic ferroelectric and multiferroics
  • Collective electron-transfer and excitations

Published Papers (6 papers)

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Research

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Open AccessArticle Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules
Crystals 2017, 7(5), 144; https://doi.org/10.3390/cryst7050144
Received: 3 April 2017 / Revised: 2 May 2017 / Accepted: 7 May 2017 / Published: 16 May 2017
Cited by 4 | PDF Full-text (14066 KB) | HTML Full-text | XML Full-text
Abstract
Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π-electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with
[...] Read more.
Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π -electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with variable ionicity 0 < ρ < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous ρ on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessArticle Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals
Crystals 2017, 7(4), 108; https://doi.org/10.3390/cryst7040108
Received: 15 March 2017 / Revised: 4 April 2017 / Accepted: 6 April 2017 / Published: 11 April 2017
Cited by 6 | PDF Full-text (4332 KB) | HTML Full-text | XML Full-text
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
[...] Read more.
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. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessArticle Quantum Phenomena Emerging Near a Ferroelectric Critical Point in a Donor–Acceptor Organic Charge-Transfer Complex
Crystals 2017, 7(4), 106; https://doi.org/10.3390/cryst7040106
Received: 13 March 2017 / Revised: 1 April 2017 / Accepted: 1 April 2017 / Published: 10 April 2017
Cited by 4 | PDF Full-text (718 KB) | HTML Full-text | XML Full-text
Abstract
When a second-order transition point is decreased to zero temperature, a continuous quantum phase transition between different ground states is realized at a quantum critical point (QCP). A recently synthesized organic charge-transfer complex, TTF-2,5-QBr2I2, provides a platform for the
[...] Read more.
When a second-order transition point is decreased to zero temperature, a continuous quantum phase transition between different ground states is realized at a quantum critical point (QCP). A recently synthesized organic charge-transfer complex, TTF-2,5-QBr 2 I 2 , provides a platform for the exploration of the quantum phenomena that accompany a ferroelectric QCP. Here, we summarize the recent results showing the quantum phenomena associated with the ferroelectric QCP in TTF-2,5-QBr 2 I 2 . Whereas the enhanced quantum fluctuations lead to quantitative changes in the critical exponents of the critical phenomena, they qualitatively change the nature of the domain-wall kinetics from thermally activated motion to temperature-independent tunneling motion. The present findings highlight the great influence of quantum fluctuations on the low-temperature physical properties and suggest that TTF-2,5-QBr 2 I 2 is a model system for the uniaxial ferroelectric QCP. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessArticle Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics
Crystals 2017, 7(1), 17; https://doi.org/10.3390/cryst7010017
Received: 30 November 2016 / Revised: 29 December 2016 / Accepted: 3 January 2017 / Published: 7 January 2017
Cited by 3 | PDF Full-text (3103 KB) | HTML Full-text | XML Full-text
Abstract
The neutral-ionic phase transition in TTF-CA was investigated by steady-state and time-resolved infrared spectroscopy. We describe the growth of high-quality single crystals and their characterization. Extended theoretical calculations were performed in order to obtain the band structure, the molecular vibrational modes and the
[...] Read more.
The neutral-ionic phase transition in TTF-CA was investigated by steady-state and time-resolved infrared spectroscopy. We describe the growth of high-quality single crystals and their characterization. Extended theoretical calculations were performed in order to obtain the band structure, the molecular vibrational modes and the optical spectra along all crystallographic axes. The theoretical results are compared to polarization-dependent infrared reflection experiments. The temperature-dependent optical conductivity is discussed in detail. We study the photo-induced phase transition in the vicinity of thermally-induced neutral-ionic transition. The observed temporal dynamics of the photo-induced states is attributed to the random-walk of neutral-ionic domain walls. We simulate the random-walk annihilation process of domain walls on a one-dimensional chain. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Review

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Open AccessReview Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions
Crystals 2017, 7(10), 285; https://doi.org/10.3390/cryst7100285
Received: 1 August 2017 / Revised: 12 September 2017 / Accepted: 13 September 2017 / Published: 23 September 2017
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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
[...] Read more.
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. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessReview Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions
Crystals 2017, 7(5), 132; https://doi.org/10.3390/cryst7050132
Received: 1 April 2017 / Revised: 30 April 2017 / Accepted: 8 May 2017 / Published: 11 May 2017
Cited by 3 | PDF Full-text (10089 KB) | HTML Full-text | XML Full-text
Abstract
Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied
[...] Read more.
Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied pressure, which is called an NI transition. This NI transition can also be induced by photoirradiation, in which case it is known as a prototypical ‘photoinduced phase transition’. In this paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed by molecular deformations and displacements, which play important roles in the stabilization of photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are also produced and stabilized by molecular deformations and displacements. However, the time characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization in the I phase and the generation of a large macroscopic polarization in the N phase. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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