Neutral-ionic Phase Transition Research Articles

Pressure-Induced Neutral–Ionic Phase Transition in the Mixed-Stack 2:1 Charge-Transfer Complex (EDT-TTF-I2)2TCNQF

We explored the neutral–ionic phase transition in (EDT-TTF-I2)2TCNQF by measuring the electrical conductivity as a function of temperature and hydrostatic pressure (up to 11.8 kbar). At about 4.3 k...

Incorporating Spacer Molecules into the Tetrathiafulvalene-p-Chloranil Charge-Transfer Framework: Modulating the Neutral-Ionic Phase Transition.

The charge-transfer (CT) tetrathiafulvalene-p-chloranil (TTF-CA) crystal, a representative functional organic electronic material, has been the subject of both basic and applied research. This material shows a neutral-ionic phase transition (NIPT) that induces drastic changes in its physical properties. Here, we use this crystal as a framework and demonstrate a method for modulating physical properties of TTF-CA. A number of multicomponent (ternary) CT crystals were obtained by crystallizing TTF-CA with a third molecular species. These complexes all contain molecular sheets formed with TTF-CA; however, the third molecules were differently inserted between these sheets as spacers to induce a variety of physical properties in the CT crystals. Some showed spacer-modified NIPT, while the transition to the ionic state was suppressed in one complex despite the presence of TTF-CA sheets, which indicates that spacer molecules can modulate the physical properties or functions of CT crystals.

Magnetic Sponge with Neutral-Ionic Phase Transitions.

Phase transitions caused by the charge instability between the neutral and ionic phases of compounds, i.e., N–I phase transitions, provide avenues for switching the intrinsic properties of compounds related to electron/spin correlation and dipole generation as well as charge distribution. However, it is extremely difficult to control the transition temperature (T c) for the N–I phase transition, and only chemical modification based on the original material have been investigated. Here, a design overview of the tuning of N–I phase transition by interstitial guest molecules is presented. This study reports a new chain coordination‐polymer [Ru2(3,4‐Cl2PhCO2)4TCNQ(EtO)2]∙DCE (1‐DCE; 3,4‐Cl2PhCO2 − = 3,4‐dichlorobenzoate; TCNQ(EtO)2 2,5‐diethoxy‐7,7,8,8‐tetracyanoquinodimethane; and DCE = 1,2‐dichloroethane) that exhibits a one‐step N–I transition at 230 K (= T c) with the N‐ and I‐states possessing a simple paramagnetic state and a ferrimagnetically correlated state for the high‐ and low‐temperature phases, respectively. The T c continuously decreases depending on the content of DCE, which eventually disappears with the complete evacuation of DCE, affording solvent‐free compound 1 with the N‐state in the entire temperature range (this behavior is reversible). This is an example of tuning the in situ T c for the N–I phase transition via the control of the interstitial guest molecules.

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

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.

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

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.

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

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.

Temperature-Induced Neutral-Ionic Phase Transition in the (EDT-TTF-I2)2TCNQF Mixed-Stack Charge-Transfer Salt

We report the infrared (IR) and Raman study of the 2:1 mixed-stack charge transfer salt (EDT-TTF-I2)2TCNQF, where EDT-TTF = ethylenedithiotetrathiafulvalene and TCNQ = tetracyanoquinodimethane, which undergoes a temperature-induced neutral-ionic phase transition. Polarized infrared (IR) and Raman spectra of single crystals were measured in the 8–293 K temperature range. Temperature variations of vibrational modes that are assigned to both donors and acceptors show that the average degree of charge-transfer is growing continuously from about zero at room-temperature to about 1e at T = 8 K; nevertheless, at about 100 K a regime change is observed. The coexistence of molecular species of different ionicity is detected in the whole temperature range. The IR vibrational features due to electron-molecular vibrational coupling increase their intensities considerably upon cooling giving an evidence of relatively small gradual distortions of crystal structure. Existence of ferroelectric domains is suggested. A scheme of the neutral-ionic phase transition in a 2:1 complex is proposed and discussed.

The excitonic insulator route through a dynamical phase transition induced by an optical pulse

We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor-acceptor organic compounds with a neutral-ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging.

Tuning of Stepwise Neutral-Ionic Transitions by Acceptor Site Doping in Alternating Donor/Acceptor Chains.

The stepwise neutral-ionic (N-I) phase transition found in the alternating donor/acceptor (DA) chain [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (0; 2,3,5,6-F4PhCO2(-) = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N'-dicyanoquinonediimine) was tuned by partly substituting the acceptor DMDCNQI with 2,5-dimethoxy-N,N'-dicyanoquinonediimine (DMeODCNQI), which displays a poorer electron affinity in an isostructural series. The site-doped series comprised [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)1-x(DMeODCNQI)x]·2(p-xylene) for doping rates (x) = 0.05 (0.05-MeO), 0.10 (0.10-MeO), 0.15 (0.15-MeO), and 0.20 (0.20-MeO). The neutral chain [Ru2(2,3,5,6-F4PhCO2)4(DMeODCNQI)]·4(p-xylene) (1), which only contained DMeODCNQI, was also characterized. All site-doped compounds were isostructural to 0 except 1 despite their identical DA chain motif. Except at an x value of 0.20, they displayed a two-step N-I transition involving an intermediate phase. This transition occurred at high temperatures in 0 but shifted to lower temperatures in a parallel manner with increasing doping rate. Simultaneously, each transition broadened with increasing doping rate, leading to a convergence of two transitions at an x value approximating 0.2. Donor/acceptor-site-doping techniques present somewhat different impacts in terms of interchain Coulomb effects.

Quantum ferroelectricity in charge-transfer complex crystals.

Quantum phase transition achieved by fine tuning the continuous phase transition down to zero kelvin is a challenge for solid state science. Critical phenomena distinct from the effects of thermal fluctuations can materialize when the electronic, structural or magnetic long-range order is perturbed by quantum fluctuations between degenerate ground states. Here we have developed chemically pure tetrahalo-p-benzoquinones of n iodine and 4–n bromine substituents (QBr4–nIn, n=0–4) to search for ferroelectric charge-transfer complexes with tetrathiafulvalene (TTF). Among them, TTF–QBr2I2 exhibits a ferroelectric neutral–ionic phase transition, which is continuously controlled over a wide temperature range from near-zero kelvin to room temperature under hydrostatic pressure. Quantum critical behaviour is accompanied by a much larger permittivity than those of other neutral–ionic transition compounds, such as well-known ferroelectric complex of TTF–QCl4 and quantum antiferroelectric of dimethyl–TTF–QBr4. By contrast, TTF–QBr3I complex, another member of this compound family, shows complete suppression of the ferroelectric spin-Peierls-type phase transition.

Random-walk annihilation process of photo-induced neutral-ionic domain walls in TTF-CA

The charge and lattice dynamics at the neutral-ionic phase transition in the organic mixed-stack compound TTF-CA were measured by time-resolved Fourier-transform infrared spectroscopy and modeled by Monte Carlo simulations on a one-dimensional chain. After photoexcitation in the ionic state, we observe transient changes in reflectivity, which follow a stretched-exponential behavior on a microsecond time scale. This implies that the dimerization between the TTF and CA molecules vanishes, creating metastable domains. The time, temperature and intensity dependences can be modeled by a diffusion-annihilation process of neutral-ionic domain walls. Their random walk explains the relaxation process and its characteristic decay with time.

High-frequency nuclear quadrupole resonance apparatus for use in pressure cell

A high-frequency NMR apparatus for use in pressure cell is described. All components of the resonance circuit are set in the pressure cell. This method makes the resonance frequency much less influenced by large stray capacitance residing at the electrical feedthrough of the pressure cell. With the use of this apparatus, a pressure-induced neutral-ionic phase transition in DMTTF-QBr(4) was successfully observed by (79)Br nuclear quadrupole resonance, whose resonance frequency is ∼300 MHz.

Finite temperature neutral-ionic transition and lattice dimerization in charge-transfer complexes: QMC study

We numerically investigate the neutral-ionic (NI) phase transition on the basis of the ionic extended Hubbard model with electron-lattice coupling as well as inter-chain Coulomb interaction. Finite-temperature (T) phase transitions are examined by a quantum Monte Carlo method, within adiabatic approximation for the lattice displacement together with inter-chain mean-field treatment. The NI transition either with or without the electron-lattice coupling is of first-order at low-T and transforms into a cross-over regime with increasing T. We confirm there exist two phases in the ionic region: with and without lattice dimerization, as suggested in recent experiments. The former is ferroelectric, which is rooted in the Coulomb interactions and a spin-Peierls mechanism.

Correlated electrons in soft lattices: Raman scattering evidence of the nonequilibrium dielectric divergence at the neutral-ionic phase transition

Correlated electrons in soft lattices: Raman scattering evidence of the nonequilibrium dielectric divergence at the neutral-ionic phase transition

NQR study of neutral–ionic phase transition and quantum paraelectric state in organic charge-transfer complexes

NQR measurements were performed for the charge-transfer complexes DMTTF–QBr n Cl n −4 under ambient and hydrostatic pressures in order to understand the neutral–ionic phase transition and the quantum critical behavior. The 35Cl NQR spectrum of DMTTF–QCl 4 showed a splitting and a shift below T c, indicating the occurrence of dimerization and charge transfer. The spin-lattice relaxation rate 1/ T 1 showed a peak anomaly around T c, reflecting the critical slowing down. The quantum paraelectric states in DMTTF–2,6-QBr 2Cl 2 and pressured DMTTF–QBr 4 were studied by the 79Br NQR. We found the microscopic evidence for the evolution of the critical fluctuations at low temperatures in the temperature dependence of spin-lattice relaxation rate.

Anomalous Dispersion of Optical Phonons at the Neutral-Ionic Transition: Evidence from Diffuse X-Ray Scattering

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.

Intermediate regime in pressure-induced neutral-ionic transition in tetrathiafulvalene-chloranil

We report a detailed spectroscopic study of the pressure induced neutral-ionic phase transition (NIT) of the mixed-stack charge-transfer (CT) crystal tetrathiafulvalene-chloranil (TTF-CA). We show that the pressure induced phase transition is still first-order and involves the presence of an intermediate disordered phase, defined by the coexistence of two species of different ionicity. Further application of pressure gradually converts this phase into an homogeneous ferroelectric phase with a single ionicity. In addition, we detect strong pretransitional phenomena which anticipate the intermediate phase and are indicative of a precursor dynamic regime dominated by fluctuations.

Neutral–Ionic Phase Transition in DMTTF-QCl4 Investigated by 35Cl NQR

We report a 35 Cl NQR investigation into the neutral–ionic (NI) phase transition in DMTTF-QCl 4 (4,4'-dimethyltetrathiafulvalene- p -chloranil), the end material of the family containing an NI quantum transition system. Four distinct lines with comparable intensities were observed well below T c =65 K; this observation is consistent with the antiferroelectric nature of the ionic phase. The temperature dependences of the spectral shift and splitting below T c are moderate in contrast with the case of TTF-QCl 4 . Low-energy charge-lattice fluctuations are observed around T c as a drastic increase of 1/ T 1 , allowing the observation of the quantum fluctuations in a chemically substituted complex. An analysis of the NQR spectra yields detailed temperature variations in the charge transfer and the concomitant lattice distortion around the NI transition.

Relaxation process in the photoinduced neutral-ionic paraelectric-ferroelectric phase transition in tetrathiafulvalene-p-chloranil

To find characteristic properties of relaxation process in the neutral-ionic, paraelectric-ferroelectric phase transition in the charge-transfer complex, tetrathiafulvalene-$p$-chloranil, we investigate stochastic process in the spin 1 anisotropic Blume-Emery-Griffith model [Phys. Rev. A 4, 1071 (1971)]. We assume that spins obey the Markov process which is described by the master equation with the transfer probability $\frac{1}{\ensuremath{\tau}}\phantom{\rule{0.2em}{0ex}}\mathrm{exp}[\ensuremath{-}(\ensuremath{\beta}∕2)\ensuremath{\Delta}H]$, where $\ensuremath{\Delta}H$ is the energy difference for the transfer. We derive time-evolution equations from two standpoints. One is the mean-field approximation (MFA) and the other is an extension of the Saito-Kubo treatment to the spin 1 and anisotropic case, which improves MFA. Solving the equations numerically, we have found highly anisotropic relaxations during the neutral-ionic phase transition: the interchain ordering develops much more slowly than the intrachain one. In contrast, the ionic-neutral phase transition proceeds rather isotropically.

Metastable domains and potential energy surfaces in organic charge-transfer salts with neutral-ionic phase transitions

A modified Hubbard model with linear coupling to both lattice phonons and molecular vibrations is applied to the structural and electronic instabilities of organic charge-transfer (CT) salts such as tetrathiafulvalene-chloranil (TTF-CA). The potential energy surface (PES) of the correlated model is found exactly in finite one-dimensional (1D) systems with a mean-field approximation for 3D Coulomb interactions and parameters close to first or second-order phase transitions. The PES near a first-order transition has multiple minima with different values of $\ensuremath{\rho}$, the degree of CT in the ground state. The energy of metastable domains is related to their length $L$, to the discontinuity $\ensuremath{\Delta}\ensuremath{\rho}$ at the transition, and to the energy $2{E}_{w}$ of two domain walls. Sharp and relaxed domain walls are modeled by free spinless fermions coupled to both phonons and molecular vibrations whose ground-state PES is obtained in chains of up to 1000 sites. When $\ensuremath{\Delta}\ensuremath{\rho}$ is sufficiently small, metastable domains become thermally accessible at low temperature where the Boltzmann population of electronic excitations is negligible. The pressure-induced transition of TTF-CA is discussed in terms of an equilibrium between stable and metastable domains with different $\ensuremath{\rho}$, using previous parameters for the temperature-induced transition, where metastable domains are not accessible thermally. In either correlated or uncorrelated models, linear coupling of electrons to slow, harmonic coordinates for lattice and molecular vibrations leads to strongly anharmonic PES near an electronic or structural instability. CT salts with neutral-ionic transitions illustrate competition between a lattice phonon that drives a second-order Peierls transition and molecular vibrations that favor a first-order $\ensuremath{\Delta}\ensuremath{\rho}$.