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

Abstract

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.

Probing charge transfer characteristics in a donor–acceptor metal–organic framework by Raman spectroelectrochemistry and pressure-dependence studies

“…The sensitivity of charge transfer transitions to external stimuli such as pressure has been explored in a plethora of discrete systems such as organic charge transfer complexes due to the onset of structure changes, new ground states and Peierls distortions. 39,40 To date, highpressure studies on MOFs and CPs have uncovered unprecedented structural phase transitions 41 and intermetallic charge transfer, 42 in addition to piezochromism in 1D systems whereby increased pressure has been shown to induce changes in the coordination geometry of the metal ion. 43,44 The effect of pressure on ligand-based charge transfer characteristics, and the possibility of pressure-induced N-I transitions in donor-acceptor framework materials, however, remains largely unexplored, thus spiking our interest in investigating potential pressure-dependent changes in [(Zn(DMF)) 2 (TTFTC)(DPNI)].…”

Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(H2B(pz)2)2(phen)]

“…Coupled phase transitions, driven concurrently by different physical phenomena (atomic ordering, soft modes, spin state transitions, ferroic ordering, etc. ), are widespread in crystalline solids and have been investigated in various fields of physics, chemistry, mineralogy, and materials science. − Since phase transitions in crystalline materials always involve some lattice distortion, the most common physical effect that couples them is lattice strain . In this context, spin state transitions in molecular complexes of 3d 4 –3d 7 ions appear particularly interesting due to the large volume strains involved (typically 1–10%). − Thanks to this high dilation, the SCO phenomenon can strongly couple to various instabilities, leading to a great diversity of structure–property relationships.…”

Strong Terahertz Radiation via Rapid Polarization Reduction in Photoinduced Ionic-To-Neutral Transition in Tetrathiafulvalene- p -Chloranil

“…We demonstrate that the efficiency of the terahertz radiation via the PIPT in TTF-CA is much higher than that via the OR in a typical terahertz emitter, ZnTe. Madelung potential, which drives the NI transition [19,20,22]. In the I phase, each molecule has a spin 1/2.…”

“…1(c) because of the spin-Peierls-like instability. The dimeric molecular displacements are three dimensionally ordered, breaking the space inversion symmetry along the a-axis [20,22]. The recent measurement of the polarization-electric-field (P-E) characteristic demonstrates that TTF-CA shows a spontaneous polarization PS along the a-axis with a hysteresis loop in the I phase [23].…”

“…We demonstrate that the efficiency of the terahertz radiation via the PIPT in TTF-CA is much higher than that via the OR in a typical terahertz emitter, ZnTe. along the a-axis [20,22]. The recent measurement of the polarization-electric-field (P-E) characteristic demonstrates that TTF-CA shows a spontaneous polarization PS along the a-axis with a hysteresis loop in the I phase [23].…”