Quantum ferroelectricity in charge-transfer complex crystals

Abstract

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.

Crystal Structure, Raman Spectroscopy and Dielectric Properties of New Semiorganic Crystals Based on 2-Methylbenzimidazole

“…A small value of Curie-Weiss constant in MBI-phosphate-2 reflects weakness of dielectric anomaly. Note that analogous relatively small increase of dielectric constant has been observed in [16]. The large value of T 1 can be explained by the fact that MBI-phosphate-2 consists of light atoms.…”

“…Note that quantum paraelectric behavior has been observed also in organic and semiorganic materials [14][15][16]66]. In semiorganic molecular crystal tetrathiafulvalene+halogen-substituted quinone (TTF+QBr 2 I 2 ) T 1 = 71 K, C CW = 6300 K [16]. A small value of Curie-Weiss constant in MBI-phosphate-2 reflects weakness of dielectric anomaly.…”

“…In particular, in rutile TiO 2 T 1 = 135 K, C CW = 39500 K [65]. Note that quantum paraelectric behavior has been observed also in organic and semiorganic materials [14][15][16]66]. In semiorganic molecular crystal tetrathiafulvalene+halogen-substituted quinone (TTF+QBr 2 I 2 ) T 1 = 71 K, C CW = 6300 K [16].…”

Evaluating the Energetic Driving Force for Cocrystal Formation

“…As well as the I···S halogen bonds that are formed in the cocrystal, the large stability of this cocrystal is most likely related to charge transfer between the stacked, planar donor and acceptor molecules. 44 The next most stable halogen bonded cocrystals (Figure 4b–d) have no other obvious strong intermolecular interactions, so most likely gain their stability through the halogen bonds themselves. These involve highly polarized halogen atoms, such as the iodine in 1,4-diiodo-2,3,5,6-tetrafluorobenzene (ISIHUN, DIVCUH) or the bromine in N -bromosuccinimide (IBIYUP).…”

Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules

“…Γ can implicitly accounts for the effect of electrostatic interactions and intramolecular vibrations, as discussed in Sections 2.3 and 2.5. Figure 3 shows P el , calculated with Equation (11), and the ∆P el (δ, Γ) = P el (δ, Γ) − P el (0, Γ) difference, computed for every fixed value of Γ, in panels a and b, respectively. In the regular stack P el has a discontinuity from 0 (N phase) to 0.5 (I phase) at Γ c ≈ −0.666t.…”