Plastic crystals represent a unique compound class that is often encountered in molecules with globular structures. The highly symmetric cubic crystal structure of plastic crystals endows these materials with multiaxial ferroelectricity that allows a three-dimensional realignment of the polarization axes of the crystals, which cannot be achieved using conventional molecular ferroelectric crystals with low crystal symmetry. In this work, we focused our attention on malleability as another characteristic feature of plastic crystals. We have synthesized the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate(III) and tetramethylammonium bromotrichloroferrate(III), and discovered that free-standing translucent films can be easily prepared by pressing powdered samples of these compounds. The thus obtained polycrystalline films exhibit ferroelectric polarization switching and a relatively large piezoelectric response at room temperature. The ready availability of functional films demonstrates the practical utility of such plastic/ferroelectric crystals, and considering the vast variety of possible constituent cations and anions, a wide range of applications should be expected for these unique and attractive functional materials.
An organic field-effect transistor was fabricated using an anthracene−tetracyanoquinodimethane (AN− TCNQ) single-crystal charge-transfer (CT) complex as the semiconductor material. The CT complex showed molecular dynamics related to reorientational motion of an AN molecule at room temperature. The transfer characteristics drastically changed when the temperature decreased from room temperature to 90 K: n-type → ambipolar-type → p-type → ambipolar-type. From thermal analysis, AN−TCNQ underwent two phase transitions when the temperature decreased from room temperature to 120 K. To clarify the mechanism of the drastic changes of the transfer characteristics, the crystal and electronic structures of AN−TCNQ at various temperatures were investigated by X-ray structure analysis, optical measurements, and magnetic measurements. It was found that the drastic changes of the transfer characteristics originated from changes in the electronic structure accompanied by the molecular dynamics.
Single-component crystals of both
(phthalocyaninato)nickel(II)
(Ni(Pc)) and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane
(F2TCNQ) are typical band insulators. However, the contact
interface between them demonstrates metal-like transport properties.
Although Ni(Pc) and F2TCNQ are an electron
donor and an acceptor, respectively, the combination of these two
components does not yield any charge transfer (CT) complex crystals.
Infrared spectra show that the highly conductive feature originates
from charge injection at the contact interface. The thermoelectric
power of the mixed powder reveals that the transport at the contact
interface is dominated by the holes in the Ni(Pc)
crystal.
The
charge-transfer (CT) complex 3,3′,5,5′-tetramethylbendizine-7,7,8,8-tetracyanoquinodimethane
was found to display the thermosalient effect, in which crystals exhibit
mechanical responses due to temperature changes. Such responses are
rarely observed with CT complexes. Furthermore, while this crystalline
complex was known to have an interesting phase transition in which
the charge distribution changes at low temperatures, some of the physical
properties of single crystals of the material were unknown, because
the crystal is broken during the thermosalient event. In this study,
we used a polymer-coating method to prevent cracking of the crystal,
allowing us to observe drastic changes in resistivity and ferroelectric-like
dielectric response accompanied by the phase transition at low temperature.
This result suggests that organic solids with mechanical response
and electric and magnetic functions can be produced, since CT complexes
exhibit various electric and magnetic functions and physical properties.
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