Perovskite-like materials
exhibit desirable photophysical and electric
properties that make them suitable for a remarkable breadth of applications
in electronics and physics. In this contribution, we report on the
multiphase ferroelectric and ferroelastic phenomena in a pyrrolidinium-based
hybrid metal–organic material: (C4H8NH2)3[Sb2Cl9]. The title compound
is the first pyrrolidinium derivative within the halobismuthates(III)
and haloantimonates(III) families that is featured by the ferroelectric
property. From a structural point of view, the crystal structure is
built of [Sb2Cl9]3–
∞ perovskite-like layers, interdigitated by layers of pyrrolidinium
cations. The rich solid-state dynamics of pyrrolidinium cations endowed
(C4H8NH2)3[Sb2Cl9] with a complex sequence of temperature-dependent
phase transitions. Remarkably, polar properties have been found to
occur in all six phases, including room-temperature Phase I. Insights
from variable-temperature single-crystal X-ray diffraction, dielectric
spectroscopy, and T1 spin–lattice relaxation measurements
revealed the general mechanism of most phase transitions, as related
to the progressive ordering of nonequivalent pyrrolidinium cations.
Noncentrosymmetry is probed by room-temperature second harmonic generation
(SHG), while the ferroelectric property was evidenced through P(E) and dielectric measurements. The experimental
values of spontaneous polarization were justified and analyzed in
the context of theoretical values derived from quantum-chemical calculations.
Optical measurements show that the integrity of the sample survives
all of the phase transitions, despite sometimes significant deformations
of the unit cell. The changes of symmetry associated with structural
phase transitions are accompanied by an intriguing evolution of the
ferroelastic domain structure with temperature.
The Fourier transform infrared spectra of the thin layers of 2-methyl-4-nitroaniline (MNA) and its deuterated analog were recorded in the 500-4000 cm(-1) region in the 10-300 K temperature range. Activation energies of the -CH(3), -NH(2), and -NO(2) groups reorientations were estimated. The (1)H-NMR spin-lattice relaxation time, T(1), and the second moment of (1)H-NMR resonance line, M(2), measured in the 80-298 K temperature range, were used to determine the parameters of the -CH(3) group motion. The experimental potential barriers for the amine, nitro, and methyl group reorientations are considered in the context of strengths of the N-H([ellipsis (horizontal)])O, C-H([ellipsis (horizontal)])O intermolecular hydrogen bonds, and other short contacts, recognized recently [U. Okwieka et al., J. Raman Spectrosc. 39, 849 (2008)], and they agree with the barriers calculated by quantum chemical methods. The dynamical disorder found in the MNA crystal in the large temperature range seems to be important from the point of view of its nonlinear optical and other properties.
A brief description of the thermal, structural and dielectric properties of bis(ethylammonium) pentachlorobismuthate(iii) ferroelectric with Ps that equals to 1.4 μC cm−2 at 180 K is presented.
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