Analysis of the relationship between temperature dependence of the libration mode and dielectric relaxation in NaNO2

Low-frequency Raman study of the ferroelectric phase transition in a layeredCuCl4-based organic-inorganic hybrid

“…However, it should be mentioned that the normal two-photon decay mechanism does not describe the data well, and models, such as the Andrade and Porto and Andrade et al disorder models seem to be necessary. 85,86 The behavior of the low-energy modes suggests that the phase transition has displacive character, probably in combination with the established order-disorder phenomena. The actual phase transition has a weak first-order nature, which is in line with the observation of temperature hysteresis in the heat capacity 40 and birefringence experiments.…”

Thermal analysis, phase transitions and molecular reorientations in [Fe(OS(CH3)2)6](ClO4)2

“…The Fourier transform far-infrared (FT-FIR) spectra of title compound within the wavenumber range of 500-50 cm To determine whether the observed phase transitions are correlated with changes in the reorientational dynamics of the OS(CH 3 ) 2 ligands and/or of the ClO 4 -anions, we followed the analysis of the full width at half maximum (FWHM) described by Carabatos-Nedelec and Becker, which is based on the theory used for the damping associated with an order-disorder mechanism [18,19]. The reorientational correlation time s R is the mean time between the instantaneous jumps from one potential well to the other, and it is defined by the following formula:…”

“…is the relaxation time at an infinite temperature T, energy activation E a is the potential barrier height for reorientation and k B is the Boltzmann constant. When x 2 s R 2 ) 1, where x = 2pm is the frequency of a particular phonon mode, the temperature dependence of the FWHM is determined by following expression [19,20]:…”

Raman-scattering study of the order-disorder phase transition in KSCN

“…As the temperature is raised, the triatomic SCN Ϫ ions are thermally activated to be able to flip between ͑110͒ and ͑110͒ directions within the ͓001͔ plane, which correspond to two minima in the orientational potential surface. When the temperature is increased to T c ϭ413 K, the SCN Ϫ ions become able to reorient between these two minima, like the NO 2 Ϫ ions in NaNO 2 above the high-temperature phase transition, 4 and the system undergoes a -type phase transition into a disordered tetragonal phase of D 4h 18 ϪI4/mcm symmetry ͑two units per primitive cell͒. The tetragonal symmetry of the high-temperature phase rests on, in addition to the equivalence of a and b lattice constants, a random distribution of the SCN Ϫ ions between the two orientations of equal minimum energy.…”

“…An off-zone-center phonon anomaly was discussed in the contest of the order-disorder phase transition in systems such as NH 4 Br. 11,12 For KSCN such an instability was suggested in a recent review 1 from the form of the order parameter, but no more direct proof of this has been reported.…”