Filipa Pinto

Raman scattering study of the phase transition sequence in the system

We have measured the polarized Raman spectra of betaine arsenate and deuterated betaine arsenate single crystals in the temperature range 12-300 K. On the basis of the comparative study of Raman spectra of protonated and deuterated compounds and by taking into account the relevant literature data, we proposed a mode assignment for . The isotopic effect in this system is discussed. The analysis of the internal modes of clearly shows the important role played by this unit in the phase sequence of . The strong coupling exhibited by the proton/deuteron modes with the internal modes of provides some evidence for the ordering of the hydrogen bonds at the phase transitions. In this analysis reveals the existence of two types of hydrogen bond related to the mechanisms underlying the phase transitions.

Phase transition sequence in the chiral compound 10FHBBBM7* studied by Raman scattering

Integrated Raman intensity of the stretching mode of the phenyl rings as a function of temperature has been studied in order to determine the phase sequence in a new liquid crystalline compound. It is shown that the Raman technique provides the identification of the phase transitions, although some of them were not observed by macroscopic techniques. In particular, two ferrielectric phases were found from Raman measurements between the SmC* and the SmCA* phases, while by DSC and by dielectric measurements only one phase could be detected. A detailed Raman study under d.c. electric field has shown that in this compound the Raman intensity is weakly influenced by the appearance of disclination lines.

Phase transition in betaine potassium bromide dihydrate studied by Raman scattering

Polarized Raman spectra of betaine potassium bromide dihydrate were studied in the broad spectral range 5-3600 cm −1 and in the wide temperature interval 30-292 K, with special emphasis on the structural phase transition at 160 K, recently discovered. It is shown that this phase transition is related to the freezing of the successive reorientations of the water molecules. Peculiar features and the temperature evolution of hydrogen bonding in this compound are discussed.

Dielectric relaxation behaviour of protonated and deuterated betaine arsenate

This work reports the study of the dielectric relaxation of betaine arsenate (BA) and of strongly deuterated betaine arsenate (DBA92) below room temperature for different values of d.c. electric fields, in the frequency range of 50 Hz-13 MHz. The main contributions for dielectric constant come from low-frequency relaxation modes. Some evidence for the existence of a new antiferroelectric phase in BA was found, stable in a short range of temperatures.

Raman spectra and phase transition in betaine potassium iodide dihydrate

Polarized Raman spectra of betaine potassium iodide dihydrate ([(CH3 )3 N+ CH2 COO- ]2 KI 2H2 O) have been studied in the temperature range 40 - 290 K. The behaviour of betaine external modes and of water internal vibrations provides evidence for a first order phase transition of order-disorder type at 100 K. This phase transition is related to the freezing of the successive reorientations of water molecules. The temperature dependence of the halfwidth of the water stretching band centred at 3489 cm-1 (40 K) obeys an Arrhenius-type law corresponding to different activation energies below and above the critical temperature. The comparison of Raman and dielectric data allows us to identify two types of reorientation of water molecules.

Raman studies in SmCFI* phases

The study of the Raman intensity of the ‘breathing’ mode of the benzene rings as a function of the temperature in smectic phases revealed strong anomalies of the intensities IZZ and IYY in SmCFI2* and SmCFI1* phases, as well as an important shift of the principal optical direction with respect to other smectic phases. The analysis of these results provided evidence for significant structural changes at the SmC*–SmCFI* and SmCFI*–SmCA* phase transitions, because the molecules can no longer be treated as cylindrical rods. No anomalies in the tilt angle were found. Experimental results suggest a relationship between the deviation of the optical axis and the particular structure of the SmCFI* phases. The deviation of IZZ maxima as a function of the rotation angle Ψ and the behaviour evident under an electric field in SmCFI* and SmCA* phases clearly excludes the coexistence of SmCA* and SmC* local domains in SmCFI* phases.

Raman scattering in liquid crystalline compounds displaying ferro-ferri- and antiferroelectric phase transitions

Abstract Integrated Raman intensity of the stretching mode of the benzene rings has been studied as a function of temperature in order to determine the peculiarities of the phase sequence in new chiral liquid crystals.

Betaine potassium iodide dihydrate: a new compound of betaine

Betaine potassium iodide dihydrate, , BKI for short, is a new compound of the aminoacid betaine with a triclinic symmetry and the space group at room temperature. The study of dielectric properties provided evidence for the existence of a structural phase transition occurring around 100 K. The spontaneous electric polarization is zero in both phases. A study of dielectric dispersion disclosed two relaxational modes with different relevance in the high and in the low temperature phases. The main features observed in BKI are consistently described by the Landau theory, by assuming a quadratic coupling between the primary order parameter and the electric polarization.

Hydrogen Bonds Behaviour in Brominated Betaine Calcium Chloride Dihydrate

The results of a comparative Raman scattering study of pure and Br m doped betaine calcium chloride dihydrate (BCCD) in the 12-300 K temperature range are reported. It is shown that bromine weakens hydrogen bonding, preventing in this way the localization of protons at low temperatures. Consequently for a high enough content of bromine, the non modulated ferroelectric phase is missing.

Dielectric Properties and Freezing of Water Molecules Vibrations in Betaine Potassium Bromide Dihydrate

Betaine potassium bromide dihydrate: (CH 3 ) 3 N + CH 2 COO m ·KBr·2H 2 O, for short BKBr, is a compound of the aminoacid betaine with an orthorhombic symmetry and the space group Pbcm at room temperature. The study of the dielectric properties of this compound, in the temperature range 20-300K, provides a clear evidence for the existence of a structural phase transition occurring around 160K. This phase transition is related to the freezing of water molecules degrees of freedom.