M. R. Chaves

Critical behaviour in (BPI)x(BP)1−x

Abstract Measurements of dielectric constant, dielectric loss and pyroelectric coefficient as a function of temperature have been performed in Betaine Phosphite (BPI), Betaine Phosphate (BP), and mixed crystals of (BPI)x(BP)1−x for x=0.05; 0.15; 0.30 and 0.60. The behaviour observed for the mixed compounds has a good resemblance to that of structural glasses.

Soft Mode in Heteroepitaxial (Ba, Sr)TiO3MgO Thin Films

Polarized Raman spectra of tetragonally distorted (Ba 1-x Sr,)TiO 3 /MgO ferroelectric films were obtained in special scattering geometries using a micro-Raman set-up. The underdamped E(TO) soft mode was unambiguously observed in the films with x = 0.30 and 0.45. The low-temperature behaviour of the E(TO) soft mode for x = 0.45 was found substantially different from those observed in pure crystals and bulk solid solutions.

Dielectric and pyroelectric behaviour in (BPI)x (BP)1−x

Abstract Measurements of dielectric constant, dielectric loss and pyroelectric coefficient as a function of temperature have been performed in mixed crystals of Betaine Phosphite (BPI) and Betaine Phosphate (BP) ((BPI)x (BP)1−x), for x = 0.80, 0.90, 0.94 and 0.97. The obtained results are compared with the ones previously reported for x = 0.00, 0.05, 0.15, 0.30, 0.60 and 1.00. Three regions are apparent: one where the ferroelectric properties are dominant (x > 0.90), another which can be related to a glassy phase (0.30< x <0.90) and a third one mainly associated with an antiferroelectric behaviour of BP (x < 0.30).

FERROELECTRIC AND ANTIFERROELECTRIC MODES IN A NEW CHIRAL THIOBENZOATE LIQUID CRYSTAL

Abstract This work concerns the dielectric relaxation study of a new compound containing a thiobenzoate group with a fluoro substituent, which presents a sequence of antiferro-, ferri-, ferroelectric and SCα phases. The polar character of these phases was confirmed by optical hysteresis loops. Three modes with a nearly monodispersive behaviour were identified: the soft mode, the Goldstone mode and a non-collective azimuthal mode. The soft mode softens above the SA - SCα transition and triggers another mode within the SCα phase. Moreover, a collective anti-phase azimuthal mode appears and may be the origin of the onset of the antiferroelectric SCA phase. A strong influence of surface interactions on the thermal hysteresis and on the dielectric constant was evident in this work.

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54 A is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.

Dielectric constant and phase transition sequence in betaine calcium chloride dihydrate

BCCD exhibits a sequence of structural phase transitions from a high-symmetry orthorhombic phase to different commensurate or incommensurate phases modulated along [001]. This work reports a study of the dielectric constant at a constant frequency of 10 kHz as a function of temperature. The results are discussed within the scope of Landau theory and reveal the existence of new intermediate high-order commensurate phases, in agreement with recent pyroelectric measurements.

Pyroelectric effect and freezing out of domain structure in betaine arsenate

Abstract This work reports some results concerning the study of pyroelectric effect in Betaine Arsenate ((CH3)3 NCH2 COO.H3 As O4). The pyroelectric coefficient along the crystallographic c-axis reveals two anomalies localized in the temperature range of 110–120 K. The existence of these two anomalies is confirmed by an experimental study of dielectric constant and dielectric loss in the same samples. We interpret the upper anomaly as that between a paraelectric and a mixed phase, i.e. a ferroelectric and structural glass and the lower one as a freezing temperature where the domain wall mobility is frozen out.

Study of lattice dynamics and phase transitions in betaine phosphate by comparison with betaine phosphite via infrared reflectivity spectroscopy

The temperature dependence of infrared reflection spectra of betaine phosphate single crystal is reported for polarizations parallel to the a- and b-axes. The spectra were fitted with the factorized form of the dielectric function. A mode assignment is proposed on the basis of a comparison with the spectra of betaine phosphite and deuterated betaine phosphate. Some very asymmetric phonon lines are observed, and the importance of mode couplings is emphasized. A picture of a tendency towards transverse optical mode softening of the low-frequency spectrum upon approaching either side of the phase transition temperature emerges for the polarization parallel to the b-axis, consistent with the peak observed in the temperature dependence of the dielectric constant. On the other hand, the temperature dependence of the ionic effective charges does not display any appreciable discontinuity at the phase transition, contrary to what is observed at the temperature of the transition to the ferroelectric phase of the parent compound betaine phosphite. This is consistent with the very low level of the spontaneous polarization below in betaine phosphate. The similarities of the behaviours with those of the compounds of the KDP family are discussed.

Competing exchanges and spin?phonon coupling in Eu1?xRxMnO3 (R=Y, Lu)

This work is focused on the phase diagrams and physical properties of Y-doped and Lu-doped EuMnO3. The differences in the corresponding phase boundaries in the (x,T) phase diagram could be overcome by considering a scaling of the Y(3+) and Lu(3+) concentrations to the tolerance factor. This outcome evidences that the tolerance factor is in fact a more reliable representative of the lattice deformation induced by doping. The normalization of the phase boundaries using the tolerance factor corroborates previous theoretical outcomes regarding the key role of competitive FM and AFM exchanges in determining the phase diagrams of manganite perovskites. However, significant differences in the nature and number of phases at low temperatures and concentrations could not be explained by just considering the normalization to the tolerance factor. The vertical phase boundary observed just for Lu-doped EuMnO3, close to 10% Lu, is understood by considering a low temperature Peierls-type spin-phonon coupling, which stabilizes the AFM-4 phase in Lu-doped EuMnO3.

Polar phases in chiral thiobenzoate liquid crystals

Abstract By dielectric measurements, the temperature scan method and studies of optical hysteresis loops, the phase sequences have been elucidated for two new compounds from a series of homologues containing a thiobenzoate group with a fluoro substituent in the first benzene ring (denoted nFHTBBM7). Both compounds (n = 11 and n = 12) exhibit antiferroelectric (Sc A), ferroelectric (Sc) and several ferrielectric (Sc FI) phases, as well as the (Sc α) phase. Also, in the compound 11FHTBBM7, a metastable ferrielectric phase may appear in the temperature range of the antiferroelectric phase. The dielectric relaxation study revealed contributions to the dielectric constant from several modes detected in the temperature range studied. The properties of the relaxation modes and the phase transition sequences of the two compounds have been compared with those of the homologues with n = 9 and n = 10 studied previously.