0000000000082790
AUTHOR
B. Jannot
Étude théorique de la dynamique du réseau de PbTiO3 dans la phase quadratique
Resume La dynamique du reseau du titanate de plomb PbTiO 3 est etudiee en phase quadratique a l'aide d'un modele a coquille tenant compte des polarisabilites electroniques des ions de la maille elementaire et des interactions a longue et a courte portee. Les resultats de nos calculs mettent en evidence le role important joue par l'anisotropie de la polarisabilite electronique de l'oxygene. Il est montre aussi que les courbes de dispersion des phonons calculees dans la premiere zone de Brillouin sont en bon accord avec l'experience.
Underdamped soft phonon in orthorhombic BaTiO3
Abstract Raman scattering measurements are reported in orthorhombic phase and through the orthorhombic-rhombohedral transition of BaTiO3. Spectra reveal the presence of an underdamped low frequency phonon which slightly softens with decreasing temperature and abruptly stiffens at the transition to the rhombohedral phase. Results are discussed in comparison with isomorphous KNbO3 and tetragonal BaTiO3 and are interpreted as due to a dominant displacive mechanism.
Indications of Two Separate Relaxators in the Subphonon Region of Tetragonal BaTiO 3
Significant discrepancies were generally reported in the dynamics of BaTiO3 between results deduced from various experimental techniques. Here dielectric and quasi-elastic light scattering measurements are investigated on high-quality BaTiO3 crystals. We show that two relaxators have to be introduced below the soft-phonon frequency. A complete dielectric dispersion in a very wide frequency range is thus achieved which consistently relies the high-frequency dielectric results and the Raman data. There is also a qualitative agreement between our two-relaxator model and the dynamics which were deduced some time ago from EPR experiments.
Existence of two time scales in the phase transitions of BaTiO3
Abstract We report new Raman scattering measurements in the tetragonal phase of a single domain BaTiO3 crystal. Data reveal the occurence of a large quasi elastic scattering. The Raman profile is analysed in terms of coexisting soft phonon and Debye relaxation.
Influence on the phase transitions of barium titanate of the manganese valency, introduced as a substituant in the titanium site
Abstract A small amount of manganese introduced in BaTiO3 modifies strongly the vibrational and electronic properties of the pure compound. Dielectric response and EPR measurements are reported in order to study the influence of the manganese valency on the ferroelectric-paraelectric phase transition.
DC electrical resistivity of Nb-Doped BaTiO3 and EPR measurements
DC electrical resistivity measurements along the c-axis of Nb-doped BaTiO3 single crystals are presented in a wide range of temperature, covering the four phases, with special attention to the rhomboedral one. The temperature dependence of the conductivity leads to convincing arguments in favour of a polaronic mechanism. Below the orthorhombic-rhomboedral transition point, a strong increase of the resistivity is observed. The correlation of this behavior with EPR analysis in the same temperature range, is consistent with a sharp trapping of the ''free'' electrons, when the temperature decreases.
Dielectric and E.P.R. studies of Mn-doped barium titanate
Manganese introduced in BaTiO3 as a substituant of titanium is able to trap the electrons produced during a sintering under a very low oxygen pressure. However it strongly modifies the vibrational ...
Raman measurements in the intermediate phase of [N(CH3)4]2ZnI4
Abstract Raman spectra and deviation angle of the indicatrix have been measured in the intermediate phase, between the two transition temperatures T c1 = 255 K and T c2 = 217 K. The results are consistent with the existence, in this phase, of large domains, piled up along the c-axis (Pnma notation).
Theory of birefringence and spontaneous polarization in the tetragonal phase of PbTiO3 due to lattice deformation caused by the change of temperature
The microscopic mechanism of birefringence in the tetragonal phase of PbTiO3 is discussed taking account of the dipole-dipole interaction (dipole-dipole effect) due to the ionic and electronic polarizations of the crystal and the spontaneous Kerr effect as a cause of the strong local field acting on the constituent ions, where the lattice deformation due to the change of temperature is also considered. For the spontaneous polarization we use only the first effect. It is found that the birefringence and the spontaneous polarization are in good agreement with the experimental data.
Experimental study of the crystals [N(CH3)4]2Znl4by Raman spectroscopy and optical measurements; comparison with isomorph compounds
Abstract Raman spectra and rotation angle of the indicatrix have been measured in the intermediate phase of the crystal [N(CH3)4]2Znl4 The results are consistent with the existence of large ferroelastric domains piled up along the a axis in a monoclinic phase P1121/a (Pnma notation in the paraelectric phase). The phase sequence (Pnma-P1121/a-P21ca) in this compound is different from other members of [N(CH3)4]2BX4 family.
Theory of spontaneous polarization and birefringence in the tetragonal phase of KNbO3
Abstract The microscopic mechanism of optical anisotropy in the tetragonal phase of KNbO3 is discussed taking account of the dipole-dipole effect due to the ionic and electronic polarizations of the crystal and the spontaneous Kerr effect. The last effect is a cause of the strong local field acting on the constituent ions. For the spontaneous polarization we use only the first effect. It is found that the birefringence and the spontaneous polarization are in good agreement with the experimental data.
Crystal growth and hysteresis phenomena of rubidium tetrachlorozincate Rb2ZnCl4
Abstract Phase transitions in Rb2ZnCl4 crystals grown by various methods are examined with differential scanning calorimetry and dielectric measurements. For most of the studied samples the permittivity along the ferroelectric axis shows a large thermal hysteresis over a wide temperature range including the incommensurate phase and the lock-in point. This hysteresis depends on the presence of different defects which appear during crystal growth or after a thermal and mechanical treatment. It is shown that dielectric measurements are quite well suited to characterize the crystals exhibiting on incommensurate phase and a ferroelectric phase.
Etude theorique de la dynamique du réseau de batio en phase quadratique
Abstract The lattice dynamics of barium titanate BaTiO 3 is discussed quantitatively in the framework of a shell model taking into account the electronic polarizabilities of the constituent ions and including Coulomb and short-range interactions. The results of calculations point out the important role of the anisotropic oxygen polarizability. There is a good agreement between calculated and experimental dispersion curves.
Effect of Hydrostatic Pressure on the Phase Transitions of {N(CH3)4}2ZnCl4-xBrxMixed Crystals
Single crystals of the solid solution system {N(CH 3 ) 4 } 2 ZnCl 4- x Br x have been prepared in a low x region, and the effect of hydrostatic pressure on the phase transitions were measured for x =0.2 and 0.3. The normal-to-incommensurate phase transition temperatures increase linearly with increasing pressure with the rates of 0.126 K/MPa and 0.129 K/MPa for x =0.2 and 0.3 crystals, respectively. The ferroelectric III phase disappears at about 60 MPa for x =0.2 crystal. On the other hand, phase III is not observed for the x =0.3 crystal both at 0 pressure and at high pressures. The II-IV or III-IV transition temperatures also increases with increasing pressure with the rates of 0.162 K/M…
Elastic and piezoelectric properties of BaTiO3 at room temperature
Abstract We have calculated the phonon dispersion curves of barium titanate (BaTiO 3 ) at room temperature. A lattice dynamical formalism using the shell model is used. This microscopic model includes the short-range interactions of axially symmetric type and the long-range Coulomb interactions and taking into account the electronic polarizability of constituent ions. Zone center phonon and a few phonons in the first Brillouin zone are used for fitting. The values of relevant parameters are critically analyzed. A calculation of the elastic and piezoelectric properties is presented and compared with the available experimental data.