B. Jannot

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 ...

Relationship between electro-optic, vibrational and dielectric properties in BaTiO3

Abstract The paper is concerned by the origin of the large r42 electro-optic coefficient in BaTiO3. First we establish the general relationship giving the dependence of the electro-optic coefficient in a large frequency range in terms of the elasto-optic, piezoelectric and second-harmonic coefficients as well as the dipolar ionic contribution. This last contribution is described as a function of the phonon and relaxation characteristics. We show that in BaTiO3 the relaxation process around several MHz could be responsible for the large value of the r42 electro-optic coefficient and its strong dependences on the doping and the temperature.

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.

Indications of Two Separate Relaxators in the Subphonon Region of Tetragonal BaTiO3

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.

Dielectric and D.T.A. studies of the phase transitions in Rb2ZnCl4: Influences of crystal growth conditions

Abstract Phase transitions in ferroelectric Rb2ZnCl4 crystals grown by various methods are examined by dielectric measurements and D. T.A. technique in the temperature range from -120 to 100 degres C. For most of the studied samples, small heat anomalies are seen around both of the two transitions points, and the dielectric constant along the ferroelectric axis shows a large thermal hysteresis over a wide temperature range including the commensurate - incommensurate transition point. All these anomalies were absent in some other samples; their unexpected behavior is assumed to be connected with the presence of defects.

Electrical transport properties of Mn- and Ni-doped barium titanate at high temperature

Abstract The dependence on temperature and oxygen pressure of the electrical transport properties of the ferroelectric compound BaTiO3 doped with manganese or nickel is investigated in the temperature range 800–1100°C with an oxygen partial pressure continuously varying between 0.21 and 10−15 atmosphere. A complex impedance analysis method has been improved for the measurements. A point defect model is proposed to discuss the different behaviour of the two dopants. The calculated conductivity is in very good agreement with the experimental data over the full temperature and oxygen partial pressure ranges. The band gap derived from this work (2.96 eV) is in good agreement with the already reported values. The ionisation energy levels of these two impurities in the band gap have been determined and compared with the vlues deduced from the MS-LSD calculations.

Local order in the cubic phase of ABO3 perovskites

Abstract The existence of polarization clusters in the cubic phase of the ABO3 compounds has been revealed for a long time by various experimental techniques, including now the direct measurement of the dielectric dispersion. These clusters are described in terms of correlated displacements of the B ions in the 〈111〉 directions, out of the centre of the oxygen cage. In this paper, we show how the improvement of both the crystalline quality and the performance of the experimental devices led to the actual concept of “relaxator ferroelectrics”. Some experimental results are reviewed within the scope of this relaxation concept and its microscopic origin is questioned.

Monovalent and heterovalent substitutions in BaTiO3 single crystals: Effects on phase transitions

Abstract It is well known that BaTiO3 exhibits four phase transitions (1460°C, 135°C, 5°C, -90°C). The most extensively discussed transition is the paraelectric-ferroelectric one at 135°C: displacive with an over-damped soft mode, order-disorder or relaxator ferroelectric? Homovalent and heterovalent substitutions change the phenomenological Landau approach of the transition, the lattice dynamics, the electronic structures and the covalency of oxygen bonds.

Theory of electro-optic effect in the tetragonal phase of BaTiO3

Abstract The microscopic mechanism of electro-optic effect in tetragonal phase of BaTiO3 is discussed taking account of the piezoelectric effect due to the lattice deformation and the spontaneous Kerr effect as to a cause of the to the strong local field acting in the constituant ions. It is found that the birefringence σn and the linear electro-optic coefficients (Pockels constants) r33 and r13 obtained are in good agreement with the experimental data.

Theoretical study of the electro-optic effect in the tetragonal phase of pure and Fe doped BaTiO3

Abstract The microscopic mechanism of electro-optic effect in tetragonal phase of monodomain pure and Fe doped barium titanate crystals is discussed taking account of the piezoelectric effect due to the lattice defomation and the spontaneous Kerr effect due to the strong local field acting on the constituent ions. It is found that the calculated birefringence δn and the linear electro-optic or Pockels constants r 33 and r 33 versus temperature and dopant concentration are in good agreement with the experimental data.