Shōichiro Nomura

Dielectric and Magnetic Properties of Pb(Fe1/2Ta1/2)O3

Single crystals of Pb(Fe1/2Ta1/2)O3 were grown from ternary melts with excess PbO. The crystals, having cubic perovskite structure at room temperature, undergo a ferroelectric phase transition at about 243°K, changing to rhombohedral structure. The dielectric constant passes through a maximum of nearly 9500 at 1 KHz. The spontaneous polarization along [100] direction is ca 28 µC/cm2 at liquid nitrogen temperature. A magnetic anomaly at about 180°K is related to the onset of antiferromgnetic ordering.

Electrostriction in Pb (Zn13Nb23)O3

Abstract The electrostriction in Pb (Zn 1 3 Nb 2 3 ) O 3 crystals has been investigated using a strain gauge method. In the ferroelectric phase below 140 C, the strain vs the electric field shows a hysteresis, which is ascribed to the effect of ferroelectric domains. A quadratic relation holds between the strain x and the electric polarization P as x = QP2 above about 170 C in the paraelectric phase. Values of the electrostrictive Q coefficients are determined from the measurements near 190 C, as Q11 = 1.6·10−2m4/C2, Q12 = −0.86·10−2m4/C2, and Q44 = 0.85·10−2m4/C2.

Quadratic Electro-Optic Effect in the System Pb(Zn1/3Nb2/3)O3-PbTiO3

Quadratic electro-optic effects in the system Pb(Zn1/3Nb2/3)O3–PbTiO3 (up to 40 mole percent PbTiO3) are measured by applying dc electric field in the temperature region above Curie temperature. The crystals are prepared by a flux method with PbO. For Pb(Zn1/3Nb2/3)O3, the values of g coefficients are g11-g12=0.011 m4/C2 and g44=0.007 m4/C2 at λ=632.8 mµ. With the increase of PbTiO3 mole percent, the g values increase, approaching 0.06 m4/C2 at PbTiO3. Some discussions are presented on the optical properties of the crystals.

Dielectric and Piezoelectric Properties in the Ternary System of Pb(Zn1/3Nb2/3)O3-Ba(Zn1/3Nb2/3)O3-PbTiO3

Phase diagram of the ternary system of Pb(Zn1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3–PbTiO3 is determined from structural and dielectric measurements. In the vicinity of Pb(Zn1/3Nb2/3)O3, two phases of perovskite (ferroelectric) and pyrochlore (non-ferroelectric) types are coexistent. The piezoelectric active part is found near the morphotropic (rhombohedral-tetragonal) phase boundary. The most active one is obtained for the composition of 0.8Pb(Zn1/3Nb2/3)O3–0.2Pb0.7Ba0.3(Zn0.1Nb0.2Ti0.7)O3, where the planar coupling factor reaches 0.44.

Electrostrictive effects in non-polar perovskites

Abstract Polarization-related electrostrictive coefficients Qh have been determined for relaxation dielectrics 0.856Sr TiO3-0.144 Bi2/3 TiO3 and (K3/4Bi1/4) (Zn1/6Nb5/6)O3, and for a simple non-polar perovskite BaZrO3 by measuring the hydrostatic pressure dependence of the dielectric permittivity. Anomalous variation in the electrostrictive coefficient with temperature and frequency were observed in (K3/4Bi1/4) (Zn1/6Nb5/6)O3 in the relaxation-temperature region. This behavior can be explained qualitatively with a simple model based on potential barriers separating alternative cation sites. The empirical rule noted previously for ferroelectric perovskites, that the electrostrictive Q coefficient increases with cation order from disordered, through simple and then ordered perovskites is confirmed again in non-polar perovskites.

Dielectric Relaxation and Thermal Stimulated Current in (K3/4Bi1/4)(Zn1/6Nb5/6)O3 and Its Solid Solutions

Dielectric properties of (K3/4Bi1/4)(Zn1/6Nb5/6)O3 and its solid solutions with perovskite structure are studied in the frequency range from 100 Hz to 4 GHz, at temperatures ranging from -100°C to 250°C. The dielectric relaxations are found on the single crystal of (K3/4Bi1/4)(Zn1/6Nb5/6)O3 as well as on the ceramics in a temperature region below room temperature. The region is shifted in the direction of lower temperatures by substituting the Bi ions with La ions. Neither dielectric nonlinearity nor structural change is observed in the region of dielectric relaxation. The relaxation seems to arise from the thermal motion of ions over potential barriers which are produced in the crystal by disordered arrangement of ions with different chemical valencies. The existence of the potential barriers is also confirmed by observing thermally stimulated current. The activation energy of potential barrier is estimated to be about 0.44 eV for (K3/4Bi1/4)(Zn1/6Nb5/6)O3.

Soft modes in relaxor ferroelectrics

Abstract The fact that the polarization-related electrostrictive coefficients Q in the relaxor type ferro-electrics are almost an order of magnitude smaller than in most normal ferroelectric perovskites can be explained under the assumption that the temperature dependence of the soft-mode TO phonon frequency is given as ω2 TO = A(T — Tc ), where the coefficient A in relaxor ferroelectrics is an order of magnitude smaller than the value in normal ferroelectric perovskites. The assumption also explains why the Curie-Weiss constants, C, in the relaxor crystals are several times larger than in the normal ferroelectrics. The soft-mode frequency in Pb(Mg1/3Nb2/3)O3 crystals is predicted to be below 10 cm−1 near the mean Curie temperature.

Mössbauer study of the FeV2O4Fe3O4 system

Abstract Mossbauer spectra of the Fe 1+ x V 2− x O 4 spinel solid solutions are taken to investigate the cation distribution. Room temperature spectra can be interpreted by assuming that the cation distribution is represented approximately as Fe 2+ [Fe 3+ x V 3+ 2− x ]O 4 for 0 ≦ x ≦ 0.35 and Fe 3+ [Fe 2+ Fe 3+ x −1 V 3+ 2− x ]O 4 for 1 ≦ x ≦ 2 and the ionic valence arrangement changes from the 2-3-3 type (Fe 2+ [Fe 3+ x V 3+ 2− x ]O 4 ) to the 3-2-3 one (Fe 3+ [Fe 2+ V 3+ ]O 4 ) in the range 0.35 ≦ x ≦ 1. Fe 2 VO 4 is found to be 3-2-3 spinel, Fe 3+ [Fe 2+ V 3+ ]O 4 . Its paramagnetic spectrum at 473°K is, however, composed of a broad single line with isomer shift value of 0.61 mm/sec relative to stainless steel, in which the line splitting due to the ferric and ferrous ions is rendered indistinguishable.

A Method for Measuring the Direction Angle of a Magnetization with a Vibrating Sample Magnetometer: Application to SmFeO3 in the Spin Reorientation Region

A method is described by which both the direction angle and the magnitude of a magnetization are measured with a vibrating sample magnetometer using a modified detection coil system. Applying the method to a SmFeO3 crystal, it was found that the spontaneous magnetization decreases about 5% in the spin reorientation temperature region. Measuring the magnetic anisotropy constants by a torque method, the lower and upper boundaries of the reorientation region were determined as 184±0.5 and 192.5±0.5°C respectively, and the temperature dependence of the rotation angle was calculated.

Thermal Dilatation in Pb(Zn1/3Nb2/3)O3 Crystal

The temperature dependence of the cell dimension of Pb(Zn1/3Nb2/3)03. crystal was investigated in the temperature range between liquid nitrogen temperature and 400°C by using an X-ray diffractometer and a thermal dilatometer of differential transformer type. The latticeconstant a increases with increasing temperature and the rhombohedral angle α approaches 90° at about 150°C. An anomaly in thermal dilatation was observed over a wide range of temperatures, corresponding to a diffuse transition from ferroelectric to paraelectric phase. The linear thermal expansion coefficient decreases with increasing temperature in the ferroelectric phase, reaches a minimum at about 120°C, and then gradually increases to a constant value(10.0×10-6K-1)in the higher temperature region.