Makoto Iwata

Morphotropic Phase Boundary in Solid Solution Systems of Perovskite-Type Oxide Ferroelectrics

The origin of the appearance of the morphotropic phase boundary in the perovskite-type oxide solid solution systems and the increase in the dielectric susceptibilities in the vicinity of the boundary is theoretically clarified on the basis of a Landau-type free energy function. The dielectric susceptibilities are concretely expressed in terms of the model parameters, and found to diverge at the morphotropic phase boundary within the present model.

A Theory of Morphotropic Phase Boundary in Solid-Solution Systems of Perovskite-Type Oxide Ferroelectrics.

The origin of the appearance of the morphotropic phase boundary in perovskite-type oxide solid solution systems is clarified on the basis of a Landau-type free-energy function expressed in terms of polarization and strain components. The dielectric susceptibilities and piezoelectric constants are shown to increase in the vicinity of the morphotropic phase boundary.

Raman Scattering in (1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3

Single crystals and ceramics of (1- x )PMN– x PT were synthesized in the whole concentration range from x =0 to x =1, and the temperature-concentration phase diagram of the system was determined fr...

Crystal Growth and Dielectric Properties of Solid Solutions of Pb(Yb1/2Nb1/2)O3?PbTiO3 with a High Curie Temperature near a Morphotropic Phase Boundary

The Pb(Yb1/2Nb1/2)O3–PbTiO3 (PYN–PT) binary system single crystal near the morphotropic phase boundary (MPB) composition with (100) planes of 1–2 mm length on one side was successfully grown by the flux method using PbO–PbF2–B2O3 flux. The PYN–PT (47/53) single crystal was found, by simultaneous microdifferential thermal analysis and thermogravimetric analysis, to show a peritectic melting point at 1205°C with a decomposition of the perovskite crystals into a pyrochlore phase and a liquidus point at 1253°C. The structure of the PYN–PT (47/53) single crystal was determined by X-ray diffraction study to be tetragonal with a=4.05±0.01 A and c=4.14±0.01 A at room temperature. The PYN–PT single crystal shows the maximum value of the relative permittivity er of 16000 at 1 kHz at a Curie temperature Tc of 404°C, which is as high as that in the Pb(Zr, Ti)O3 system near the MPB and the value of the remanent polarization Pr of about 26 µC/cm2 at 140°C.

Domain Observation in Pb(Zn1/3Nb2/3)O3–PbTiO3 Mixed Crystals

Domain-wall structures in the vicinity of the morphotropic phase boundary (MPB) in (1-x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 (PZN–xPT) mixed crystals (x=5% and 8%) were investigated by means of a polarizing microscope. It was found that the apparent jump of the indicatrix rotation as a function of temperature appears in the MPB region (x=8%) without the poling process, indicating the occurrence of a phase transition between the tetragonal and rhombohedral phases, despite the fact that this transition is known to be observed only in a poled sample in the case of dielectric constant measurements. We concluded that the sharp phase transition between the tetragonal and rhombohedral phases in the PZN–PT crystal can be observed in the small region of 50 µm order even without the poling process. It was also found that the principal axis of the indicatrix slightly rotates in the MPB region, implying the appearance of the lower symmetry phase such as a monoclinic structure.

Optical Absorption in Band-Edge Region of ( CH3NH3)3Bi2I9 Single Crystals

Optical absorption spectra were measured around the band edge in ( CH 3 NH 3 ) 3 Bi 2 I 9 single crystals. In the very thin crystals, an intrinsic exciton band appears clearly at 2.51 eV at low temperatures, and the absorption peak remains even at room temperature reflecting a large binding energy of more than 300 meV. The exciton band does not show a discontinuous shift near the temperature where the methylammonium ions induce a structural phase transition. This fact indicates that the exciton states are independent of the methylammonium ions. The origin of the exciton band is examined on the basis of the annealing effect on the absorption bands in vacuum-evaporated films. The lower energy tail of the exciton band obeys the Urbach rule between 78 and 350 K. The detailed analysis by the Urbach rule reveals that the exciton-phonon interaction in the present system is relatively strong.

Theory of Morphotropic Phase Boundary in Solid-Solution Systems of Perovskite-Type Oxide Ferroelectrics: Elastic Properties

Elastic stiffness constants in a perovskite-type oxide solid-solution system are discussed on the basis of the Landau-type free energy function expressed in terms of polarization and strain components. It is found that some of the elastic stiffness constants become extremely small in the vicinity of the morphotropic phase boundary.

Theoretical Analysis of the Temperature–Field Phase Diagrams of Perovskite-Type Ferroelectrics

The temperature–field phase diagrams are discussed within the Landau theory for perovskite-type ferroelectrics, which undergo successive transitions of first order. The sequences of the cubic–tetragonal–orthorhombic–rhombohedral phases and the cubic–rhombohedral–orthorhombic–tetragonal phases are considered, and the field is applied along the [100], [110], or [111] direction. The topology of the phase diagrams obtained are found to depend much on the degree of the anisotropy of the Landau potential functions in the order parameter space. When the free energy function is nearly isotropic, the critical end points associated with all the possible ferroelectric phases, stabilized successively with decreasing temperature, appear at a certain field and temperature. Only the one associated with the highest temperature ferroelectric phase does that at a fairly high field. It is pointed out that the critical end points experimentally observed in the relaxor ferroelectrics like Pb(Mg 1/3 Nb 2/3 )O 3 suggests that t...

The Temperature and Pressure Dependence of the Dielectric Properties of Disordered and Ordered Pb(In1/2Nb1/2)O3 Single Crystals.

Dielectric properties of the disordered and ordered Pb(In 1/2 Nb 1/2 )O 3 (abbreviated as PIN) single crystals were investigated in the range of temperature T from -70 to 200°C and of hydrostatic pressures p up to 0.7 GPa. For the disordered PIN, a typical dielectric dispersion which obeys the empirical Vogel-Fulcher relation was observed. Such dielectric dispersion strength was weakened with an increase in the degree of chemical ordering (S) at the B-site cations in the perovskite ABO 3 , and then relaxor behavior disappears. The relaxor behavior was also weakened with increasing pressure up to 0.7 GPa for the quenched disordered PIN with S=0.2, whereas for the disordered PIN with S=0.4, the relaxor behavior was weakened with increasing pressure and then the antiferroelectric phase transition was induced by the application of pressure above 0.4 GPa. For the ordered PIN, the phase below the transition temperature was confirmed to be antiferroelectric by observation of P - E double hysteresis loops. The st...

Dielectric dispersion in (CH3NH3)5Bi2X11 (X=Cl, Br) single crystals

The temperature dependences of dielectric dispersion in (CH 3 NH 3 ) 5 Bi 2 Cl 11 and (CH 3 NH 3 ) 5 Bi 2 Br 11 single crystals, which undergo the para-ferroelectric phase transition (I→II) at 307 K and 311.5 K, respectively, were measured near the phase transition point in the frequency range from 1 MHz to 1 GHz. The dielectric dispersion exhibited was found to be of the Debye-type. The critical slowing down was observed near the phase transition point.