Toshiharu Mitsui

Grain-Size Development in the Crystallization Process from Amorphous PbTiO3

The size of PbTiO3 grains crystallized from amorphous state has been investigated. The devitrification of the sample by suitable annealing has been observed by an optical method and the pronounced effect by the grain growth, in the grain-size region from 50 nm to 1000 nm, has been definitely confirmed by the scanning electron microscopy.

Ferroelectricity and structural phase transitions in a hexagonal ABX3-type antiferromagnetic compound: KNiCl3

Dielectric properties of a hexagonal ABX3-type antiferromagnet KNiCl3 are studied by measuring dielectric constants and observing D - E hysteresis loops. Dielectric anomalies indicating structural phase transitions are found exclusively along the c-axis at 274 K, 285 K, 561 K and 762 K. Ferroelectric D - E hysteresis loops are observed in the phase above 274 K, though the D - E loop cannot be observed at temperatures higher than 600 K owing to enhancement of electric conductivity of this compound. Within the observed temperature range, the spontaneous polarization Ps as a function of temperature behaves in a manner wherein as temperature is raised, Ps appears suddenly at 274 K, decreases gradually towards 285 K after substantial growth and then monotonically increases up to the phase above 561 K. The magnitudes of Ps at 274.5 K and 533.0 K are estimated as 0.048 μC/cm2 and 0.871 μC/cm2, respectively. Such behavior of Ps is attributed to a possible ferrielectric alignment of sublattice polarizations inherent to the crystal structure with the space group P63cm.

Dielectric anomaly involving magnetic phase transition in a hexagonal antiferromagnet RbCoBr3

Abstract The dielectric and magnetic phase transitions in a triangular lattice antiferromagnet RbCoBr3 were studied by means of some dielectric, magnetic and calorimetric measurements. In this compound, which is a typical Ising-like antiferromagnet with antiferromagnetic moment along the hexagonal c-axis, a ferroelectric phase transition involving magnetic ordering with weak ferromagnetic moments confined in the c-plane was found to occur at T N(=37.OK). Those results imply that the magnetic ordering of this system is assisted by a polar lattice distortion, which can release partially the spin frustration due to the antiferromagnetic spin arrangement on a triangular lattice. This co-ordering is certainly ascribed to a probable competition between lattice-system and spin-system. Experimental data on this interesting phenomenon are presented and a possible mechanism is proposed.

Dielectric and Magnetic Properties of the Distorted Triangular Lattice Ising-Like Antiferromagnet TlCoCl3

Successive structural phase transitions with ferroelectricity were found in TlCoCl 3 through dielectric constant measurements and D – E hysteresis loop observations. The sequence of the transitions is identical with that in RbMnBr 3 and TlFeCl 3 , which undergo successive transitions from the prototype crystal structure of CsNiCl 3 . The temperature dependence of the magnetic susceptibility below T N1 indicates the presence of successive magnetic orderings, which has not been observed in the Ising-like antiferromagnets CsCoCl 3 and CsCoBr 3 without structural transitions. Dielectric dispersion attributable to the Ising-spin dynamics on a linear-chain spin system was observed at low temperatures. Several problems to be solved are proposed for understanding structural and magnetic properties of TlCoCl 3 .

Magnetic and Dielectric Properties of RbCoBr3 Inspected from Nonmagnetic Impurity Effect

Through magnetic and dielectric measurements, a simultaneous phase transition of spin and lattice systems observed previously at 37.2 K of RbCoBr 3 have been investigated by doping with nonmagnetic Mg impurities. A mutual reduction in the ferroelectric Curie temperature T C and the Neel temperature T N due to the enhancement of Mg doping confirms the occurrence of the synergistic ordering of both. A broad peak of the real part of dielectric constant along the c axis, e ′ c ( T ), at approximately 32 K of the pure system splits into two sharp peaks in mixed systems. This suggests the presence of successive structural phase transitions at around this temperature. A probable sequence of crystal and magnetic structures was discussed for the successive phase transitions of RbCo 1- x Mg x Br 3 . The interplay between the spin and lattice variables of RbCoBr 3 was interpreted by taking account of spin-dependent polarizability and spin-dependent spontaneous polarization. A dielectric anomaly observed at 9.0 K in ...

Dilatometric Study on Monoclinic Crystals of Ferroelectric TGS Down to Cryogenic Temperature Region

We have further developed the clip-type strain gauge dilatometer in order to measure an angular deformation by applying the cosine theorem. In this research, not only a linear thermal expansion along three crystallographic axes but also a monoclinic angle β of ferroelectric TGS in the monoclinic system were measured below 380 K down to the cryogenic temperature region. Each of these components showed a clear bend and a typical feature of the second order transition at the Curie temperature. On the other hand, the volume thermal expansion did not show a big change at the Curie temperature.

Phase transitions and dielectric properties of hexagonal KNiCl3-type antiferromagnets

Abstract It is found that KNiCl3, RbFeBr3, RbMnBr3, RbCoBr3 and TlFeCl3 belonging to a KNiCl3-type antiferromagnet undergo novel class of structural phase transitions which comprise magnetic phase transitions accompanied by spin-frustration in the c-plane and ferroelectric one. They have a prototype crystal structure of space group symmetry P63 Immc and change this symmetry to hexagonal P63 cm or trigonal P Cl. Simultaneously, the configuration of the three kinds of the linear chains of face-sharing NiCl6-octahedra (h-h-h-packing) is displaced along the c-axis at the structural phase transitions. These compounds exhibit characteristic variations of the dielectric constant ε c , lattice constant c and spontaneous polarization P s in the structural and magnetic transitions. The compounds mentioned above and RbVBr3 are classified into three groups with respect to their dielectric and magnetic properties associated with the transitions. It is in particular interesting that RbCoBr3 develops simultaneously the ferroelectric and antiferromagnetic phase transitions at 37.OK.

Dielectric anomaly in hexagonal ABX 3-type antiferromagnets with successive structural phase transitions

Abstract Dielectric properties around the structural and magnetic phase transtions of ABX 3-type compounds, such as KNiCl3, RbMnBr3, TlFeCl3, RbFeBr3, RbVBr3 and RbCoBr3, with distorted CsNiCl3-type crystal structures were summarized. At certain temperatures those crystals take the structure with space group symmetry P63 cm (RT-KNiCl3-type, where RT means the room temperature phase) or P3cl, of which the lattice deformation from the prototype P63/mmc (CsNiCl3-type) structure is characterized by a displacement of -BX 3- chain along the c-axis. The dielectric behavior of those crystals was classified into three types.

Investigations of amorphous PbTiO3 and its crystallization

Abstract Amorphous state of PbTiO3 has been prepared. DTA, optical and electron-microscopic investigations have clarified the crystallization temperature and the devitrification phenomenon. Certain dielectric anomalies were found in the amorphous state. In Raman spectra, low-lying response is found in the amorphous state, and the ferroelectric soft mode is found when it is crystallized.

Thermal Expansion and Dielectric Properties of Triangular Lattice Antiferromagnet RbCoBr3

Thermal expansion and dielectric measurements of RbCoBr3 were performed to examine the lattice distortion accompanied by structural phase transitions. It was confirmed that RbCoBr3 undergoes structural phase transitions at 90, 37, and 10 K because its linear thermal expansion and the temperature dependence of its linear thermal expansion coefficients showed clear anomalies at these temperatures. The pyroelectric charge measurement showed that a spontaneous electric polarization appears between 37 and 31 K and below 10 K. We propose a successive structural phase transition sequence and crystal symmetries at each phase.