Masaaki Takashige

Raman Scattering Study of Bismuth Layer-Structure Ferroelectrics

The lattice dynamical properties have been studied by Raman spectroscopy in bismuth layer-structure oxides Bi4Ti3O12, and ABi4Ti4O15 (A=Ba, Ca, Pb, Sr). The rigid-layer modes were observed in all compounds in the range 57–62.5 cm-1. The lowest mode of Bi4Ti3O12 shows marked softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi4Ti4O15 and SrBi4Ti4O15 the underdamped soft modes were also clearly observed. In contrast, in BaBi4Ti4O15 only low-frequency overdamped modes have been observed. In PbBi4Ti4O15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.

Ferroelectricity in {N(CH3)4}2ZnCl4

Tetramethylammonium tetrachlorozincate, {N(CH 3 ) 4 } 2 ZnCl 4 has been found to undergo successive phase transitions at about -112, -92, 3.3, 6.0 and 20°C and to show ferroelectricity along the a axis in the temperature range between 3.3 and 6.0°C.

Ferroelectricity in NH4LiSO4

It has been found that in NH 4 LiSO 4 another transition point 186.5°C exists in addition to the already known one 10°C and this substance is ferroelectric between these two transition points. Anomalies of dielectric constant at the transition points are very small as compared with those of proper ferroelectrics. The Curie-Weiss law holds within a narrow temperature range in the phase above the upper transition, where the Curie constant has a very small value of 5.6 K. Moreover the spontaneous polarization shows a curious increase with lowering temperature. Thus NH 4 LiSO 4 can be said to be one of improper ferroelectrics.

Ferroelectricity in Rb2ZnBr4

Rb 2 ZnBr 4 has been found to show phase transitions at about -86 and 18°C, respectively and to be ferroelectric in the a direction below -86°C.

Effects of hydrostatic pressure on the successive phase transitions in ferroelectric {N(Ch3)4}2 CoCl4 and {N(CH3)4}2ZnCl4

Abstract The hydrostatic pressure effect on the stability of I, II, III, IV and V phases in {N(CH 3 ) 4 } 2 CoCl 4 is found to be quite similar to that of the corresponding I, II, III, VII and IV phases in {N(CH 3 ) 4 } 2 ZnCl 4 ; the ferroelectric phase III disappears at 600 bar for Co-salt and at 1150 bar for Zn-salt, and a new phase VII in Zn-salt induced by pressures above 400 bar corresponds to the phase IV in Co-salt which has already appeared at 1 atm.

New Ferroelectric K2ZnBr4

The phase transition sequence of a K 2 ZnBr 4 crystal was investigated by DTA and dielectric measurements. It was found that the crystal undergoes at least two phase transitions, at -118°C and around 200°C, on heating. Ferroelectricity was observed below the lower transition point.

The Structural, Dielectric, Raman-Spectral and Low-Temperature Properties of Amorphous PbTiO3

Amorphous ribbons of pure PbTiO3 were obtained by the rapid solidification technique and were confirmed to be in the amorphous state by several different experimental techniques. Structural studies revealed clusters with radii up to about 150 A, larger than those so far found in other materials such as metals or semiconductors in the amorphous state. Dielectric and Raman spectral studies confirmed that the material remains in the amorphous state indefinitely. Low-temperature studies showed that not only the amorphous state but also the crystalline state of PbTiO3 has a dielectric constant minimum in the mK region, a characteristic of the amorphous state.

Crystallization of amorphous bismuth titanate

We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The differential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identified as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at first a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure.

Raman study of ferroelectric bismuth layer-oxides ABi4Ti4O15

Abstract The optical phonon modes have been studied by Raman spectroscopy in bismuth layer-structure oxides ABi 4 Ti 4 O 15 (ABa, Ca, Pb, Sr). Although the bands higher than 50cm −1 reveal no remarkable temperature dependence, the low frequency modes change markedly. As for CaBi 4 Ti 4 O 15 and SrBi 4 Ti 4 O 15 the underdamped soft modes were clearly observed. In contrast, in BaBi 4 Ti 4 O 15 only low-frequency overdamped modes have been observed. In PbBi 4 Ti 4 O 15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.

Crystallization Process from Amorphous PbTiO3

Investigation has been made of the crystallization from the rapidly quenched amorphous state of PbTiO3 utilizing DTA and Raman spectroscopy. The DTA curves reveal sharp exothermic peaks between 500 and 600°C on heating the as-quenched samples. Essential differences between the Raman spectra from the as-quenched state and those from the crystalline state have been recognized: the soft mode observed in the crystalline state is found not to exist in the as-quenched state. By annealing samples, a dramatic change in the Raman spectra due to the structural relaxation is clearly visible.