Eiji Nakamura

Crystal structure of BaTiO3 in the cubic phase

Abstract The crystal structure of cubic barium titanate was determined at five temperatures up to 180 °C, using the data collected by an X-ray automatic four-circle diffractometer. At temperatures between the Curie temperature and about 180 °C, the Ti atom is displaced from the center of the unit cell along the direction and has eight local disorder positions. This result is the same as the model proposed by Comes et al. In addition to this disorder, we find that the Ba atom is displaced from the cell corner along the direction leading to twelve local disorder positions and the O atoms are uniformly distributed on rings in their face centered planes. At temperatures higher than about 180 °C, the Ti atom is no longer located at any local disorder positions but lies rather on the center of the unit cell.

A Note on the Classification of Ferroelectrics

It has been tried to classify newly discovered ferroelectrics as well as old ones according to their Curie-Weiss constants. Results have suggested that the displacive type ferroelectrics are not necessarily oxide crystals, and have also shown that small Curie-Weiss constants are not very rare in new ferroelectrics. Classification of ferroelectrics by the transition entropy is misleading in some cases.

Dielectric Critical Slowing-Down in Ferroelectric Ca2Sr(C2H5CO2)6

Measurements of dielectric constant e and dielectric loss e of Ca 2 Sr(C 2 H 5 CO 2 ) 6 along the ferroelectric axis have been made between 100 kHz and 3.3 kMHz as functions of temperature. Dielectric dispersion arising from the dipolar reorientation takes place mainly in the frequency range from about 10 7 Hz to 10 8 Hz, where e( T ) shows double peaks with minimums at the Curie point. e( T ) becomes maximum at the Curie point, and the maximum value decreases with the increase of the frequency. The results obtained can be explained by the critical slowing-down of the polarization relaxation process.

Critical phenomena in ferroelectric phase transitions I. Experimental observations and qualitative discussion

A review is made of experimental studies of the critical phenomena in ferroelectric phase transitions of the order-disorder type.The static critical indices obtained so far agree well with those given by the mean field theory except in the extreme vicinity of the critical point. Recent observations on the critical x-ray scattering, the critical slowing-down and the anomalies in transport processes are surveyed and their significance is discussed qualitatively.

X-Ray Study of High-Temperature Phase Transitions in KH2PO4

High-temperature phase transitions in KH 2 PO 4 near 110°C and 180°Chave been studied by X-ray diffraction. The extinction rule observed at 123°C is the same as that in the room-temperature phase. It is concluded that on heating through 187°C crystal system changes from tetragonal to monoclinic whose space group is P 2 1 or P 2 1 / m . The lattice parameters at 195°C are given by a =7.47A, b =7.33A, c =14.49A, α=β=90° and γ=92.2° .

Determination of Sign of Surface Charges of Ferroelectric TGS Using Electrostatic Force Microscope Combined with the Voltage Modulation Technique

The sign of surface charges and the surface topography around a domain wall on a cleaved (010) surface of ferroelectric TGS [ (NH2CH2COOH)3H2SO4] were studied in air at room temperature. Using an electrostatic force microscope (EFM) combined with the voltage modulation technique, we determined the location of the domain wall and the sign of the surface charges around it. At the domain wall, we found a ridge structure with a large peak in the spatial distribution of a feedback signal used to keep the amplitude of the 2ω component of electric force constant. This means that the dielectric constant has a large peak value at the domain wall. Furthermore, in an atomic force microscope (AFM) image in a contact mode, we observed a small step 2–3 A high at the center of the domain wall.

Anomalous dielectric behavior of KH2PO4 type crystals in the ferroelectric phase

Abstract Experimental studies on the dielectric constants of KH2PO4 type ferroelectrics below the Curie temperature Tc are reviewed with special emphasis to the anomalously large dielectric constant and its abrupt decrease in the low temperature region called domain freezing. The origin of the large dielectric constant below Tc is explained in connection with the elastic softening in the multidomain crystals. The domain freezing is closely related to the dielectric dispersion observed in the multidomain crystals. The relaxation time τ of the dispersion obeys approximately the Vogel-Fulcher law, which is widely found in dipole glasses near their freezing temperatures. The anomalous increase in τ provides the abrupt decrease in the dielectric constant around the domain freezing temperature.

Refinement of Crystal Structure of SbSI in the Ferroelectric Phase

Crystal structure analysis of SbSI in the ferroelectric phase has been made at 1.3°C by using full three-dimensional Bragg reflection data collected with X-ray automatic four-circle diffractometer. The discrepancy factor is 0.030. The atomic shifts from the positions in the paraelectric phase take place along the ferroelectric c -axis by 0.103 A for Sb atoms and 0.019 A for S atoms. A large thermal parameter of 0.16 A in amplitude is seen for Sb atoms along the c -axis. The split atom method has shown that the nature of the phase transition is very close to the displacive type.

Measurement of Microwave Dielectric Constants of Ferroelectrics Part I. Dielectric Constants of BaTiO3 Single Crystal at 3.3 KMc/s

Horner et als method of dielectric measurement at microwave region has been extended so that it can be applied for cylindrical specimens of arbitrary cross-sections with relatively high loss tangent and high permittivity. Small amount of Specimen is good enough for our method. As the first application of our method, dielectric constants of BaTiO 3 single crystal have been measured from room temperature to 170°C at a frequency of 3.3KMc/s. Above the Curie point, dielectric constants measured at 3.3KMc/s agree well with those measured at 24KMc/s by Benedict and Durand, while the loss tangents slightly above the Curie point are 0.01±0.007 in our case compared to 0.1 in the case of Benedict and Durand.

Dielectric Properties of SbSI at Microwave Frequencies

The complex dielectric constants κ=κ-iκ along the c axis of SbSI at 3.3 and 9.8 GHz have been measured between 0°C and 50°C. The value of κ in paraelectric phase is found to show as high value as 3×104 even in this frequency range. No remarkable dielectric dispersion was observed below 9.8 GHz. No essential difference was found between 3.3 and 9.8 GHz. The values of tan δ at 3.3 and 9.8 GHz are about 0.1 and 0.3, respectively, near the Curie temperature. To ascertain the accuracy of the cavity perturbation method, a new expression is proposed.