Yukikuni Akishige

Electrical properties of multiferroic BiFeO3 ceramics synthesized by spark plasma sintering

Single-phase multiferroic BiFeO3 powder was prepared by improved solid-state synthesis, and dense BiFeO3 ceramics were fabricated by a spark plasma sintering (SPS) process. The sintering temperature of SPS ranged from 650 to 800 °C under 50 MPa. It is found that the sample is not only single phased but also of high density when sintered at 650 °C. The dielectric constant and loss of the samples were tested between 100 Hz and 20 MHz at several temperatures. Oxygen and vacuum annealing conditions impact the dielectric behaviour of the BiFeO3 ceramics prepared by the SPS technique obviously. The piezoelectric coefficient d33 of the sample annealed in vacuum is 4.2 pC N−1 at room temperature.

New Ferroelectric BaTi2O5

A needle-shaped BaTi2O5 crystal was grown from a Ti-rich solution. The chemical composition of the crystal was analyzed by an electron probe micro-analyzer (EPMA). X-ray Weissenberg photographs were taken at room temperature (RT): the crystal is monoclinic with a=16.91 A, b=3.94 A, c=9.49 A, β=103.0° and Z=6. A broad and large dielectric anomaly reaching 30,000 was observed at 430°C for the dielectric constant along the b-axis. Also, spontaneous polarization was estimated to be approximately 7 µC/cm2 from the D-E loops at RT. Thus the crystal is a new ferroelectric substance with a high Curie temperature of 430°C and the space group C2 at RT.

Structural Phase Transition in Hexagonal Barium Titanate

Distinct dielectric anomalies have been found in the hexagonal modification of BaTiO 3 at 60 K and 222 K. X-ray studies have revealed that a structural distortion from the prototypic hexagonal structure occurs at T 0 =222 K. Below T 0 , a very weak diffraction line has been detected, which is the forbidden line for the hexagonal space group of P6 3 /mmc; and the diffraction intensity I in the new phase is given approximately by I ∝ T 0 - T . The transition at 222 K is very probably of the second order. A possibility is speculated that a ferroic transition takes place below T 0 .

Synthesis and Physical Properties of Single Crystals and Ceramics of New Ferroelectric BaTi2O5

Single crystals of ferroelectric BaTi2O5 were grown by a floating zone method, and the dielectric constant and specific heat were measured in a temperature range of 300–800 K. The dielectric constant and dielectric loss along the b-axis show a large dielectric dispersion at TC=730 K, similar to the ferroelectric phase transition of the order-disorder type. Transition enthalpy and transition entropy were estimated to be ΔH=98.6 J/mol and ΔS=0.136 J/molK, respectively. The optical absorption edge of the crystals exists near 365 nm. Single-phase BaTi2O5 powder was successfully synthesized through a sol-gel route, using barium diethoxide and titanium tetraisopropoxide, and the porous ceramics sintered at 1375 K show a broad maximum in the electrical capacitance vs T curve at approximately 600 K.

Ferroelectric Behavior in Hexagonal Type Barium Titanate

An investigation on dielectric anomalies in a hexagonal modification of barium titanate has revealed that this crystal is also ferroelectric as in the well known perovskite type barium titanate. The hexagonal single crystals have been grown successfully. The dielectric constant along c-axis ec shows two distinct peaks at 222 K and 74 K; a general feature of ec vs temperature relation is in agreement with that of ceramic samples (J. Phys. Soc. Jpn. 54 (1985) 480). Above 74 K Curie-Weiss law holds with the curie constant C=1.2×104 degrees. The dielectric constant perpendicular to c-axis is constant e⊥~70. Below about 74 K ferroelectric hysteresis loop occurs. The c-component of the spontaneous polarization is ~2×10-2 C/m2 at 5 K. Barium titanate affords a rare and an interesting example that both polymorphic states show strong ferroelectric activity despite the quite difference in crystal structures.

Metallization of magnetite at high pressures

We report on the first finding that an insulator to metal transition (i.e. Verwey transition) of magnetite (Fe 3 O 4 ) takes place at absolute zero temperature as a function of pressure. In order to investigate the Verwey transition, we measured the electrical resistivity on two single crystalline samples with distinct oxygen compositions of Fe 3 O 4.002 and Fe 3 O 4.007 at temperatures from 300 K down to 3 K under a pressure up to 9 GPa using a cubic anvil apparatus. The Verwey transition temperature of these samples suddenly disappears at a certain critical pressure P c in between 6 and 8 GPa. We confirmed that a metallic ground state is realized in magnetite under pressure above P c .

Ferroelectric and Piezoelectric Properties of Single Crystals of Ba1-xKxTiO3-xFx with x=0.1

Cubic and hexagonal BaTiO 3 single crystals doped with KF (Ba 1- x K x TiO 3- x F x with x =0.1) were grown from a solution with a molar ratio of BaO+2TiO 2 +10KF. The cubic BaTiO 3 crystals with x =0.1 undergo a second order ferroelectric phase transition at the Curie temperature T C of 35 °C; the dielectric constant e ′ along the [100] direction shows a peak at T C reaching 12000. Further, a spontaneous polarization appears below T C ; the remanent polarization P r and the coercive field E c are 7 µC cm -2 and 1.3 kV/cm at 20 °C, respectively. Furthermore, a large piezoelectric constant d 33 =300 pC/N was observed at 20 °C. On the other hand, in hexagonal BaTiO 3 with x =0.1, e ′ along the hexagonal c -axis increases monotonously with decreasing temperature and saturates at low temperatures like quantum paraelectrics.

Ferroelectric Activity on Organic Crystal Trichloroacetamide

Dielectric constants and D - E hysteresis loops have been measured on trichloroacetamide (TCAA) single crystals. Only along the crystallographic b -axis are remarkable dielectric anomalies found at T C =354.6 K and T I =356.9 K upon heating. The Curie-Weiss relation holds in the high-temperature phase with a very small Curie constant of about 5.6 K. Typical ferroelectric D - E hysteresis loops are observed only in a limited temperature range of a few degrees just below T C ; the spontaneous polarization P s and the coercive field E c are estimated to be about 0.2 µC/cm 2 and 4 kV/cm, respectively, at 350 K. Since the E c steeply increases with decreasing temperature, it is difficult to reverse the P s at room temperature. Some discussions are presented for the causes of the weak ferroelectricity on TCAA.

Order-disorder nature of ferroelectric BaTi2O5

Ferroelectric single crystals of BaTi2O5 have been studied in detail by Raman and high-resolution micro-Brillouin scattering with a focus on the central component of the relaxation mode over a wide temperature range (20–600°C). A relaxation mode has been clearly observed in these single crystals. The temperature dependences of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model. The ferroelectric phase transition shows, at least near the vicinity of Tc, an order-disorder nature. Our results indicate that the spontaneous polarization mainly resulted from the dynamic disorder of the Ti ions in the octahedra along the polar b axis of BaTi2O5.

Relaxor in KF-doped BaTi2O5 ceramics by spark plasma sintering

Dense BaTi2O5 ceramics with KF doping up to 5at.% were synthesized by spark plasma sintering (SPS). The SPS method enabled us to overcome the densifying difficulty in conventional sintering which is caused by the limit of the decomposing temperature of this metastable phase. We observed drastic KF-doping effect on the ferroelectric (FE) properties of BaTi2O5. Although there is only very small change in lattice parameters upon KF doping, the peak temperature in dielectric constant strongly decreases in an exponential way. The diffuseness of phase transition increases and FE relaxor state sets in for KF content as low as 1at.%. Possible structural reasons for the effect and the implication for the nature of FE phase transition in BaTi2O5 are discussed.