Etsuro Sawaguchi

Optical properties of hexagonal BaTiO3 around the 222 K structural phase transition

Abstract The refractive indices n of the hexagonal type BaTiO3 have been measured. The crystal is optically positive (na = 2.31, nc = 2.39) at 20°C, and the Sellmeier relation holds: n 2 − 1 = S 0 λ 2 0 ( 1 − λ 2 0 g l 2 ), where S0a =794 μm-2, λoa = 0.218 μm for a- polarized light; and Soc = 797 μm−2,λoc = 0.226 μm for c-polarized light. Below the structural transition at T0 = 222 K, the spontaneous birefringence δ(Δn) occurs and critically depends on temperature: δ(Δn) ∝ (T0 - T) 2β with β = 0.36 ± 0.2. Comparisons are made with the perovskite BaTiO3.

Dielectric and Ferroelectric Properties in the Low Temperature Phase of Strontium Niobate

Temperature dependence of the dielectric constant, D – E hysteresis loops and pyroelectric charges in Sr 2 Nb 2 O 7 have been studied in the temperature range from 4.2 K to 300 K. In addition to the well known ferroelectric transition at 1615 K, another ferroelectric phase transition occurs at T 0 ∼100 K. The dielectric constant along the b -axis shows a large ferroelectric anomaly at T 0 and the Curie-Weiss constant above T 0 is 8.0×10 3 K. On cooling, the direction of the spontaneous polarization P s begins to deviate at T 0 from the original c -axis of the high temperature ferroelectric state and the new b -component of P s appears; the b -component is 2×10 -2 C/m 2 at 4.2 K. Effects of a dc biasing field on the dielectric constant have been examined. With increasing the field strength, the dielectric constant decreases and the transition temperature shifts to the high temperature side. The results are analyzed with the phenomenological theory developed by Devonshire.

Effects of hydrostatic pressure on the dielectric properties of hexagonal barium titanate around the 222K structural phase transition

Temperature dependence of the dielectric constant of hexagonal BaTiO3 has been studied at a variety of hydrostatic pressures up to about 10kbar. The phase transition point T0 = 222K decreases with pressure: dT0/dp = -3.4K kbar-1. The hexagonal crystal deforms slightly and it changes to orthorhombic below T0 (a′(ortho.) ∼ a(hex.) ∼ 5.72A, b′(ortho.) ∼ √3a(hex.) and c′(ortho.) ∼ c(hex.) ∼ 13.96A). Temperature dependence of lattice parameters has been examined. No anomaly has been detected in c′ parameter at T0, however thermal expansion coefficient αa′ for a′ axis changes the sign below T0. The difference of the volume thermal expansion coefficient above and below T0(Δβv = 19 × 10-6K-1) and the dT0/dp are used in the Ehrenfest relation; the specific heat anomaly at T0 is estimated as ΔCp = 4.7 J mol-1·K-1.

Ferroelectric phase transition in hexagonal type BaTiO3

Abstract A polarizing microscope is used to investigate the ferroelectric phase transition of hexagonal type BaTi03. Extinction positions of thin crystal c-plates change rather steeply with temperature in the ferroelectric phase near the Curie point of Tc ∼ 70K. suggesting that a cooperative phenomenon is taking place. Orthorhombic crystal symmetry above Tc changes to a lower one, probably monoclinic P21 in the ferroelectric phase.

Twins in the intermediate phase of hexagonal BaTiO3

Abstract Six kinds of twins appear in the intermediate phase of hexagonal BaTiO3 (74K < T< 222K). The six can be classified into three pairs, each consisting of enantiomorphic twins which are indistinguishable by conventional means with a polarizing microscope. The six kinds of twins can be transformed into one another by applying both a suitable uniaxial stress and a dc electric field. The transformations are explained as a compound effect of piezoelectric strain and of uniaxial stress.

Lead zirconate: Half a century ago

Synopsis Around 1950, a group guided by Prof. Yutaka Takagi succeeded in disclosing mysterious dielectric behavior of lead zirconate; remarkable contrasts with barium titanate have been found.

Phase transition in hexagonal type BaTiO3

Abstract A review is given on the structural phase transition of hexagonal type BaTiO3 at 222 K and on the ferroelectric · ferroelastic phase transition at ∼ 70 K. X-ray studies and Raman studies have disclosed that an E2u phonon mode condenses below 222 K where crystal changes from hexagonal P63/mmc to orthorhombic C2221-D5 2. As for the ferroelectric transition at ∼70 K, dielectric and elastic behaviors strongly suggest that the transition is primarily of a ferroelastic nature associated with softening of the elastic stiffness c66. A probable space group below 70 K is monoclinic P21.

Elastic and Piezoelectric Properties of Gd2(MoO4)3

The elastic and piezoelectric constants of Gd 2 (MoO 4 ) 3 have been measured in a temperature range from 100°C to 400°C by means of piezoelectric resonance-antiresonance method. The elastic compliance s 11 (refered to the orthorhombic axes) of the paraelectric phase can be fitted in well with a logarithmic law; however an inverse-power law holds as well in s 11 if the temperature range is limited to 40°C from the T c . In the ferroelectric phase, the elastic compliances s 11 , s 22 and the piezoelectric constants d 31 , d 32 have been analyzed on the basis of a phenomenological theory proposed by Aizu and Dvořak: the temperature dependences of these constants are well expressed by the formulae A ( T 1 - T ) -1/2 + B , being consistent with the theoretical predictions. Behavior of the spontaneous polarization is also examined.

Effects of magnetic field cooling on the dielectric properties of magnetite at low temperatures

Abstract Dielectric and conducting properties of single crystal of magnetite have been studied below the Verwey point Tv = 122 K. A magnetic field cooling treatment has been applied on the crystal for controlling the alignment of pseudo-orthorhombic a-and b-axes (magnetic hard axis and intermediately hard axis, respectively). A large effect of the field cooling on the dielectric and conducting properties has been found.

Lattice strain of hexagonal BaTiO3 under hydrostatic pressures

Abstract X-ray diffraction pattern of hexagonal BaTiO3 has been examined under hydrostatic pressures with a diamond-anvil cell. The hexagonal modification is still stable under the pressure of as high as 180kbar. The volume of the unit cell decreases by about 10% at 150kbar. From the pressure-volume data, the bulk modulus B has been estimated by using the Birch relation; the B in the hexagonal BaTiO3 turns out to be 1.10 ± 0.05 Mbar, and is smaller by about 10% than that of the perovskite BaTiO3. The difference of bulk modulus between the two polymorphic forms may be explained by a difference of the stacking sequence of Ba-O layers.