Akikatsu Sawada

The Origin of the Ferroelectric Phase Transition in Ammonium Sulfate

Long-wavelength lattice vibrations in ammonium sulfate are analysed group-theoretically, and symmetry coordinates of external modes belonging to the irreducible representation B 1 u , which is responsible for the phase transition from D 2 h to C 2 v , are obtained. They are composed of two groups; one is made of the purely translational and polar modes, and the other is made of the purely librational and nonpolar modes. Some peculiar properties of ammonium sulfate, including, especially, the very small value of the Curie-Weiss constant, are explained phenomenologically by a model in which a normal mode with a large component of a librational nonpolar mode admixed with a small contribution of a translational polar mode becomes soft.

The Lower Frequency Soft-Mode in the Ferroelectric Phase of K2SeO4

In the ferroelectric phase below T 3 (93 K) of K 2 SeO 4 a new soft-mode, belonging to the B 2 representation, has been observed by means of Raman scattering experiment. This mode is different from the soft A 1 mode previously reported to soften as T 2 (129.5 K) was approached from below. The frequency of this newly observed mode is abotut 13.2 cm -1 at 9.8 K, and decreases as the temperature increases. The observed result is discussed in connection with the successive phase transitions in the K 2 SeO 4 crystal.

Raman Scattering Spectra of K2SeO4

The Raman spectra in potassium selenate single crystal were measured over the temperature range 77∼300 K. The spectra contain components due to internal vibrations of selenate radicals in the frequency region 300∼900 cm -1 , and those due to external lattice vibrations in the region 0∼200 cm -1 . The soft-mode was observed whose frequency decreases as the transition point T 1 (129.5 K) is approached from below. On the basis of these results the relationship between the soft-mode and the phase transition is discussed.

The Origin of Mechanical Twins in (NH4)2SO4

We have found that, if in (NH 4 ) 2 SO 4 a hexagonal phase happened to exist somewhere on higher temperature side, we can explain group-theoretically various experimental results of mechanical twins observed in (NH 4 ) 2 SO 4 crystals as straightforward consequences of a hypothetical hexagonal-orthorhombic phase transition. The transition occurs through an instability at the M point of the Brillouin zone. The microscopic picture of the transition may be the ordering in the orientations of the tetrahedral SO 4 - ions.

Successive Phase Transitions in {N(CH3)4}2CuCl4

Successive phase transitions in {N(CH 3 ) 4 } 2 CUCl 4 single crystal were studied by the measurements of the angle between two extinction positions, thermal expansions, dielectric constants and ultrasound velocities, and also by the weissenberg photographs. It has turned out that {N(CH 3 ) 4 } 2 CUCl 4 undergoes the phase transitions successively from the prototypic phase (Pmcn), to the incommensurate phase, to the ferroelastic commensurate monoclinic phase (P12 1 /c1) and then to another monoclinic phase (P112 1 /n) at T 1 =24°C, T 2 =18°C (on heating) and T 3 = -10°C, respectively. The phase transition sequence of {N(CH 3 ) 4 } 2 CuCl 4 , including the incommensurate-commensurate (ferroelastic) transition, is discussed in terms of the Landau-type free energy.

Composite Structure of BaTiO3 Nanoparticle Investigated by SR X-Ray Diffraction

The size effects on the crystal structure and phase transitions have been investigated for BaTiO 3 fine particles of 700, 400, 300 and 100 nm size, by means of powder diffraction using synchrotron ...

Evidence of Incommensurate-Ferroelastic (Commensurate) Phase Transition in {N(CH 3 ) 4 } 2 CuCl 4 Crystal

Successive phase transition in {N(CH 3 ) 4 } 2 CuCl 4 has been studied by polarization microscopic observation and by the measurement of dielectric constant. A new phase transition is found to take place at intermediate temperature (about 8°C on cooling, about 18°C on heating) between the two phase transition points at 24°C and -9.4°C previously reported. Evidence is presented that the phase transition is the one from an incommensurate phase to a ferroelastic (commensurate) phase.

Hexagonal-Orthorhombic Phase Transition and Ferroelasticity in K2SO4 and K2SeO4

It is shown that (011) and (031) twin planes can be introduced in the orthorhombic phase of K 2 SO 4 and K 2 SeO 4 , and both twin planes can be moved smoothly through crystals by appropriate external stress in the temperature range from about 300°C to the orthorhombic-hexagonal phase transition point. It is inferred from the crystal structure in the orthorhombic phase that SO 4 - (or SeO 4 - ) radicals must have two stable states (the up and down states), and change from one state to another during the passage of twin planes. It is suggested that the space group of the hexagonal phase is D 6h 4 –P6 3 /mmc, and the hexagonal-orthorhombic phase transition is essentially due to the ordering of SO 4 - (or SeO 4 - ) radicals.

Twin Plane Motion in (NH4)2SO4

Dynamical properties of mechanical twins in (NH 4 ) 2 SO 4 have been studied using a polarizing microscope both in the paraelectric and ferroelectric phases. It has been found that both {011} and {031} twins can be introduced into an originally untwinned crystal, and twin planes once introduced can propagate through the crystal by application of mechanical stress. The threshold stress necessary to move twin planes was found to increase with decreasing temperature, but did not show any abrupt jump at the ferroelectric transition point. It is suggested by considering twinning condition that propagation of twin planes is accompanied with the reorientation of tetrahedral molecules SO 4 - .

Observation of the Soft Polar Mode in the Paraelectric Phase of Li2Ge7O15

The dielectric dispersion associated with the ferroelectric phase transition in Li 2 Ge 7 O 15 has been measured in the paraelectric phase by means of a backward-wave oscillator spectrometer in the frequency region 8-24 cm -1 . A temperature dependent resonant mode was found, which corresponds to the soft mode previously observed in the ferroelectric phase by Raman scattering. This mode has a very small oscillator strength and its contribution to the dielectric constant does not exceed 0.1 even in the vicinity of the transition point. A relaxational mode with the characteristic frequency of about 0.03 cm -1 is required for consistently explaining the reported dielectric anomaly.