Toshio Osaka

Study of the Phase Transition in KH3(SeO3)2. I. Acoustic Phonon Softening

The phase transition of KH 3 (SeO 3 ) 2 , was studied by ultrasonic velocity measurements at 5 and 10 MHz together with X-ray diffraction measurement and pyroelectric examination. It was clarified that the crystal below the transition point ( T c ) is nonferroelectric and belongs to the centrosymmetric space group P2 1 / b . All of the elastic stiffnesses c i j were determined front the velocity data. The phase transition accompanies with a large phonon softening only in the c 44 mode. It was shown that the ferroelastic behavior above and below T c can well be explained by a phenomenological theory.

Dielectric and Dilatometric Studies of Phase Transitions in (ND4)3D(SeO4)2 and Isotope Effect

Dielectric and dilatometric measurements were made for the deuterated crystal (ND 4 ) 3 D(SeO 4 ) 2 . For comparison the dilatometric measurement was also made for the undeuterated crystal (NH 4 ) 3 (SeO 4 ) 2 . Two phase transitions were found at +55^° C and at about +29^° C for (NH 4 ) 3 H(SeO 4 ) 2 besides the two phase transitions previously reported. Deuteration rises the Curie temperature by about 34°C, showing important role of the O–H–O bonds playing in the ferroelectric transition. The Curie-Weiss constant and spontaneous polarization of the deuterated crystal are 3.7×10 3 K and 2.4 µC·cm -2 at -170^° C , respectively.

Acoustic Phonon Softening and Spontaneous Strains in KD3(SeO3)2

Ultrasonic velocities at 5 and 10 MHz and spontaneous strains of KD 3 (SeO 3 ) 2 were measured and the results were compared with those for KH 3 (SeO 3 ) 2 . Elastic constants were obtained from the velocity data. The phase transition of KD 3 (SeO 3 ) 2 accompanies an instability of the soft s 44 mode alone, as in the case of KH 3 (SeO 3 ) 2 . Deuteration yields no significant isotope effect in the Curie-Weiss constant C s , although it gives large isotope effect in the transition temperature T c .

New Phase Transitions of Monoclinic RbD_2PO_4 and Its Forced Ttansition to the Ferroelectric State

Dielectric and dilatometric properties of monoclinic RbD 2 PO 4 have been studied. The crystal undergoes successive phase transitions accompanied with a large peak of the dielectric constant along the unique b axis e b at 44°C ( T II ), and a change in temperature gradient of e b -1 at 104°C ( T I ) respectively. The phase below T II is antiferroelectric or approximately antiferroelectric. The forced transition to a ferroelectric phase is observed below T II as a double hysteresis loop. The critical electric field to cause the forced transition and spontaneous polarization of a sublattice are about 12 kV/cm and 1.8 µC/cm 2 at 30°C respectively.

Dielectric Studies on the Phase Transitions of {(NH4)3H(SO4)2}1-x{(ND4)3D(SO4)2}x System –Ferroelectricity in (NH4)3H(SO4)2–

Dielectric constant along the direction perpendicular to the (001) plane in crystals of {(NH 4 ) 3 H(SO 4 ) 2 } 1- x {(ND 4 ) 3 D(SO 4 ) 2 } x system was measured for various x values in a temperature range from liquid nitrogen temperature to room temperature. The crystal has two ferroelectric phases in the range of x ≥0.9 and one ferroelectric phase in the range of x <0.9. It is found from the phase diagram that the normal crystal (NH 4 ) 3 H(SO 4 ) 2 undergoes a ferroelectric phase transition at about -210°C, besides four non-ferroelectric transitions reported previously. Saturated value of spontaneous polarization estimated as a function of x is about 1.4 µC/cm 2 for (NH 4 ) 3 H(SO 4 ) 2 and about 2.5 µC/cm 2 for (ND 4 ) 3 D(SO 4 ) 2 in the lowest ferroelectric phase. There is practically no isotope effect on non-ferroelectric transitions, in contrast with large isotope effect on ferroelectric ones.

Ferroelectricity of Deuterated Triammonium Deuterium Disulfate and Isotope Effect on Ferroelectric Activity

The dielectric properties of deuterated triammonium deuterium disulfate have been studied at 1 atm in the temperature region between -170° C and 25° C . It has been found that the crystal is ferroelectric in two phases. The spontaneous polarization is 2.15 µCoul·cm -2 at -170° C . The present results indicate a large isotope effect on ferroelectric activity.

Ferroelastic Phase Transition in (NH4)3H(SeO4)2

Optical observation of twin structures, X-ray examinations and the measurement of the dielectric constant of (NH 4 ) 3 H(SeO 4 ) 2 were carried out in reference to the phase transition at 29°C ( T c ). It was found that (NH 4 ) 3 H(SeO 4 ) 2 undergoes the improper ferro-elastic phase transition at T c . The crystal is determined to be monoclinic withthe space group A 2/ a below T c and trigonal with the space group R \bar3 m above T c . The crystal has spontaneous strain ( e 22 - e 11 )/2=-3.0×10 -3 and e 13 =3.3×10 -3 at room temperature. It was clarified by both observation of domains and symmetry consideration that the crystal exhibits twin structures with two types of domain walls and three kinds of domain orientations at room temperature. The behaviour of the dielectric constant observed near T c is well explained by the Landau theory.

Successive Phase Transitions in Ferroelectric RbHSeO4

The dielectric and dilatometric properties of RbHSeO 4 are studied. It is found that RbHSeO 4 undergoes a phase transition at 173°C besides the 98°C phase transition and it is ferroelectric below the 173°C transition.

Optical and X-Ray Studies in Ferroelastic Cs3H(SeO4)2

Optical observation of domain structures and X-ray examinations of Cs 3 H(SeO 4 ) 2 were carried out above room temperature. The effect of an external force on the rearrangements of the constituent ions accompanied by the movement of domain boundary was studied at room temperature. The interrelations between unit cells in each phase were obtained above room temperature. When the crystal is warmed up through the phase transition at T II , all the hydrogen bonds are instantaneously broken, and then the hydrogen atoms are so rearranged as to form new hydrogen bonds between two diffrent adjacent SeO 4 -2 tetrahedra. When the domain boundary is moved, the SeO 4 -2 tetrahedra are tilted and displaced to new positions, and simultaneously hydrogen bonds are broken, and then new hydrogen bonds are reconstructed.

Dielectric Study on the Effect of Hydrostatic Pressure on the Phase Transitions in the System of {(NH4)3H(SO4)2}1-x{(ND4)3D(SO4)2}x

The concentration-pressure-temperature three dimensional phase diagram of {(NH 4 ) 3 H(SO 4 ) 2 } 1- x {(ND 4 ) 3 D(SO 4 ) 2 } x system was studied by means of dielectric measurements in a pressure-temperature range of \(p{\lesssim}10\) kbar and -100^°\(C{\lesssim}T{ 0.90 show the ferroelectric VI phase even at atmospheric pressure. A good correspondence between the effects of hydrostatic pressure and deuterium-substitution was established as regards the stability of the ferroelectric phases. Two pressure-induced phases (Phases VIII, IX) were newly observed in the normal-deuterated mixed crystal system.