Yasuharu Makita

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.

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.

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 - .

Ferroelectric and Optical Properties of Na(K-NH4)-tartrate Mixed Crystals

The phase diagram of the mixed crystals composed of NaK-tartrate and its isomorph NaNH 4 -tartrate has been investigated over the whole range of the concentration. The properties of each phase have been studied by dielectric and pyroelectric measurements and by optical observations. In 90.5 per cent NaNH 4 -tartrate crystal three kinds of polymorphic modification have been found, that is, successively, a ferroelectric phase with spontaneous polarization along the crystallographic b axis, another ferroelectric phase with spontaneous polarization along the crystallographic a axis and a paraelectric phase. The phase of compositions near the pure NaNH 4 -tartrate has been confirmed to be ferroelectric with spontaneous polarization along the b axis.

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 .

Calorimetric Study on Phase Transitions in (NH4)3H(SO4)2

Specific heat of (NH 4 ) 3 H(SO 4 ) 2 was measured in a temperature range \(90\,\text{K} \lesssim T \lesssim 440\) K covering the four phase transitions. The transition entropies were estimated to be Δ S I–II =2.5 ±0.2, Δ S II–III =2.10 ±0.08, Δ S III–IV =0.51 ±0.03 and Δ S IV–V =0.53 ±0.03 in the units of cal ·mol -1 ·deg -1 for the I–II, II–III, III–IV and IV–V phase transitions, respectively.

Ferroelasticity and Hypothetical Hexagonal-Orthorhombic Phase Transition in (CH3NHCH2COOH)3·CaCl2

It has been found that the (011) and (031) twin planes observed in tris-sarcosine calcium chloride (TSCC) can be moved by an appropriate external stress just like those found recently in (NH 4 ) 2 SO 4 . The ferroelastic properties of TSCC in the orthorhombic phase can be explained if the existence of a hypothetical hexagonal phase is assumed just as in the case of (NH 4 ) 2 SO 4 . The space group of the hypothetical phase is inferred to be D 6h 3 –P6 3 /mcm, in which sarcosine molecules are in disordered state, occupying either one of two stable states with equal probability.

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.