Junko Hatori

Phase Transition in (CH3)2NH2H2PO4 and Its Ferroelectric Activity

Dielectric, AC calorimetric and X-ray investigations were carried out for a single crystal of (CH 3 ) 2 NH 2 H 2 PO 4 . The specific heat shows a λ-type anomaly associated with a phase transition at the Curie temperature of 259.15 K ( T c ). A large dielectric anomaly was observed at T c and a D - E hysteresis loop was observed below this temperature. It was found that the crystal undergoes a second-order phase transition and becomes ferroelectric below T c . The results of X-ray examination indicate that the crystal belongs to monoclinic P 2 1 / n with the lattice parameters of a =9.290(1) A, b =9.592(3) A, c =7.001(1) A and β=90.35(3)° at room temperature, and to P n with a =9.267(4) A, b =9.560(1) A, c =6.988(2) A and β=90.42(3)° at 193 K.

Phase Transition in Ferroelectric (CH3)2NH2H2AsO4 and Its Dielectric Property.

Dielectric, AC calorimetric and X-ray investigations were carried out for the single crystal of (CH 3 ) 2 NH 2 H 2 AsO 4 . It was found that the crystal undergoes the ferroelectric phase transition of second order at 274.5 K(= T c ), and has a spontaneous polarization P s =4.5 µC/cm 2 at 93 K. The phase transition of (CH 3 ) 2 NH 2 H 2 AsO 4 is characterized by the symmetry change from the monoclinic space group P 2 1 / n to P n . The temperature dependence of the static dielectric constant e 0 observed for (CH 3 ) 2 NH 2 H 2 AsO 4 , which shows deviation from the Curie-Weiss law, can be described well above T c except near T c by the expression based on the quasi-one-dimensional Ising model.

Electrical properties and memory effect in the field effect transistor based on organic ferroelectric insulator and pentacene

We have fabricated a field effect transistor (FET) based on an organic ferroelectric insulator and molecular conductor, and investigated the electrical properties and memory effects on the PEN-FET. We have observed a drastic change in the drain current at around the coercive electric fieldEc of the organic ferroelectric insulator in not only a FET (PEN-FET) based on a pentacene (PEN) film but also a FET (IPEN-FET) based on an iodine doped PEN film. The magnitude of the change of the drain current for the IPEN-FET is 200 times larger than that for the PEN-FET. It is expected from these results that the PEN-FET (especially the IPEN-FET) is an improvement in such devices, since it operates at a low gate electric field accompanied by the appearance of the spontaneous polarization in the organic ferroelectric insulator. In addition, we have found that the drain current for the PEN-FET does not return to the initial drain current ofEG=0 V/cm for more than one week, even if the gate electric field is changed to 0 V/cm from 500 V/cm(>Ec). From these results, it is suggested that the PEN-FET becomes a memory device.

Ferroelastic phase transition in Tl2SeO4

Thermal, dielectric, X-ray and optical investigations of Tl 2 SeO 4 were carried out above room temperature. It was found that Tl 2 SeO 4 undergoes an improper ferroelastic phase transition at 388°C , and has spontaneous strain a s =( e 22 - e 33 )/2 = 1.97 ×10 -2 at room temperature. Tl 2 SeO 4 shows domain structures composed of three kinds of orientations with two types of domain boundaries. The crystal belongs to the space group P 6 3 / m m c with the hexagonal unit cell parameters of a h =6.295(6) A and c h =8.189(5) A at 410°C . It was deduced that the improper ferroelastic phase transition is characterized by the symmetry change from P 6 3 / m m c to Pnam .

Thermal and NMR Studies on Phase Transitions of Tl2SeO4.

The M2XO4-type crystals where M=K, Rb, Cs, NH4, Tl; X= S, Se have interesting characteristics, for example ferroelectricity, ferroelasticity and incommensurate modification. These crystals crystallize to an orthorhombic system with the space group Pnam at room temperature. K2SeO4 shows ferroelectricity along the c axis below 93K and ferroelasticity below 745K. Moreover, this crystal has an incommensurate modification along the a axis in the temperature range between 93K and 130K. On the other hand, (NH4)2SO4 shows ferroelectricity below 223K and ferroelasticity at room temperature. 4) However, an incommensurate modification in (NH4)2SO4 was not observed in contrast with K2SeO4. Thallium selenate Tl2SeO4, where the M-site is replaced by thallium, undergoes an improper ferroelastic phase transition at 661K. 6) This ferroelastic one is characterized by the symmetry change from the hexagonal space group P63/mmc to the orthorhombic Pnam in common with many other crystals of a M2XO4 type. Moreover, it has been found by Unrch that Tl2SeO4 undergoes a phase transition at 70K on cooling and at 75K on warming, accompanied by a discontinuous change in the dielectric constant and a thermal hysteresis of 5K which are indicative of first order. The space group of Tl2SeO4 below the phase transition temperature of 70K was reported to be P212121, 8) which is clearly different from the space group Pna21 in the cases of the K2SeO4 and (NH4)2SO4 crystals. Although the M2XO4-type crystals exhibit a common nature in the mechanism of ferroelasticity, a variety of different phase transition has been reported for some of them below room temperature. Recently we examined the thermal property and the spin-lattice relaxation time T1 by means of NMR spectrometer for the Tl2SeO4 crystal, and found a new phase transition at 97K, except for the phase transitions at 70K and 661K. The present paper reports the results obtained for the specific heat and the temperature dependence of T1 for Tl2SeO4 Single crystals Tl2SeO4 were grown by a slow evaporation method at 303K from an aqueous solution of thallium selenate. The single crystals thus grown are of pseudo-hexagonal plate with the predominant (100) faces and are transparent. The specific heat was measured with an AC calorimetric apparatus (ACC-1, Ulvac

Pressure effect on the phase transition in (CH3)2NH2H2AsO4 and its isotope effect

The effect of hydrostatic pressure on the phase transition of (CH 3 ) 2 NH 2 H 2 AsO 4 was investigated by dielectric measurement at high pressure up to about 1.5 GPa. A pressure-induced phase for (CH 3 ) 2 NH 2 H 2 AsO 4 was newly observed above 1.36GPa at room temperature. The transition to the pressure-induced phase shows first order character. The effects of hydrostatic pressure on the ferroelectric transition temperatures for both crystals of (CH 3 ) 2 NH 2 H 2 AsO 4 and (CH 3 ) 2 N(H 0.5 D 0.5 ) 2 (H 0.5 D 0.5 ) 2 AsO 4 were dielectrically investigated. As the pressure increases, the transition temperatures for both crystals monotonously decrease. Their pressure derivatives, dT c /dp, were obtained to be -49 ± 1 K/GPa for (CH 3 ) 2 NH 2 H 2 AsO 4 and -52 ± 1 K/GPa for (CH 3 ) 2 N(H 0.5 D 0.5 ) 2 (H 0.5 D 0.5 ) 2 AsO 4 . From the present experimental results, the lack of an appreciable isotope effect on dT c /dp was observed in (CH 3 ) 2 NH 2 H 2 AsO 4 .The effect of hydrostatic pressure on the phase transition of (CH 3 ) 2 NH 2 H 2 AsO 4 was investigated by dielectric measurement at high pressure up to about 1.5 GPa. A pressure-induced phase for (CH 3 ) 2 NH 2 H 2 AsO 4 was newly observed above 1.36 GPa at room temperature. The transition to the pressure-induced phase shows first order character. The effects of hydrostatic pressure on the ferroelectric transition temperatures for both crystals of (CH 3 ) 2 NH 2 H 2 AsO 4 and (CH 3 ) 2 N(H 0.5 D 0.5 ) 2 (H 0.5 D 0.5 ) 2 AsO 4 were dielectrically investigated. As the pressure increases, the transition temperatures for both crystals monotonously decrease. Their pressure derivatives, d T c / d p , were obtained to be -49±1 K/GPa for (CH 3 ) 2 NH 2 H 2 AsO 4 and -52±1 K/GPa for (CH 3 ) 2 N(H 0.5 D 0.5 ) 2 (H 0.5 D 0.5 ) 2 AsO 4 . From the present experimental results, the lack of an appreciable isotope effect on d T c / d p was observed in (CH 3 ) 2 NH 2 H 2 AsO 4 .

19F-NMR Study in Phase Transition of [(CH2OH)3CNH3]2SiF6

Measurements of 19F and 1H-NMR absorption lines have been carried out in [(CH2OH)3CNH3]2SiF6 below room temperature. It was found that the 19F-NMR absorption line width becomes narrow with the increase of temperature above around 100 K and becomes almost constant above Tc= 178 K. This result indicates that motional narrowing of 19F-NMR line accompanied by the phase transition at 178 K is observed. It is deduced from this narrowing that the SiF6 2− anions are reoriented with the correlation time which is fast enough to narrow the 19F-NMR absorption line above Tc. Moreover, we have observed the drastic change of 1H-NMR absorption line above Tc. This result indicates that the ─CH2OH groups begin to reorient by the phase transition at Tc. It is deduced from these results that the motions of SiF6 2− anions and ─CH2OH groups play an important role in the phase transition at 178 K.

Thallium selenate (Tl2SeO4) in a paraelastic phase by X-ray powder diffraction.

The structure of thallium selenate, Tl2SeO4, in a paraelastic phase (above 661 K) has been analysed by Rietveld analysis of the X-ray powder diffraction pattern. Atomic parameters based on the isomorphic K2SO4 crystal in the paraelastic phase were used as the starting model. The structure was determined in the hexagonal space group P6(3)/mmc, with a = 6.2916 (2) A and c = 8.1964 (2) A. From the Rietveld refinement it was found that two orientations are possible for the SeO4 tetrahedra, in which one of their apices points randomly up and down with respect to [001]. One Tl atom lies at the origin with 3m symmetry, the other Tl and one of the O atoms occupy sites with 3m symmetry, the Se atom is at a site with 6m2 symmetry and the remaining O atom is at a site with m symmetry. Furthermore, it was also found that the Tl atoms display anomalously large positional disorder along [001] in the paraelastic phase.

Dielectric Dispersion Studies in Pyridinium Periodate Single Crystal

The static dielectric constant of the pyridinium periodate crystal was measured at the frequency of 50 kHz in the temperature range from room temperature to 365 K. In addition, the real and imagina...

LINEAR BIREFRINGENCE AND ULTRASONIC STUDIES OF PHASE TRANSITION IN (CH3)2NH2H2PO4 CRYSTAL

Abstract Linear birefringence (LB) and ultrasonic wave (USW) velocity measurements were performed for (CH3)2NH2H2PO4 single crystal in the temperature range 300-230 K. The observed anomalies of LB (Δni) and USW velocity (vii) are characteristic for continuous phase transition and they are explained within a framework of phenomenological approach. The birefringence increments Δ(Δni) and ultrasound velocity increment δV6 are proportional to the square of spontaneous polarisation. Critical exponent obtained from both methods has practically the same value β ≈ 0.43