Masaru Komukae

Dielectric relaxation in ferroelectric (CH3)2NH2H2PO4 and its isotope effect

Abstract The temperature dependences of the real and imaginary parts of the complex dielectric constant of (CH3)2NH2H2PO4 and its partially deuterated crystal, (CH3)2N(H1-x D x )2(H1-x D x )2PO4, were measured in a frequency range between 1 MHz and 1 GHz. The results obtained in the present study indicate that the crystal of (CH3)2N(H1-x D x )2(H1-x D x )2PO4 exhibits remarkable dielectric relaxation in the microwave frequency range, and exhibits critical slowing-down near Tc . It is therefore concluded that the ferroelectric phase transition is of order-disorder type, and the transition temperature and the relaxation time are enhanced by the increase of the deuterium concentration.

DIELECTRIC AND THERMAL STUDIES OF PHASE TRANSITION IN (CH3)2NH2H2ASO4

Abstract Dielectric and AC calorimetric investigations were carried out for the single crystal of dimethylammonium dihydrogen arsenate, (CH3)2NH2H2AsO4. The complex dielectric constant e* was measured in a frequency range between 10 MHz and 13 GHz. The critical slowing-down phenomenon of polarization was observed above Tc . In the paraelectric phase, the relaxation time τ can be well explained by the quasi-one-dimensional Ising model. The activation energy was estimated to be Ea = 64.3 meV. The transition heat and the transition entropy were obtained as ΔQ = 240 J/mol and ΔS = 0.95 J/mol·K, respectively.

Phase transition and dielectric properties of a new ferroelectric, N2H5Al(s04)212H2O

Abstract The dielectric property of N2H5A1(SO4)2·12H2O was studied below room temperature. The dielectric constant measured along the [100] direction shows a discontinuous change at the Curie temperature of Tc = -109.6 °C on cooling and obeys the Curie-Weiss law above T. The pyroelectric charge was measured after poling the crystal. It was found that the crystal becomes ferroelectric below Tc = -107.8 °C on warming, and has the spontaneous polarization of 3.3 μC/cm2 at −186°C.

Structure Determination of Monoclinic RbD 2 PO 4 in the High-Temperature Phase

Crystal structure of monoclinic RbD 2 PO 4 is determined in the high-temperature phase. It is found that the structure is quite similar to the paraelectric structure of CsH 2 PO 4 . On the basis of the structure, we discuss why negative and positive H/D isotope effects are induced at higher and lower phase transition temperatures, respectively.

NMR Study on (CH 3 ) 2 NH 2 H 2 AsO 4

Proton NMR line width and spin-lattice relaxation time T 1 for the ferroelectric (CH 3 ) 2 NH 2 H 2 AsO 4 have been measured at the temperature range between 77 K and 300 K. The temperature dependence of T 1 is well described by the expressions based on rotation of the methyl group using the value of activation energy E a = 99 meV and elementary correlation time 0 = 1.5 2 10 m 13 sec. Moreover, a sharp dip of T 1 near ferrroelectric phase transition temperature ( T c = 274.5 K) results from the ordering of protons. In addition, the temperature dependence of T 1 deviates from the expression for molecular rotation above around 200 K. It is deduced that the deviation is caused by the suppression of some kind of proton motion.

Phase Transitions in Monoclinic RbH 2 PO 4

Dielectric and thermal measurements were carried out for the RbH 2 PO 4 crystal belonging to the tetragonal system at room temperature. The monoclinic RbH 2 PO 4 crystal, which could be obtained by heating the tetragonal RbH 2 PO 4 crystal above the tetragonal-monoclinic transformation temperature, undergoes successive phase transitions at 390.1 K and 248.8 K. Dielectric constant of monoclinic RbD 2 PO 4 in the phase I is described by the quasi-one-dimensional Ising model as well as in the phase II.

Pressure effect on the phase transition temperature in (CH3)2NH2H2AsO4

Abstract The effect of hydrostatic pressure on the transition temperatures in (CH3)2NH2H2AsO4 and (CH3)2N(H05D0.5)2(H0.5D0.5)2AsO4 were investigated by dielectric measurements. A pressure-induced phase was newly observed for (CH3)2NH2H2AsO4 above 1.36 GPa at room temperature. The transition to the pressure-induced phase shows first order character. As pressure increases, the ferroelectric transition temperatures for both crystals monotonously decrease. Their pressure derivatives, dTc /d p . were obtained to he −49 ± 1 K/GPa for (CH3)2NH2H2AsO4and −52 ± 1 K/GPa for (CH3)2N(H0.5D0.5) 2(H0.5D0.5)2AsO4. From the present experimental results. the lack of an appreciable isotope effect on dTc /dp was observed in (CH3)2NH2H2AsO4.

Neutron Structure Analysis of Paraelectric and Ferroelectric Phases of (CH3)2NH2H2PO4

Crystal structure of dimethylammonium dihydrogen phosphate has been determined in the paraelectric phase at 298 K and in the ferroelectric phase at 223 K by a single crystal neutron diffraction method. Two inequivalent protons in the hydrogen bonds forming the H 2 PO 4 − chains are disordered over two-equilibrium positions each other with separations 0.47 and 0.49 Å in the paraelectric phase. In the ferroelectric phase, they order at a position nearly equal to one of two positions related to the proton disorder. Accompanying the proton order, the PO 4 tetrahedra deform the structure in the paraelectric phase.

Dielectric relaxation of NH4Al(SO4)2 · 12H2O

Abstract The pyroelectric charge of NH4Al(SO4)2·12H2O was measured after poling the crystal. It was found that the crystal becomes ferroelectric below the Curie temperature of Tc = -197°C on warming and has spontaneous polarization of 0.10μ C/cm2 at-240°C. The complex dielectric constant was measured at various frequencies in a frequency range between 330 Hz and 1 × 106 Hz. The observed temperature and frequency dependences of the complex dielectric constant can be approximated in terms of single relaxation time τ(T) of the Debye-type. Above the Curie temperature, τ(T) is described well by the relation τ(T) =(h/kB)·(T-To)-1·exp(ΔF/kbT) with the activation free energy ΔF =ΔFo[1 -α (T - Tc)]. The values of ΔFo and a are estimated to be 1.49 × 10-20 J and 0.0028 deg-1, respectively.