Hiroshi Motegi

X-Ray Study on the Phase Transition of NaNO2

The precise temperature dependence of x-ray diffraction effects near the transition temperature of ferroelectric NaNO2 crystal has been examined. The satellite reflections, 1±σ 5 0, start to appear at Ts, 0.2°C below the well-known first-order transition point Tt. Marked asymmetry of the satellite reflections was sometimes observed when a multi-domain crystal was measured. The period of the sinusoidal modulation along the a-axis varies from 10.3 a0 to 8.4 a0 with increasing temperature. The effect of a d. c. external field along the b-axis on the x-ray diffraction effect was also examined. The experimental evidence indicates the presence of a new phase between Ts and Tt. Some thermodynamical calculations for the phase transition of NaNO2 were carried out. To explain the intensity asymmetry of the satellite reflections the diffraction effect for a distorted crystal was also calculated.

X-Ray Study of High-Temperature Phase Transitions in KH2PO4

High-temperature phase transitions in KH 2 PO 4 near 110°C and 180°Chave been studied by X-ray diffraction. The extinction rule observed at 123°C is the same as that in the room-temperature phase. It is concluded that on heating through 187°C crystal system changes from tetragonal to monoclinic whose space group is P 2 1 or P 2 1 / m . The lattice parameters at 195°C are given by a =7.47A, b =7.33A, c =14.49A, α=β=90° and γ=92.2° .

X-Ray Study of the Surface Layer on Barium Titanate Single Crystal

The surface of the single crystal of BaTiO 3 was studied by X-ray diffraction method under a d.c. electric field. In the paraelectric phase, tetragonal surface layers are induced by the electric field. It is concluded that a large electric field exists in the surface part of the crystal whereas the interior part contributes to only a small potential drop. The migration of the negative charge is shown to play an important role for such a distribution of the electric field in the crystal. The thickness of the surface layer was determined to be 1.5×10 -4 cm. Similar surface layer was observed in the ferroelectric phase. By taking the existence of the surface layer into account, an “actual” coercive field for the specimen of 0.32 mm thick used in the experiment was estimated to be about 1.2×10 5 V/cm, which is about 200 times as large as the usually reported value.

Crystal Structure and Phase Transition of Hydrogen Chloride

Detailed structural studies of HCl single crystal were performed by X-ray and neutron diffraction experiments. A face-centered cubic lattice with a twelve-fold disordered orientation of HCl molecules was found only above 120 K. Below 120 K, the molecules form hydrogen bonded zigzag chains and a synthetic twin structure of an orthorhombic lattice with the (101) twin boundary is realized. No appreciable change of the crystal structure was seen in the diffraction experiments at the transition point 98 K. The mechanism of the phase transition can be explained by considering a fluctuation of chains and molecular flippings.

Effect of Heat-Treatment on Dielectric Dispersion of Ferroelectric Ca2Sr(C2H5CO2)6

Complex dielectric constants of Ca 2 Sr(C 2 H 5 CO 2 ) 5 along the ferroelectric c -axis have been measured for as-grown and annealed crystals between 1 kHz and 1 GHz around their Curie temperatures, T c . The magnitude of the resonant type dispersion which appears in the low frequency region decreases by annealing. The ratio of the relaxation time of the Debye dispersion, τ, to the dipolar part of the static dielectric constant, e s - e ∞ , can be expressed as τ/( e s - e ∞ )= A exp ( U / k T ) except in the vicinity of T e for the as-grown crystal as well as for the annealed one. The activation energy U decreases whereas the value of A increases slightly by annealing. A narrow distribution of τ is found in the as-grown crystal contrary to the annealed one which shows the monodispersive Debye relaxation.

Dielectric Dispersion in KH2PO4 below 10 K

Complex dielectric constants of KH 2 PO 4 along the c - and a -axes have been measured in the frequency range from 10 Hz to 20 kHz. Dielectric dispersions of relaxation type with small loss peaks have been observed at temperatures below 10 K in both directions. Arrhenius plots of the relaxation frequencies give extremely small values of activation energies, 0.014 eV for the c -axis and 0.004 eV for the a -axis, respectively.

Neutron diffraction study on molecular libration in solid DCl

Neutron diffraction measurements of DCI using a single crystal revealed that such an anomalous transition as observed in the higher temperature cubic phase of HCl did not occur. Mean square angular displacements due to the molecular libration of DCl were obtained from anisotropic temperature factors of deuterium atom. Their temperature variation showed a rapid increase in the rate of the 60° molecular flipping with increasing temperature. The difference of the molecular motion between DCl and HCl was observed, which could not be explained by a simple harmonic oscillator model.

Lattice Deformations in the Successive Phase Transitions of (NH4)2ZnCl4 and {N(CH3)4}2ZnCl4

Polarization microscopic observations of (NH4)2ZnCl4 have revealed a domain structure in o-plates in the phases III (46~91°C), IV (-5~46°C) and V (-10~-5°C) and another one in c-plates in the phase V. The former domain structure does not change at the III–IV and IV–V phase transitions and disappears at about 70°C. Similar observations of [N(CH3)4]2ZnCl4 have revealed different domain structures in a- and c-plates in the phase IV (-92~3°C) and in a- and b-plates in the phase V (-112~-92°C). The lattice strains associated with these domains except two in [N(CH3)4]2ZnCl4 are determined with the X-ray double-crystal method. Discussions are made on the crystal symmetries of the intermediate phases of both materials. The temperature dependences of the spontaneous shear strains observed in the intermediate phases of both materials are found to be different from those expected from the usual Landau-type free energy.

Dielectric Dispersion of Triglycine Sulfate (TGS) Powders

The complex dielectric constant of TGS powders 74~105 µm in particle size has been measured between 77 K and 340 K in the frequency range from 10 Hz to 105 Hz. Four types of dielectric relaxations are found in the ferroelectric phase. The relaxation frequencies obey the formula, fr=foexp (-U/kT). The values of fo and U are determined to be 1×108 Hz and 0.33 eV, 4×1012 Hz and 0.35 eV, 9×1013 Hz and 0.33 eV, and 2×108 Hz and 0.06 eV, respectively, which are quite different from those of multidomain single crystals.

Periodic Motion of Laminar Domains in BaTiO 3 under Constant Electric Field

A periodic motion of laminaar domains is observed in BaTiO 3 near the Curie temperature under a static electric field. A competition between minimization process of the strain energy of the crystal plate and a movement of mobile charge on domain boundaries by the electric field is considered to be a possible origin of the motion.