Kiyofumi Nagata

The II-I crystal transformation of poly(vinylidene fluoride) under tensile and compressional stresses

Abstract The mechanism for a crystal transformation in poly(vinylidene fluoride) by a tensile deformation at atmospheric pressure was investigated in the temperature range 25–150 °C. Simultaneous X-ray and stress-strain relationship measurements during the drawing process revealed that the crystal transformation from Form II to Form I occurred at the temperatures below 130°C where the sample deformed by cold-drawing, and always initiated at the deformation stage where necking was completed at the centre of the tensile sample. Above 140°C, on the other hand, the sample deformed uniformly without necking and did not perform the crystal transformation. Thus, it was suggested that a heterogeneous stress distribution in the sample played a critically important role in the crystal transformation phenomenon. A uniaxial compressional deformation also caused the crystal transformation from Form II to Form I in this sample. The crystal conversion ratio varied with the conditions of compressional pressure and temperature, and the molecular orientation in resultant samples depended on the shapes of the anvils employed in the compression experiment. By applying a high d.c. voltage during the compressional deformation, a highly uniaxially oriented Form I film with prominent piezoelectric properties was obtained.

Raman and Infrared Spectra of Rhombohedral Selenium

Raman and infrared spectra of rhombohedral selenium have been measured and the fundamental bands observed in these spectra are assigned by analogy to the S6 ring molecule. Calculation of the normal mode vibration for the Se6 ring using a modified Urey-Bradley force field confirms this assignment. Anomalous softening of the A1g stretching mode relative to other modes and to the corresponding A1 mode in the Se8 ring was observed. This phenomenon is explained by the idea of interference of inter-molecular interaction with the intra-molecular bonding. Comparison of the observed spectra of rhombohedral selenium with those of other allotropes (α-monoclinic, trigonal, and amorphous selenium) suggests that Se6 ring molecules may exist in amorphous selenium.

Direct Observation of Crystal Transformation Process of Poly (vinylidene fluoride) under High Pressure by PSPC X-Ray System

A PSPC (position sensitive proportional counter) X-ray system was employed to directly observe the crystal transformation processes of poly (vinylidene fluoride) at high pressures and high temperatures. X-ray diffraction measurements performed with a heating rate of 5°C/min have revealed that Form I structure is formed after the melting of Form II structure at high pressures above 3,500 kg/cm2. This result provides a solid proof for the origins of multiple endothermic peaks observed in the DTA melting trace at high pressures. Changes in X-ray diffraction patterns during annealing process at a temperature below the melting temperature of Form II structure under 4,000 kg/cm2 were followed and it has been shown that Form II structure performs a crystal transformation into Form I structure with an activation energy of about 30 kcal/mol.

Raman Spectra of Rhombohedral and α-Monoclinic Selenium under High Hydrostatic Pressure

The Raman spectra of rhombohedral, α-monoclinic, and trigonal selenium have been measured under high hydrostatic pressures up to about 8 kbar at room temperature. The frequencies of the A1-type stretching modes in both rhombohedral and α-monoclinic selenium have been observed to decrease with increasing pressure, as has that of trigonal selenium. This indicates that interference between inter- and intramolecular bondings also exists in crystals composed of ring molecules such as Se6 or Se8.

X-Ray, Raman and Photoluminescence Study of Vacancy Ordered β-Ga2Se3 under High Pressure

The effect of pressure on the structure and optical properties of vacancy-ordered β-Ga2Se3 has been investigated by X-ray diffraction, Raman and photoluminescence spectroscopies using diamond anvil cell. From the shift of the photoluminescence peak with pressure, the pressure dependence of the band gap energy (dEg/dP) is derived to be 46 meV/GPa. From Rietveld analysis, the pressure dependencies of the lattice parameters and the atomic positions are obtained. The relation between these crystal data and the pressure dependence of the band gap energy is discussed. From the X-ray measurements, a structural phase transition at 14 GPa is observed, as is observed in α-Ga2Se3. In the Raman scattering measurement, pressure-tuned resonant phenomenon is observed near 2.5 GPa, where the band gap energy of β-Ga2Se3 coincides with the photon energy of the argon ion laser (λ = 514.5 nm) used for Raman measurement as excitation light.

X-ray structural analysis of the high-pressure phase III of tellurium

A structure of a high-pressure phase of tellurium (Te-III) has been re-examined by an x-ray diffraction method. This phase appears at a pressure between 7 and 27 GPa, where a second-order phase transition to Te-IV occurs. The newly obtained crystal lattice of Te-III at 8 GPa is monoclinic with a = 8.4682(14) A, b = 4.7424(8) A, c = 3.9595(7) A, β = 88.112(11)°. The space group is C2/m, with six atoms in the unit cell, two in positions 2a at (0, 0, 0) and four in positions 4i at (x, 0, z) with x = 0.324(11), z = 0.675(3).

Raman Spectroscopic and X-Ray Diffraction Study of Sulfur under High Pressure

Raman spectrum and X-ray powder pattern of orthorhombic sulfur have been examined at pressures up to about 10 GPa. The Raman spectrum shows reversible phase transition at 5.2 GPa, while the X-ray diffraction pattern shows no sign of phase change up to 8.3 GPa. These contradictory results are explained as follows. A high-pressure phase exists above 5.2 GPa. In this pressure region, the metastable orthorhombic phase can also exist. The argon ion laser (λ=514.5 nm) used for Raman measurement induces a structural transformation from the orthorhombic phase to the high-pressure phase. The Raman spectrum suggests that the high-pressure phase is composed of chain molecules. In addition, evidence for interference between inter- and intramolecular bondings has been observed in this phase.

Raman Spectrum of Selenium Dissolved in an Aqueous Solution of Sodium Sulfide

The Raman spectrum of selenium dissolved in an aqueous solution of sodium sulfide was measured using an YAG laser and a multiple-reflection cell, and an intense broad band was observed at 264 cm-1. This frequency is higher than those of the corresponding symmetric bond-stretclaing modes in any solid form of selenium. This is explained by considering the decrease of the interference between inter- and intramolecular bondings in the solution.

Ultrasonic Study of High Pressure Phase in Polyethylene

Behaviors of transitions and the nature of high pressure phase in polyethylene are studied using methods of longitudinal and transverse ultrasonic waves, differential thermal analysis (DTA) and X-ray diffraction by PSPC X-ray system. Two anomalous absorption peaks of longitudinal wave agree with the normal phase to high pressure phase transition temperature and the high pressure phase to fused phase transition temperature as observed by DTA and X-ray diffraction, respectively. For the folded-chain crystal sample, the recrystallization phenomena by lamella thickening during the heating process has been discovered by the anomaly in longitudinal sound velocity. The result of transverse wave shows that the modulus of rigidity of high pressure phase is of the same order in magnitude as that of the fused state. Therefore, the high pressure phase should be a nematic liquid crystal-like state.

Structural Phase Transitions of Ga2Se3 and GaSe under High Pressure

High pressure X-ray powder diffraction studies have been made on α-Ga 2 Se 3 up to 27 GPa and e-GaSe up to 64 GPa by using synchrotron radiation. A structural phase transition is observed for α-Ga 2 Se 3 at 14 GPa, as is observed in β-Ga 2 Se 3 , The diffraction patterns of the high pressure phases of α- and β-Ga 2 Se 3 are almost the same, showing that the structures of the high pressure phases of both α- and β-Ga 2 Se 3 are the same. On the other hand, the structural phase transition for e-GaSe is observed near 25 GPa. The diffraction pattern of the high pressure phase of GaSe is similar to that of Ga 2 Se 3 . This shows that the high pressure phase of GaSe is similar to that of Ga 2 Se 3 , although the atomic concentrations in the alloys are different between Ga 2 Se 3 and GaSe. Most of the diffraction profile of the high pressure phase is explained by a NaCI type structural model, suggesting that the high pressure phase has a structure similar to NaCI.