Hideyuki Nakayama

Crystallization and glass-transition phenomena were studied for amorphous chlorobenzene (CB)/toluene (TL) binary systems as the function of composition. Samples were prepared by vapor-deposition onto cold substrates, and their structural changes due to temperature elevation were monitored with Raman scattering and light transmission. It was found that the crystallization temperature (T-c) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of glass-transition temperatures (T-g) of many organic compounds on the concentration of additive. Also found was that T-c of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of T-c are discussed briefly.

Crystallization and glass-transition phenomena were studied for amorphous chlorobenzene (CB)/toluene (TL) binary systems as the function of composition. Samples were prepared by vapor-deposition onto cold substrates, and their structural changes due to temperature elevation were monitored with Raman scattering and light transmission. It was found that the crystallization temperature (Tc) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of glass-transition temperatures (Tg) of many organic compounds on the concentration of additive. Also found was that Tc of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of Tc are discussed briefly.

Light transmission through amorphous chlorobenzene was studied during the vacuum deposition of the vapor onto cold substrates. It was found that samples prepared at 78 K were transparent, but those prepared at 15 K were slightly opaque. Raman and X-ray studies indicated that both kinds of samples were amorphous, but they showed different behavior against annealing. The transparent samples needed some induction period to start crystallization when the temperature was raised for annealing, while the slightly opaque samples underwent crystallization by annealing almost without any induction period. Samples prepared at very-low temperatures are considered to incorporate density inhomogeneity, and the resultant free space around molecular aggregates may allow the samples to start crystallization easily.

By the continuous elevation of temperature, ethylbenzene in the supercooled liquid state first turns opaque and then becomes transparent again before it finally crystallizes. These observations are possibly related to the appearance of the second amorphous state with a correlated structure that has recently been reported for triphenyl phosphite.

Density dependence of Raman spectra for carbon dioxide was studied along the isotherm in the temperature region 0.96 less than or equal to T/T-c less than or equal to 1.06 and the density region 0 < rho/rho(c) < 2. Spectrum in the small-wavenumber region below 100 cm(-1) and that in the region of the 2 upsilon(2) intramolecular band, i.e. one of the Fermi dyad, were carefully measured and analyzed. The results are interpreted by assuming the presence of the gas- and liquid-like regions in the supercritical region separated by the ridge of density fluctuation. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

The temperature evolution of fluorescence spectra was measured to study the behavior of electronic excitation in amorphous naphthalene prepared by vacuum deposition on cold substrates. Monomer- and excimer-type fluorescence was observed in the amorphous state. To study the effect of structural relaxation, spectra were measured by raising the temperature stepwise and also measured at a constant temperature each time after the annealing at different temperatures. No serious structural relaxation was observed if we keep the annealing temperature below the crystallization temperature. From the behavior of the intensities of the monomer and excimer fluorescence, it was found that thermally activated energy transfer is efficient even in the amorphous state. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Depolarized Raman spectra of simple organic liquids in the wavenumber region 0-200 cm(-1) were measured. The data were analyzed by assuming model spectral functions in this entire wavenumber region but by putting stress on the relaxational mode which lies in the lowest wavenumber region. For 11 simple organic liquids at room temperature, the relaxation time tau was obtained and also the parameter beta which represents the distribution of tau; tau ranges from 1 to 5 ps. For toluene and tetrahydrofuran, the temperature evolution of the spectra was measured. It was found that tau of these liquids increases as the temperature is lowered, This is in harmony with the increasing viscosity with decrease in temperature. It was found that beta is close to unity for most of the liquids studied, which implies a small distribution of tau in simple organic liquids. (C) 1997 by John Wiley & Sons, Ltd.

Molecular conformation of butanenitrile in gas, liquid, glass, and crystalline states was studied to clarify the cause of the stability of its glass state against crystallization. The crystal comprises gauche molecules. However, the trans and gauche conformations have energies very close to each other, with an in-between energy barrier that cannot be surmounted at low temperatures. The multiple conformation that exists in gas and liquid is frozen in the glass, keeping the glass of butanenitrile in the disordered state even at temperatures as high as 100 K.

A new crystal form of an organic nonlinear optical material, 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (APDA) was found in the crystal growth by plate sublimation. This new crystal structure belongs to the noncentrosymmetric class, orthorhombic P2(1)2(1)2(1), and the unit-cell dimensions are a = 8.428 (8) Angstrom, b = 28.349 (3) Angstrom, c = 5.634 (2) Angstrom, V = 1346.0 (6) Angstrom(3), Z = 4, which are different from previously reported, orthorhombic Pna2(1). The molecule in the P2(1)2(1)2(1) crystal has less twisting and more bending in the structure than in Pna2(1). The APDA molecules are coupled in an antiparallel system in the P2(1)2(1)2(1) crystal as if to compensate for the repulsion arising from dipole-dipole interactions. This is in contrast to the molecular alignment of the Pna2(1) crystal in which the molecules are arranged in the same direction. The molecular hyperpolarizabilities (beta) and a coefficients of the new crystal form were investigated theoretically by semiempirical CNDO/S-CI.

The structure of benzene films that were vacuum-deposited on cold-metal substrates to be 5-20 mu m in thickness was studied with Raman spectroscopy and X-ray diffraction. Films deposited below 30 K were amorphous. They crystallized when the temperature was raised to about 60 K. If the substrate temperature at sample deposition was set above 45 K, films including crystalline particles were obtained. Structural relaxation in the amorphous state did not take place in an appreciable manner before the crystallization. Only an increase in intramolecular Raman-band intensities was observed during the relaxation in the amorphous state. Such behavior of amorphous benzene is compared with that of amorphous states of other simple organic compounds. The above crystallization temperature is discussed in relation to the previously estimated glass-transition temperature of benzene by referring to NMR data.

Amorphous states of three types of simple organic molecules were prepared by vacuum deposition on cold metal substrates. Relaxation and crystallization processes in these amorphous states were studied by Raman and IR measurements. Characteristic relaxations which reflect the molecular structures were observed during the annealing. The features of the relaxation processes in amorphous molecular systems are discussed.

Raman spectra were studied for naphthalene films prepared by vacuum deposition onto cold substrates. Films prepared at 7 and 25 K showed spectra similar to those of liquid, suggesting that they were amorphous. Structural relaxation due to stepwise elevation of the sample temperature was studied by monitoring the spectral evolution. Abrupt spectral changes which suggest that the sample underwent crystallization were observed around 105 K after a gradual increase of Raman intensity. This increase is discussed in connection with the change in structural coherence in the amorphous state.

8-(4'-Acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (APDA) was found to be a new organic material for nonlinear optical devices. Prior to its application to devices, material purification, single crystal growth, characterization and optical properties were studied, and second harmonic generation from the grown single crystal was demonstrated. Recrystallization from ethyl acetate solution by slow cooling, zone refining and sublimation under reduced pressure were explored. Among them, a combination of recrystallization and sublimation was the most effective for APDA purification. Single crystals were grown by the Bridgman technique with different growth parameters for the furnace temperature, the growing rate and the cooling rate of grown crystals. Single crystals with a diameter larger than 12 mm and length of 28 mm were obtained from the melt, but had some cracks. Characterization, measurements of optical properties of APDA crystals and second harmonic generation demonstration revealed that the crystals had good quality suitable for application to nonlinear optical devices, for example, frequency doublers of laser diodes in the blue region.

Anthracene films deposited onto cold metal substrates in vacuum were studied by Raman spectroscopy. They were found to be amorphous but weakly indicated the formation of some molecular pairs with close contacts when the substrate temperature at deposition was set at around 120 K. Fluorescence measurements showed that these molecular pairs in the electronic ground state are the source of the previously reported excimer fluorescence. This local ordering in amorphous anthracene seems to inhibit the system from crystallization which starts at around 200 K if the sample is highly disordered.

Temperature dependences of polarized Raman spectra and birefringence of bis(4-chlorophenyl) sulfone (BCPS) crystal were measured. Gas-phase Raman spectra of BCPS were also measured. Raman bands observed in the low-frequency region at room temperature were shown to have partial nature of intramolecular vibrations. The lowest-frequency Raman band observed at low temperatures softened as the temperature was raised up to T1 = 150 K. A gradual change of birefringence was also observed below T1. These are in harmony with previous reports on the normal-incommensurate phase transition of this crystal at T1. The critical exponents for the changes in the soft-mode frequency and the birefringence were compared with that for the previously reported intensity change of the satellite X-ray diffraction. These data were explained by assuming an incommensurate orientational modulation of the phenyl groups of BCPS molecules. However, the incommensurate-commensurate transition which has been expected to take place around 115 K was not detected by the present measurements.

A detailed Brillouin scattering study of the low temperature phase of phenothiazine reasonably explains the slightly 1rst order character of this structural phase transition (SPT) and the non observation of any soft optical mode due to a non negligible order parameter relaxation time. Results also confirm the possibility of a pure ferroelastic SPF.

The ferroelastic structural phase transition (SPT) and the elastic properties of the phenothiazine acoustic soft mode have been studied by Brillouin scattering. A pronounced softening of the C55-related acoustic modes is observed for directions close to a or c, but a strong dispersion, with respect to ultrasonic results, is observed for the longitudinal-acoustic mode propagating along c. This dispersion and the behavior of C55 in the low-temperature phase are characterized by a value of the relaxation time of the order parameter at T(c) in the 100-ps range, which suggests that the regime of this SPT is of the order-disorder type. Nevertheless the C55(T) variation in the low-temperature phase looks like that predicted in a true ferroelastic SPT.

Anisotropy and temperature dependence of elastic constants of phenothiazine crystal, which undergoes a ferroelastic phase transition at T(c) = 248.8 K, were obtained by ultrasonic velocity measurements. The characteristic anisotropy of this crystal against shear stress is suggested to have a relation to weak intermolecular hydrogen bonds extended along b-axis. A phenomenological analysis of free energy of the crystal indicates that the novel temperature dependence exhibited by the elastic constant C33 arises from a coupling of the strain e3 with the strain e5 along which a softening is observed as the temperature approaches T(c). Temperature dependence of the spontaneous deformation in the low-temperature phase and the entropy of transition are qualitatively explained in terms of the above analysis.

N-methylacetamide (NMA) crystal forms one-dimensional hydrogen-bond chains, which are similar to those in an acetanilide (ACN) crystal for which an unconventional vibrational band accompanying the amide-I band has been observed. Infrared spectra of NMA crystals show an additional band on the small-wave-number side of the amide-II band as the temperature is lowered. There is a close resemblance between this band and the band of ACN. It is likely that these bands appear by the same mechanism. The polaron model, which has been employed to explain the band of ACN, was found to be applicable also to the case of NMA, although the main vibrational mode is amide I in ACN and amide II in NMA.