仲山 英之

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.

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.

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.

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.