No abstract

No abstract

A new magnetic symmetry of the rare-earth orthochromites and orthoferrites (RMO3) below the second Neel temperature due to the reordering of the rare-earth spins is proposed. It is shown that the magnetic symmetry group is lowered to the subgroup of index two, thereby keeping the unit cell invariant. The proposed symmetry is shown to be consistent with the previously observed spin configuration of RMO3 such as GdCrO"DyFeO"and DyCr03, and would also be consistent with those of other RMO3. Although symmetry arguments and the calculation of the free energy show that two spin configurations are stable, in

We have measured the absorption and the resonance Raman spectra of a solvatochromic dye, phenol blue, in liquid and supercritical solvents. We have found anomalous solvent dependence of the absorption bandwidth in liquid solvents: the width has apparently no correlation with the absorption peak shift. On the other hand, we have found good linear correlation between the absorption peak shift and the peak position of the resonance Raman bands (the CdN and the CdO stretching modes). The relative intensities of the Raman bands and the bandwidth of the CdN stretching mode also show correlation with the absorption peak shift. Incorporating these Raman data, the anomalous bandwidth of the absorption spectrum is explained by the change of the intramolecular vibrational contribution to the absorption bandwidth due to the electronic structure change by the solvent, which cancels the change of the solvent contribution. We have estimated the solvent reorganization energy assuming linear dependence of the intramolecular contribution on the absorption band center and neglecting the solvent reorganization energy in alkanes such as ethane and cyclohexane. In liquid solution, the estimated solvent reorganization energy is correlated fairly well with the absorption peak shift. Solvent dependence of the Raman bandwidth of the CdN stretching mode resembles the solvent dependence of the solvent reorganization energy estimated in this way. Relatively large bandwidths of both absorption and resonance Raman spectra have been observed in supercritical solvents compared with those in liquid solvents of a similar absorption peak shift. We interpreted this as due to the small refractive indices of the supercritical solvents relative to the liquid solvents; the large refractive indices of the liquid solvents only make the absorption peak shifts without broadening the absorption spectra.

The frequency-dependent shear viscosities of four representative imidazolium-based room-temperature ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([bmim][TFSA]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]), and 1-methyl-3-octylimidazolium hexafluorophosphate ([omim][PF(6)]), are measured from 5 to 205 MHz with shear impedance spectroscopy. A relaxation is observed in the measured frequency range in all cases. This is the first report on the shear relaxation of ionic liquids at room temperature, to our best knowledge. Comparing the spectra of the common cations, [bmim][TFSA] and [bmim][PF(6)], the normalized relaxation spectra, eta(nu)/eta(0), reduce to a single curve when plotted against eta(0)nu, where nu and eta(0) stand for the frequency and shear viscosity, respectively. The lower viscosity of the TFSA salt is thus elucidated by the shorter relaxation time. The lower viscosity at higher temperature is also attributed to the shorter relaxation time. On the other hand, the increase in the length of the alkyl chain of the cation leads to the lower-frequency shift of the relaxation frequency on the eta(0)nu scale. Therefore, the higher viscosity of the omim salt is the result of the compensation of the longer relaxation time for the smaller high-frequency shear modulus. In addition, the relaxation time distribution becomes broader with increasing chain length, which can be ascribed to the heterogeneity of the liquid structure.

No abstract

Molecular dynamics (MD) simulation is performed on two imidazolium-based ionic liquids, one of which possesses a long alkyl chain whereas the other does not, and the coupling between the liquid structure and the shear stress is examined by means of cross-correlation analysis. The former liquid exhibits a prepeak representing the polar-nonpolar domain structure whereas the latter does not. The relaxation of the intermediate scattering function at the prepeak is slower than those of other microscopic structures. Nonetheless, the principal part of the viscoelastic relaxation is ascribed to the relaxation of the charge-alternation mode in both liquids. In particular, the dynamics of the domain structure hardly seems to affect the viscosity of the ionic liquid with a long alkyl chain, although the cross correlation between the shear stress and the domain mode exists and the relaxation of the cross correlation with the domain mode is far slower than that with the charge alternation mode. The absence of the domain dynamics in the viscoelastic relaxation is ascribed to the decoupling between the relaxations of the short- and the long-range structures.

We have measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C=N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, no dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C=O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlated with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR absorption spectra of PB are measured in chloroform and carbon tetrachloride, and compared with the Raman spectra in the same solvents. The excitation energy dependence is explained by the solvation state selective excitation in the resonance Raman spectroscopy, that is, the inhomogeneity that appears in the absorption spectra of PB also appears in the resonance Raman process.