Kohji Abe

Photoluminescence Properties of Eu3+-Doped ZnO Nanoneedles

Eu3+-doped ZnO nanoneedles were fabricated by thermal evaporation. In order to include Eu in ZnO nanostructures, europium nitrate was used to produce starting droplets from which ZnO nanoneedles were grown. Photoluminescence spectra included a sharp blue peak corresponding to the ZnO band gap and a broad red band due to defect states. In addition, sharp intra-4f transitions of Eu3+ ions were observed. Energy transfer from the ZnO matrix to Eu3+ was also observed. Study of the photoluminescence excitation revealed absorption tail states below the ZnO band gap induced by the inclusion of Eu ions in the ZnO nanoneedles. The location of Eu ions was assumed to be near the surfaces of the ZnO nanoneedles.

Vibrational modes of hydrogens in the proton ordered phase XI of ice: Raman spectra above 400 cm−1

Polarized Raman spectra of the proton ordered phase of ice Ih, i.e., ice XI, were measured above 400 cm(-1) in the range of librational, bending, and stretching vibrations. Vibrational modes in ice XI, of which symmetry is C(2v) (12)(Cmc2(1)), were discussed from the group theoretical point of view. In the librational mode spectra below 1200 cm(-1), several new peaks and clear polarization dependencies were observed. Assignments of the librational modes agree reasonably well with the recent MD calculations by Iwano et al. (J. Phys. Soc. Jpn. 79, 063601 (2010)). In contrast, the spectra for bands above 1200 cm(-1) show no distinct polarization dependencies and the spectra resemble those in ice Ih. In ice XI, however, fine structure composed of several weak peaks appear on the broad bending and the combination band. No direct evidence of the LO-TO splitting of the ν(3) anti-symmetric stretching mode was obtained. It is contrary to the case of the translational modes Abe and Shigenari (J. Chem. Phys. 134, 104506 (2011)). Present results suggest that the influence of the proton ordering in ice XI is weaker than the effect of inter- and intra-molecular couplings in the stretching vibrations of ice Ih.

One-Dimensional Crystal Model for Incommensurate Phase. I. Small Displacement Limit

A simple one-dimensional crystal model was proposed in order to study the incommensurate phase formation and its properties were numerically analyzed. In this model each particle (molecule) of the crystal has two degrees of freedom so that a longitudinal sound wave propagation is possible. In the case of small displacements and incompressible molecules the model has the Hamiltonian identical to that for a linear chain with a local fourth order anharmonic potential and the harmonic nearest- and next-nearest-neighbor interactions. Particular emphasis has been placed on the analysis of the incommensurate phases caused by the soft modes with short wavelength which has not been studied in most of the previous investigations.

Raman spectra of proton ordered phase XI of ICE I. Translational vibrations below 350 cm−1

Polarized Raman spectra from single crystals of ice XI (proton ordered phase of ice Ih) were measured and assigned for the modes below 350 cm(-1) in the translational vibration region. In contrast to the proton disordered ice Ih, the spectra in ice XI show clear polarization dependence and several new peaks are observed. Most of the vibrational modes were successfully assigned by the simplified point mass model with the symmetry C(2v) (12)(Cmc2(1)) and by the depolarization effect. In particular, LO-TO splitting of the mode near 240 cm(-1) was experimentally confirmed for the first time, which indicates that the long range force effect appears distinctly in ice XI.

Raman Spectra in Ferroelectric SrTi 18 O 3

Raman spectra in a ferroelectrics SrTiO 3 crystal in which 84% 16 O is replaced by the isotope 18 O were studied. Dielectric measurement shows that the sample undergoes a ferroelectric phase transition at 24K. Softening of a peak at 11cm -1 was found on heating to the transition temperature T c . New peaks also appear at 170cm -1 and 545cm -1 below 24K as the result of the condensation of the zone center F 1u modes. Analyzing the spectra in different scattering geometries for the twinned sample, we tentatively assigned the phonon modes assuming that the ferroelectric phase has the symmetry C 2v with the polar axis perpendicular to the tetragonal c axis.

Effect of Uniaxial Stress on Central Peaks in the 1q and 3q Incommensurate Phase of Quartz

The effects of uniaxial stress σ y y on the incommensurate (INC) phase transition of quartz have been investigated by light scattering spectroscopy. The 1q INC phase is induced between β and the 3q INC phase and its region becomes wider with the increase of stress. Anomalies of the central peak in the Rayleigh-Brillouin spectra are observed at three temperatures, corresponding to T i 1 (β to 1q INC phase), T i 3 (1q to 3q INC phase) and T c (3q to α phase). Only at T i 1 , is the central peak found to have a dynamical component.

Poisson Ratio Beyond the Limits of the Elasticity Theory

Elastic media can be stable only if the Lame coefficients are positive. This suggests that the Poisson ratio (PR) σ of a regular isotropic elastic media can range from −1 to 0.5. The regular elastic media, described by conventional elasticity theory, means the media which is homogeneous and deforms affinely, that is without formation of hollows or overlapping. In contrast, there are examples of the so-called micropolar media or Cosserat continua, where each microscopic element itself has rotational degrees of freedom. For most of the materials in nature σ lies in the range from 0 to 0.5. For example, cork shows almost zero σ, and σ = 0.5 (constant volume media) is observed for rubber or for the plastic deformation of metals. The materials with a negative σ are quite rare. Usually, the negative σ is attributed to the anisotropy or to the microscopic rotations. The anisotropy is the reason for the negative σ in highly anisotropic crystals like arsenic, antimony and bismuth and also in many single crystals of cubic metals deformed in an oblique direction with respect to the cubic axis. The rotational degrees of freedom cause the negative σ of foams and near the α−β phase transition of quartz and cristobalite. A microscopic model with particles having rotational degrees of freedom has been offered by Ishibashi and Iwata. The model nicely explains the variation of PR from −1 to 0. In the present Short Note we present a microscopic model which demonstrates PR beyond the limits of conventional elasticity. We consider the two-dimensional microscopic model of a crystal shown in Fig. 1. This structure can be found in ref. 11. The model consists of the rigid bars joined to each other by the hinges (open circles in Fig. 1). The bars simulate the rigid clusters of atoms and they have rotational degrees of freedom. The three parameters, the bar lengths a, b and the angle γ (|γ| < π/2), define the geometry of the model. The unit cell has parameters X = 2b cos γ, Y = 2(a− b sin γ). (1)

Line Shape of the Raman Spectrum and the Disorder Mechanism of the Methylammonium Group in a [(CH 3) 2NH 2] 5Cd 3Cl 11 Crystal

The Raman line shape of the symmetric stretching vibration of the dimethylammonium (DMA) group at 890 cm -1 in the spectrum for a [(CH 3 ) 2 NH 2 ] 5 Cd 3 Cl 11 (DMAPCC) crystal was analyzed. The temperature dependence of this vibration mode is explained in terms of the motional narrowing due to the disordered NH 2 group in DMA(3), which results in splitting of the DMA(2) line in the internal mode region. The disorder mechanism is discussed by comparing the results with the two relaxation times obtained from the dielectric dispersion data.

Debye-Waller factors for incommensurate structures

The Debye-Waller factors for displacive-type incommensurate (IC) structures are calculated taking into account the correlation of the phases of the modulation. It is shown that and that IC satellites should not be observed in the immediate vicinities of transitions.

On the origin of the inhomogeneities and birefringence in the incommensurate phase of quartz

It is shown that the incommensurate (IC) phase near the transition point in quartz is an improper ferroelastic one and should be split into domains. The birefringence induced by such a ferroelastic phase is estimated, and it is shown that it can be as strong as is observed in experiments only if one accepts a new model for the IC transition in quartz.