Neutron Rietveld Analysis for Optimized CaMgSi2O6:Eu2+ and its Luminescent Properties

We optimized synthesis conditions of blue-emitting CaMgSi2O6:Eu2+ (CMS:Eu2+) with conventional solid-state reaction and successfully determined structure parameters by Rietveld refinement method with neutron powder diffraction data. The final weighted R-factor Rwp was 6.42% and the goodness-of-fit indicator S (= Rwp/Re) was 1.34. The refined lattice parameters of CMS:Eu2+ were a = 9.7472(3) Å, b = 8.9394(2) Å, and c = 5.2484(1) Å. The β angle was 105.87(1)°. The concentration quenching process was observed, and the critical quenching concentration of Eu2+ in CMS:Eu2+ was about 0.01 mol and critical transfer distance was calculated as 12 Å. With the help of the Rietveld refinement and Dexter theory, the critical transfer distance was also calculated as 27 Å. In addition, the dominant multipolar interaction of CMS:Eu2+ was investigated from the relationship between the emission intensity per activator concentration and activator concentration. The dipole–dipole interaction was a dominant energy transfer mechanism of electric multipolar character of CMS:Eu2+.

Luminescence and decay behaviors of Tb-doped yttrium silicate

The photoluminescence of terbium-activated yttrium silicate with the general formula Y2−XTbxSiO5 was investigated as a function of Tb3+ concentration. Especially, the main attention was focused on the 5D3 fluorescence and its energy transfer behavior. The emission and excitation spectra were measured in terms of Tb3+ concentration. The diffuse reflectance spectrum was also measured in the range from VUV to UV. As a result, yttrium silicate was found to have a broad absorption band extended from the VUV to UV range. The concentration quenching was investigated in terms of luminance and decay time both for 5D3 and 5D4 fluorescence. The energy transfer was also investigated by analyzing the decay curve of 5D3 emission on the basis of the multipolar interaction. The decay curves of 5D3 emission, for which well-known cross-relaxation has been accepted as a main factor, were analyzed by Inokuti and Hirayama's formula on the basis of the direct quenching scheme. Furthermore, the rate equations including a newly proposed quenching scheme were taken into consideration. The rate equations accept the emission quenching as due to the cross-relaxation from 5D3 or 4 to some upper levels such as 7D and the charge-transfer band.

MANY-BODY EFFECT IN ELECTRORHEOLOGICAL RESPONSES

The many-body effect in the kinetic responses of ER fluids is studied by a moleculardynamic simulation method. The mutual polarization effects of the particles are considered by self-consistently calculating the dipole strength on each particle according to the external field and the dipole field due to all the other particles in the fluids. The many-body effect is found to increase with the enhancement of the particle concentration and the permittivity ratio between the solvent and the particles. The calculated response times are shorter than that predicted with the ‘point-dipole’ model and agree very well with experimental results. The many-body effect enhances the shear stresses of the fluids by several times. But they are not proportional to the many-body correction factor λ as expected. This is due to the fact that larger interaction forces between the particles lead to coarsening of the fibers formed in the suspensions. The results show that the many-body and multipolar interaction between the particles must be treated comprehensively in the simulations in order to get more reliable results.