N. Soga

Relation between the Ω6 intensity parameter of Er3+ ions and the 151Eu isomer shift in oxide glasses

The Ωt intensity parameters (t=2,4,6) of Er3+ ions in several oxide glasses and the isomer shift (IS) of 151Eu Mossbauer spectra in glasses of the same composition were determined. Among these Ωt’s, the Ω6 parameter was found to have a good relation with IS; Ω6 decreases with an increase of IS which reflects the 6s electron density of rare‐earth ions. From the theoretical expression of Ωt, Ω6 is considered to be more affected by the overlap integrals of the 4f and 5d orbitals than Ω2 and Ω4, and to increase with an increase of these overlap integrals. These overlap integrals are supposed to decrease when the 6s electron density is larger, since the 6s electron density shields the 5d electron orbital.

Phonon sideband spectra and local structure around Eu3+ ions in sodium silicate glasses

The local structure around Eu3+ ions in sodium silicate glasses was investigated by the phonon sideband associated with 5D2 ← 7F0 transition of Eu3+. Since the shapes of the electronic transition were asymmetric due to the Stark splitting of 5D2 level, the spectra were analyzed by assuming the superposition of the Gaussian distributions of phonon energy and coordination of non-bridging oxygen of the Qn (n = 3, 2, 1) unit in silicate glass. The phonon energy almost corresponded to those of the stretching vibrations of Qn units by Raman spectra. The type and fraction of the Qn units coordinating Eu3+ varied with alkaline content. The compositional variations of Qn units were similar to those determined by NMR for undoped alkaline silicate glasses. It is shown that the local structure around Eu3+ is affected by the composition of matrix glass and the phonon sideband analysis can become a powerful technique to estimate the local distribution of Qn in silicate glasses.

Crystallization and glass formation in 50Li2O·50Nb2O5 and 25Li2O·25Nb2O5·50SiO2

Abstract Quenching of 50Li 2 O·50Nb 2 O 5 melt was investigated with X-ray diffraction (XRD) and surface morphological analysis. Glass formation of 25Li 2 O·25Nb 2 O 5 ·50SiO 2 and devitrification of LiNbO 3 from the 25Li 2 O·25Nb 2 O 5 ·50SiO 2 glass system were studied using DTA, XRD and electrolysis techniques. High quench rate yields smaller crystal grain size, lower crystallinity and more LiNbO 3 crystal dendritic growth than the slow quenching. Three-fold symmetry was observed and a preferred crystallographic orientation of [0001] was confirmed for the quenched surfaces of LiNbO 3 . High Li + mobility was observed in 25Li 2 O·25Nb 2 O 5 ·50SiO 2 glass. By firing the 25Li 2 O·25Nb 2 O 5 ·50SiO 2 at 800°C, glass ceramics of LiNbO 3 –SiO 2 can be prepared.

ESR and Mössbauer studies of the precipitation process of various ferrites from silicate glasses

ESR and Mössbauer studies of the precipitation process of various ferrites from silicate glasses were made to characterize the precipitation mechanism. The changes in linewidth (ΔH1/2) and effectiveg-value (geff) in the ESR spectra of the precipitation process are well explained in terms of super-exchange interaction between magnetic ions and interparticle dipolar interaction between precipitated ferrites. The precipitation tendency of spinel type ferrites from silicate glasses was found to be in the following order: NiFe2O4≫CoFe2O4>Fe3O4≧ZnFe2O4, MnFe2O4. The above order coincided with the order of octahedral site preference energies of divalent transition metal ions.

High temperature persistent spectral hole burning of Sm2+ in fluorohafnate glasses

Abstract Persistent spectral hole burning of Sm2+ in a fluorohafnate glass was observed. The hole was burned even at 180 K. In this system, the photochemical process is likely to be dominant because of the absence of an anti-hole adjacent to the hole.

Preparation and optical properties of transparent glass-ceramics containing LiGa5O8∶ Cr3+

Transparent glass-ceramics containing LiGa5O8∶Cr3+ crystallites have been prepared by heat treatment of Li2O-Ga2O3-SiO2-Cr2O3 glassy material. Average crystallite size evaluated from the full-width at half-maximum of the X-ray diffraction lines varies from about 3–7 nm as the heat-treatment temperature increases from 650 °C to 800 °C. The ligand field strength of the Cr3+ ion estimated from optical absorption measurements increases with an increase in the heat-treatment temperature. The fluorescence spectra of the glass-ceramics resemble that of LiGa5O8∶ Cr3+ polycrystal. In particular, for the specimen containing a crystallite of 7 nm, intense emission due to the 2E→4A2 transition has been observed. These optical measurements demonstrate that the Cr3+ ions are incorporated into LiGa5O8 microcrystals in the present glass-ceramics. For the transparent glass-ceramics and LiGa5O8∶Cr3+ polycrystal, the temperature dependence of the peak position of the zero-phonon line (R-line) has been analysed assuming that the electronic transition is described by the Raman process and the vibrational density of state is expressed by the Debye model. The Debye temperature of the transparent glass-ceramics is slightly lower than that of LiGa5O8∶ Cr3+ polycrystal, indicating that the softening of the phonon occurs in the LiGa5O8∶Cr3+ microcrystal precipitated in the transparent glass-ceramics.

Frequency upconversion and its new mechanism in Tm3+-doped fluoroaluminate glasses

Upconversion fluorescences have been observed in the UV (363 nm) and the blue region (451 and 476 nm) in Tm3+-doped fluoroaluminate glass by pumping with a DCM dye laser. The two-photon absorption process of upconversion fluorescences was confirmed by the quadratic dependence of the emission intensity on incident pumping power. The excitation spectra for upconversion fluorescences show that the mechanism of upconversion is a simple ESA (excited state absorption) process. In addition to ESA, it was found that the energy transfer by cross-relaxation (3H4, 3H6 → 3F4, 3F4) is also involved in the mechanism of the 476 nm blue upconversion fluorescence which differs from that of the UV and the 451 nm blue upconversion fluorescence. The difference was confirmed by measuring both the radiative and the non-radiative decay rate of the 3H4 level.

Young's modulus of RF-sputtered amorphous thin films in the SiO2-Y2O3 system at high temperature

Abstract High-temperature Youngs moduli of amorphous films in the SiO2-Y2O3 system were determined by using a newly developed measuring system for Youngs modulus. It was found that the modulus of amorphous SiO2 film at high temperature was close to that of silica glass. As Y2O3 was added to amorphous SiO2( the mean temperature coefficient of Youngs modulus of the amorphous Alms decreased monotonically from a positive value to a negative one. From this result, it was considered that yttrium ions act as a network modifier in the structure of amorphous films. In addition, from measurements of Youngs modulus at room temperature and its temperature coefficient, it is found that the difference in Youngs modulus due to the Y2O3 content observed at room temperature becomes extinguished with increasing temperature.

Preparation and physical properties of rf-sputtered amorphous films in the Al2O3AlN system

Abstract Oxynitride amorphous films in the system of Al 2 O 3 AlN were prepared by rf sputtering in the range 0–79 mol% AlN, and their density, refractive index, Youngs modulus, Poissons ratio and dynamic hardness were measured. The atomic ratio of oxygen to nitrogen in the films increased linearly with the partial pressure ratio of oxygen to nitrogen during sputtering. The density and Poissons ratio decreased monotonicly with AlN content, while the refractive index increased. Youngs modulus and dynamic hardness decreased with AlN content below about 50 mol% AlN, beyond which they increased. The compositional dependence was explained by the change of the coordination state of aluminum ions with the nitrogen content, and was confirmed by IR spectra and the molar refractivity.

A study of the local structure around Eu3+ ions in oxide glasses using Mössbauer spectroscopy

The local structure around Eu3+ ions in several oxide glasses (silicate, germanate and borophosphate glasses) was investigated by using 151Eu Mossbauer spectroscopy. It was found that the isomer shift (IS) of silicate and borophosphate glasses was independent of the sodium content, but that of germanate glasses was not. This means the first coordination sphere around Eu3+ ions in silicate glasses is insensitive to the composition of the glass matrix. It is assumed that, regardless of the sodium content, Eu3+ ions in silicate glasses attract a certain amount of nonbridging oxygen (NBO, SiO⊖) when incorporated stably into silicate glass matrix, because NBO is the only species donating negative charge. For germanate glasses, the behavior of IS is considered to be related to the presence of GeO62 octahedra. On the basis of experimental results, the coordination models of Eu3+ in these systems are proposed.