Tetsuaki Nishida

Mössbauer spectroscopic study of the formation of non-bridging oxygen in the potassium borate glasses

Abstract Mossbauer spectroscopic study was performed to investigate the formation of non-bridging oxygen in the potassium borate glasses denoted by the formula of x K 2 O(100 − x ) B 2 O 3 · 7 Fe 2 O 3 (0 ⩽ x ⩽ 40 mol%). In the range of alkali content higher than 20 mol%, the quadrupole splitting for Fe 3+ decreased continuously with the concentration of the alkali oxide in the glass, showing the increase of the symmetry around the Fe 3+ ions. The isomer shift value also decreased in almost the same x region. The drastic decrease of Mossbauer parameters was attributed to the formation of non-bridging oxygen at the adjacent site of the Fe 3+ ions. In the region of lower alkali content (less than 10 mol%), Mossbauer spectra only showed the absorption due to α-Fe 2 O 3 . X-ray powder diffraction patterns also displayed the existence of α-Fe 2 O 3 , together with the microcrystal of potassium borate.

Mössbauer and ESR studies of non-bridging oxygens in potassium phosphate glasses

Abstract Mossbauer spectroscopic study was performed to investigate the behavior of non-bridging oxygens in the potassium phosphate glasses denoted by a formula of x K 2 O(100− x )P 2 O 5 7 Fe 2 O 3 . The quadrupole splitting values for Fe 2+ and Fe 3+ ions decreased continuously with the alkali content of the glasses in the alkali region of 0–30 mol.%, and increased in the higher alkali region. On the other hand, the Mossbauer absorption area for Fe 2+ increased continuously in the same alkali region ( x = 0–30 mol.%), and then decreased in the higher alkali region. The continuous decrease of the quadrupole splitting, i.e., the increase of the symmetry around the iron and the increase of the absorption area for the Fe 2+ ions are discussed in connection with the increase of non-bridging oxygens at site adjacent to the iron. ESR measurements for the γ-ray irradiated potassium phosphate glasses free from iron were also carried out to confirm the formation of the non-bridging oxygens in the glasses by observing PO 4 2− radicals produced by γ-ray irradiations. We conclude that the ESR absorption intensity for the PO 4 2− radicals linearly increases with the alkali content of the glasses, which was ascribed to the linear increase of non-bridging oxygens in the PO 4 tetrahedra.

Advances in the Mössbauer effect for the structural study of glasses

Abstract Topics of recent Mossbauer studies of glasses are summarized briefly together with the principle of the Mossbauer effect. Structural information can be obtained through the Mossbauer parameters since a small amount of Fe3+ or Sn4+ can easily substitute for network formers in several oxide glasses. Mossbauer ions in phosphate glasses are present at interstitial sites and play the role of network modifier. The γ-ray irradiation effect is effective for determining the structural role of Mossbauer ions, because oxidation and reduction take place when they are network modifier and network former, respectively. The Debye temperature, θD, obtained from low-temperature Mossbauer measurements also indicates the structural role of Mossbauer ions, e.g., the θD of Na2Oue5f8 O3ue5f8SnO2 glass is 360 K because Sn4+ substitutes for W6+ and W5+. On the contrary, the values of θD of SnOue5f8P2O5 and Bi2Sr2Ca(Cu,Sn)2O8−y glasses are 155 and 240 K, respectively, because Sn2+ and Sn4+ occupy interstitial sites. A linear relationship between glass transition temperature, Tg, and quadrupole splitting, Δ, named the Tg − Δ rule, reveals the structural role of Fe3+ in glasses.

Mössbauer study of the fraction of non-bridging oxygens in potassium borate glasses☆

Abstract A Mossbauer study was performed to find the fraction of non-bridging oxygens in potassium borate glasses containing a small amount of iron. The borate glasses with alkali contents of 15, 20, 25, 30 and 40 mol% were irradiated with 60 Co-γ rays at room temperature in a nitrogen atmosphere. The Mossbauer absorption area for the irradiation-induced Fe 2+ increased with the total γ-ray dose in the range of 1 × 10 6 −2 × 10 8 R, and became constant after irradiation ranging from 2 × 10 8 R − 1 × 10 9 R. The absorption area for the Fe 2+ at the saturated region was correlated with the difference in the glass structure, i.e., the fraction of the non-bridging oxygens was estimated from the increased absorption area for the Fe 2+ as a function of the alkali content of the glasses. The fraction of the non-bridging oxygen increased with the alkali content of the glasses in the alkali region above 20 mol%, and about 14% of the fraction was obtained for an alkali content of 40 mol%.

Mössbauer spectroscopic study of potassium borate glasses at low temperatures

Abstract Mossbauer studies of potassium borate glasses containing a small amount of iron were performed at dry ice and liquid nitrogen temperatures in order to investigate the physical properties of non-bridging oxygens in the glasses. The Mossbauer spectra at the temperature of dry ice were almost the same as those at room temperature. On the other hand, the spectra at the temperature of liquid nitrogen consisted of quadrupole doublet and hyperfine structure due to Fe 3+ ions with tetrahedral symmetry. Magnetic suceptibility measurements revealed that the hyperfine structure was observed because of a paramagnetic relaxation effect. Isomer shift values for both the quadrupole doublet and the hyperfine structure were constant in the alkali region below 20 mol.%, and continuously decreased when the alkali content of the glasses was in the region ⩾ 20 mol.%. The absorption areas for the hyperfine structure were also constant in the alkali region below 20 mol.%, and linearly decreased with alkali content in the region ⩾ 20 mol.%. The internal magnetic field for the hyperfine structure also showed the same tendency as the absorption area. These results were attributed to the formation of non-bridging oxygen at the site adjacent to the iron, and the decrease of the absorption area for the hyperfine structure seemed to be directly related to the fraction of non-bridging oxygen in the oxygens constituting FeO 4 tetrahedra.

Correlation between the structure and glass transition temperature of potassium, magnesium and barium tellurite glasses

The Mössbauer spectrum of tellurite glasses, containing 5 mol% Fe2O3 as a probe, consists of a paramagnetic quadrupole doublet with an isomer shift of 0.39 ± 0.01 mmsec−1. This indicates that Fe3+ ions are present at substitutional sites of Te4+ ions constituting distorted TeO4 trigonal bipyramids, each of which has one oxygen vacancy at an equatorial site. On increasing the K2O content from O to 35 mol%, the quadrupole splitting (Λ) for potassium tellurite glasses decreases continuously from 0.76 to 0.44 mm sec−1. On the other hand, Λ for magnesium and barium tellurite glasses increases with increasing MgO and BaO content, respectively. When the alkali or alkaline earth oxide contents are the same as each other, Λ increases in proportion to the ionic potential (Z/r) of the alkali or alkaline earth metal ion. These results suggest that the glass matrices of alkaliv and alkaline earth tellurite glasses are continuously changed into a chain and a three-dimensional network structure, respectively. Differential thermal analysis studies reveal that there exists a linear relationship between the glass transition temperatureTg and the quadrupole splitting, indicating thatTg is primarily determined by the magnitude of the distortion of TeO4 trigonal bipyramids. This relationship is also applicable to several oxide glasses.

Mössbauer spectroscopic study of gamma-ray irradiated potassium phosphate glasses

Abstract A Mossbauer spectroscopic study was performed to investigate the effect of γ-ray irradiation on the formation of non-bridging oxygens in the potassium phosphate glasses denoted by the formula x K 2 O · (100 − x )P 2 O 5 · 7 Fe 2 O 3 (0 ⩽ x ⩽ 50 mol.%). Mossbauer spectra for these glass samples consisted of two kinds of doublets due to Fe 2+ and Fe 3+ ions of octahedral symmetries. Only small changes occurred in the Mossbauer parameters as a result of irradiation at room temperature in a dry nitrogen atmosphere, except for the decrease in the absorption area for the Fe 2+ ions. The decrease in the absorption area was attributed to the electron transfer from the Fe 2+ ions to the neighboring oxygens. Thermal annealing experiments for a few non-irradiated glass samples indicated that the decrease in the absorption area was confirmed to be due to γ-rays rather than heating during the irradiation.

Occupation of tungsten site by iron in sodium tungstate glasses

Abstract 57 Fe Mossbauer spectra of x Na 2 O · (99 − x )WO 3 · 57 Fe 2 O 3 glasses (30 ≤ x ≤ 42) comprised of a doublet due to octahedral iron, Fe 3+ (O h ), and a weak doublet due to Fe 2+ (O h ); a large Debye temperature of 580 K was obtained at low temperatures. Fourier transform infrared spectra of the sodium tungstate glasses showed a gradual increase in the fraction of WO 4 tetrahedra (T d ) when Na 2 O content was increased. Mossbauer spectra of the tungstate glasses irradiated with 60 Co γ-rays showed an increase in the fraction of Fe 2+ , due to electron transfer (scattering) from Wue5f8O and Feue5f8O bonds to Fe 3+ , as the electron spin resonance spectra of iron-free sodium tungstate glass indicated a simultaneous formation of W 5+ and the ‘hole’ trapped on the oxygen atom. From the linear relationship between glass transition temperature, T g , and quadrupole splitting, Δ, of Fe 3+ (‘ T g − Δ rule’), the slope of the straight line was estimated to be 260°C/(mm s −1 ). Mossbauer spectra of heat-treated tungstate glasses showed a gradual change from Fe 3+ (O h ) to Fe 3+ (T d ), along with the precipitation of the Na 2 W 2 O 7 phase composed of W 6+ (O h ) and W 6+ (T d ). These results indicate that iron occupies W 6+ (O h ) sites in glasses, while it occupies W 6+ (T d ) sites in glass-ceramics.

Annealing of fission tracks in zircons

Abstract Etching conditions and thermal annealing effects have been studied for fission tracks in four kinds of zircons. The observation of fission tracks under an optical microscope was made with respect to the mean pit length as well as the mean pit density. Apparent rate constants were obtained by substituting the isochronal (60 minutes) annealing data into the ordinary equation of the rate constant. The order of reaction (almost 3rd or 4th) and the activation energy (1.7–2.9 eV) of annealing were determined from the Arrhenius plot of the apparent rate constants.

Transformation of Egg-White Glass into Partially Crystallized Glass Induced by Heat Treatment and Gamma-Ray Irradiation

Egg-white gel is easily transformed into transparent glass by drying at ambient temperature, e.g., at 25° C for 140 min. The optical transmittance is increased from 2 to 25% accompanied with a decrease in the weight by more than 70%. Broad and intense IR absorption peaks observed at 3288–3308 and 1653–1657 cm-1 were respectively assigned to ν (NH) and να(C=O) stretching modes in the amido (RCONH-) groups, and the δ(NH) bending mode was observed at 1521–1546 cm-1. Differential thermal analysis (DTA) study showed a distinct endothermic peak due to the glass transition, and the peak position was located at 36, 42, 58 and 69° C when the heating rates were 2, 5, 10 and 20° C min-1, respectively. Heat treatment of egg-white glass at 60° C or γ-ray irradiation resulted in partial crystallization, and the XRD patterns showed a few diffraction peaks superimposed on the halo peak due to the glassy phase.