C E Johnson

Tin oxidation state, depth profiles of Sn2+ and Sn4+ and oxygen diffusivity in float glass by Mössbauer spectroscopy

Abstract Mossbauer transmission spectra of tin in commercially produced soda-lime-silica float glass have been obtained at low temperatures. It was found that between 20% and 37% of the tin which had diffused into the lower surface of the ribbon during manufacture was in the Sn 4+ state. An estimate of the depth profiles for each oxidation state was obtained by measuring the Mossbauer spectra of samples after increasing thicknesses of the tin surface had been removed by polishing. These revealed that about 60% of the total Sn 2+ occurs in the first 3.5 μm, when the total tin penetration depth was 17 μm. Heat treatments were conducted to investigate the rate of oxidation of Sn 2+ to Sn 4+ in the glass. The diffusivity of oxygen into the glass was estimated to vary between 1.29 × 10 −18 m 2 s −1 at 500°C and 1.92 × 10 −16 m 2 s −1 at 730°C. Measurements of the decrease in recoil-free fraction with temperature enabled values for the Debye temperatures Θ D of Sn 2+ and Sn 4+ to be found to be 185 and 260 K respectively. These results show that the binding of tin at the surface of float glass is weaker than in bulk tin-containing glasses where Θ D was 200 K and 319 K.

Ultrafine particles of barium ferrite from a citrate precursor

Ultrafine particles of barium ferrite produced by the precursor method have sizes between 5 and 100 nm. These particles have relatively low magnetisations and high coercivities. Mossbauer spectra exhibit different relative areas for the 4fiv and 2b sites compared to the bulk which partly explains the low magnetisations.

An investigation of particle size effects in ultrafine barium ferrite

Abstract Ultrafine particles of barium ferrite in the size range 5–100 nm have been synthesized by thermal decomposition of a citrate precursor. The precursor decomposed at 425°C is amorphous, but crystalline barium ferrite starts forming at temperatures of 550°C and above. Barium ferrite which shows a monophase X-ray diffraction pattern and well-resolved Mossbauer spectra is obtained at 700°C. The Mossbauer spectra at both liquid helium and room temperature of samples annealed at 700, 750 and 800°C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be satisfactorily resolved into five components corresponding to the five sublattice sites in barium ferrite. The Mossbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift, all show particle size dependence. The magnetic hyperfine field and quadrupole splitting are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing temperature for all the five sublattice sites. Isomer shifts also differ from bulk values but the variation with particle size is also dependent on the sublattice sites.

Characterization of tin at the surface of float glass

Abstract Transmission Mossbauer spectroscopy (TMS) and conversion electron Mossbauer spectroscopy (CEMS) have been used to obtain spectra of 119Sn in three commercially produced soda–lime–silica float glasses. Measurements have been made of the f-factor of both tin oxidation states from a series of results at temperatures between 290 and 13 K. TM spectra yielded data on the whole tin layer, which extends to a depth of about 20 μm, while CEM spectra gave information on the near-surface region with a depth of about 2 μm. The isomer shift and quadrupole splitting of Sn4+ in the near-surface were higher than the shift and splitting measured for the whole region, and the Debye temperature, θD, was found to be significantly less in the surface than in the full depth containing tin, showing that it was less tightly bound. The shift, splitting and Debye temperature of Sn2+ remained the same throughout the glass surface within experimental errors. These results are discussed in terms of the conditions of the float process and differences in the coordination number of Sn4+.

Mossbauer spectra of tin in binary Si-Sn oxide glasses

The Mossbauer spectra of 119Sn in binary glasses of SnO and SiO2, in varying proportions of between 16.8 and 71.5 mol% SnO, have been obtained at 77 K. From these spectra the tin was found to be predominantly in the 2+ state. The isomer shift and the quadrupole splitting were both significantly greater than those in crystalline SnO and were found to decrease with increasing tin content, which also corresponds to decreasing molar volume. A second series of spectra of two of the samples was taken at temperatures between 10 and 300 K. These spectra show a decrease in shift with temperature which was the same for the two samples and which could only partly be accounted for by the second-order Doppler shift. The remainder was ascribed to the temperature dependence of the isomer shift. By using these data together with the measured expansivity and increase in density with tin content, it was possible to correct the isomer shift for the effects of the change in volume, and it was found that the volume-corrected isomer shift increased with increasing tin content. This showed that there was an increase in s-electron density at the tin nuclei, while the accompanying decrease in quadrupole splitting showed a decrease in the p-electron character, as expected for a decrease in covalency. From the decrease in absorption area with temperature the Debye temperature of tin was estimated to be ~190 K, which is slightly less than that of crystalline SnO, and decreased with increasing tin content. All the data suggest that the Sn-O bonds become progressively less covalent as more tin is added to SnO-SiO2 glasses, and possibly indicate a change from 4-fold to 6-fold co-ordination. A third series of spectra was obtained for the glass containing 40.9 mol% of SnO after a series of heat treatments. The spectra show changes in the oxidation state of the tin which depended upon the different conditions applied.

Synthesis and characterization of ultrafine lithium ferrite from a citrate precursor

Abstract Ultrafine particles of lithium ferrite have been prepared at the unusually low temperatures of 240 and 455°C by a citrate precursor method. The materials obtained after annealing at temperatures between 455 and 700°C consist of ordered lithium ferrite as shown by the characteristics infrared absorptions, the X-ray diffraction superstructure reflections, and the 4.2 K Mossbauer spectra. The values of magnetization obtained are somewhat low, due to the presence of small quantities of an α-Fe2O3 phase, which can also be observed in the X-ray diffraction and the Mossbauer spectra.

Spin reorientation transitions in DyFeO3 induced by magnetic fields

Mossbauer spectrometry was used to study spin reorientation transitions in single crystals samples of DyFeO3. In agreement with magnetisation data of Belov (1974), a Morin transition was observed at 35+or-3K where the coupled Fe3+ ions reorient from a magnetic structure having an antiferromagnetic vector parallel to the crystal a axis and weak ferromagnetism along the crystal c axis ( Gamma 4) to a pure antiferromagnetic structure along the crystal b axis ( Gamma 1). Spin reorientations were induced with an external field. At 270, 130 and 77K fields applied parallel to the a axis caused continuous rotation of the Fe3+ antiferromagnetic vector from the a to the c axis ( Gamma 4 to Gamma 2) the rotation being complete as fields of 7.5, 6.7 and 5.5 T respectively. At 4.2K a field parallel to the b axis induced a transition Gamma 1 to Gamma 4 at a critical field Hcr=0.75+or-0.1 T. Reorientations were also observed at 4.2K in which the antiferromagnetic vector rotated in a plane perpendicular to the applied field: a field parallel to the c axis caused a rotation of the antiferromagnetic axis from b to a ( Gamma 1 to Gamma 4 with Hcr=0.7+or-0.1 T) and a field parallel to the a axis caused the antiferromagnetic axis to rotate from b to c ( Gamma 1 to Gamma 2 with Hcr=1.6+or-0.4 T). A model incorporating Dy3+-Fe3+ interactions is discussed to explain the reorientations.

Mössbauer spectroscopy measurement of iron oxidation states in float composition silica glasses

Mossbauer spectroscopy and chemical analysis were used to determine the Fe2+/(Fe2++Fe3+) ratio of iron ions in soda-lime silica float composition glass. Apparent disagreement between these ratios determined by these techniques in glasses of low (<0.5 mol% Fe2O3) iron concentration was resolved by measuring the Mossbauer spectra at 4.2 K and including as Fe3+ the observed magnetic sextet component. The sextet is explained by slow relaxation of Fe3+ ions; conditions that give rise to this in the glass environment are discussed.

A Mossbauer study of magnetic phase transitions in alpha -Fe2O3 crystals

The Mossbauer spectra of single crystals of alpha -Fe2O3 have been measured (i) close to the Morin transition at TM approximately=260K, (ii) at 4.2K in magnetic fields parallel to the (111) axis in the region of the spin-flop transition, and (iii) at temperatures below TM in magnetic fields perpendicular to the (111) axis. The first two transitions show a coexistence of two magnetic phases over a range of temperatures and fields respectively, confirming their first-order character, although no hysteresis was observed. The width of the transitions showed a strong dependence on small levels of impurity and on strains. In the third transition a gradual reorientation of all the spins towards a direction in the (111) plane perpendicular to the applied field was observed. This transition was incomplete at the highest fields and temperatures used (10.0 T at 245K).

Mössbauer spectra of tin in float glass

Tin is not a major constituent of window glass, but is found at high concentrations in the lower surface of float glass which has been in contact with the molten tin bath. It does not extend far into the surface, but causes the physical and chemical behaviour to differ from that of the upper surface. It is important, therefore, to understand the structural role of tin in silicate glasses and thus its effect on various properties. Mössbauer spectra were taken of three series of glassy materials, namely binary glasses (SnO and SiO2) in varying proportions, re-melted float glass containing tin, and float glass from a production plant. The binary glasses contained between 20 and 70% tin which was found to be mainly Sn2+, with very small amounts of Sn4+ in some of them. The spectra showed a small decrease in isomer shift with increase in tin content, which is ascribed to the change in molar volume. The re-melted samples were float glass which was mixed with stannous oxalate in appropriate conditions to try and maintain tin in the 2+ state, and contained up to 15% tin by weight. The spectra show both Sn4+ and Sn2+ with rather more in the 4+ oxidation state. The change in the spectra as a function of temperature revealed a large difference in thef-factor (and hence the chemical binding) of the two states. A series of spectra was taken between 17.5 and 900 K for the sample containing 15% tin. From the absorption as a function of temperature thef-factor was determined for both oxidation states, and hence enabled the relative amounts of Sn4+ and Sn2+ present in each sample to be estimated. Measurements of the shift as a function of temperature were also made. The float samples were surface material produced by grinding away all but 0.1 mm of the lower surface of industrially produced float glass. The Mössbauer spectra showed them to be predominantly Sn2+, as expected from the reducing atmosphere in the float plant. The concentration and oxidation state of the tin may be estimated from the value of thef-factors and isomer shifts.