Mössbauer studies of phosphate glasses for the immobilisation of toxic and nuclear wastes

Various iron-containing phosphate glasses were investigated by Mössbauer spectroscopy. Iron was found to occur predominantly as Fe³⁺ in all glasses, and largely occupied sites with distorted octahedral coordination for both redox states. Using a base glass of nominal composition 60 P₂O₅–40 Fe₂O₃ (mol%), stepwise molar replacement of Fe₂O₃ by (0.67 Na₂O?×?0.33 Al₂O₃) increased the redox ratio, Fe²⁺/ΣFe, from 0.13 at 40% Fe₂O₃ to 0.25 at 10% Fe₂O₃. The centre shift increased and quadrupole splitting decreased by up to ～20% over this range, interpreted as a decrease in the average distortion of Fe sites from cubic symmetry, and an increase in average iron coordination. Literature revealed that recoil-free fraction ratio f (Fe³⁺)?/?f (Fe²⁺)?≈?1.3 in iron phosphate glasses, and this was considered when assessing redox. Mössbauer parameters of these and other glasses demonstrated a combination of structural stability and compositional flexibility which makes them so suitable for waste immobilisation.     

Structural studies of nanocrystalline oxides

We report the results of structural studies of samples of nanocrystalline tin oxide, zirconia, magnesia, alumina and silica, prepared by sol–gel techniques (including the addition of silica to restrict grain growth) and high-energy ball milling. XRD, EXAFS/XANES and MAS-NMR analyses were used to characterise the materials. EXAFS showed that nanocrystals of ZrO₂ and SnO₂ prepared by sol–gel methods are highly crystalline, consistent with previous EXAFS studies of sol–gel prepared nanocrystalline oxides [A.V. Chadwick, M.J. Pooley, K.E. Rammutla, S.L.P. Savin, A. Rougier, J. Phys.: Condens. Matter, 15 (2003) 431.; A.V. Chadwick, G.E. Rush, in Nanocrystalline Metals and Oxides: Selected Properties and Applications, (Kluwer, New York), (2002), p.133.]. In contrast, the sol–gel prepared SiO₂ appeared amorphous, even after prolonged heating at 1200 °C. The EXAFS of the ball-milled samples showed clearly attenuated signals that cannot be attributed solely to particle size. Ball milling of quartz crystals (which have a simple Q₄) first broadens the NMR line and then generates Q₃ and then Q₂ lines indicating the generation of amorphous materials in the samples.     

Structural studies of tetrachloride liquids

The differential cross-sections for neutron scattering from liquid carbon tetrachloride have been measured with the TSS instrument at the Harwell Electron Linac. Data were taken at seven different scattering angles for a wavelength range of 0·2–3·5 Å. The observed diffraction patterns at high momentum transfer (> 8–30 Å^-1) have been analysed in terms of the molecular form factor f ₁(Q). It was found that the oscillation amplitudes could be satisfactorily described only by introducing an energy-dependent term into the Debye-Waller factors of the form factor. The f ₁(Q) data were fitted with a four-parameter function for measurements at scattering angles of 150°, 90° and 58°. The carbon-chlorine bond length parameter was accurately defined in all cases and had a mean value of 1·766 ± 0·002 Å. The inclusion of an anharmonicity constant in the form factor gave an improved χ²-fit to the data with an increased value of 1·770 ± 0·002 Å for the bond length. The results are in excellent agreement with other measurements and show the importance of pulsed neutron techniques for molecular structure studies of disordered materials.     

Mössbauer studies of materials used to immobilise industrial wastes

The necessity to immobilise waste safely requires the development of stable materials. Mössbauer spectroscopy has been used to help understand and obtain desirable properties in alkali borosilicate glasses, phosphate glasses and vitrified sewage sludge ash. Phosphate glasses suitable for waste immobilisation have been microwaved and conventionally melted and differences reported. The environment of Fe in promising ceramics has also been studied. Mössbauer studies of irradiated vitrified wasteforms show their resistance to radiation damage.     

Structural investigations of semiconducting barium-vanadate glasses containing iron

Mössbauer and EPR spectroscopic studies are performed on (0.9-z)V₂O₅.ZBaO. 0.1Fe₂O₃ amorphous semiconductors. It is observed that the addition of BaO results in the formation of non-bridging oxygens. For low Z-values the formed non-bridging oxygens are attached to vanadium ions whereas for higher Z-values some of the oxygens attached to Fe³⁺ ions are also converted into non-bridging ones.     

Structural and optical properties of porous iron oxide

The structural and optical properties of the novel porous iron oxide fabricated by wood template have been investigated. The obtained porous iron oxide was characterized to be α- Fe₂O₃ by Fourier transform infrared and Raman spectroscopy. X-ray absorption fine structure measurement revealed that the bond length of Fe-O₁ of the porous iron oxide has good agreement with that reported for the α- Fe₂O₃ crystal structure while the bond lengths for Fe-O₂ and Fe-Fe deviate slightly from those of the α- Fe₂O₃ crystal structure. Photoluminescence from the porous iron oxide exhibited broad emission bands around 760 and 890 nm, which are believed to be due to the unique nanoscale structure of the porous iron oxide.     

Structural studies of nano silica employing on-line ultrasonic studies

Monodispersed, nano silica particles have been prepared by sol-gel hydrolysis and condensation of the metal alkoxide using pH buffer. The prepared particles are characterised by XRD, FTIR, BET, SEM, TEM measurements. The measurements reveal that the size and shape of silica particles depend on concentration of water. In addition, the ultrasonic longitudinal velocity and attenuation of the nano silica particles have been measured at a frequency of 5?MHz over a wide range of temperatures from 300?K to 1150?K in nano silica. The different structural transitions, such as monoclinic, orthorhombic, orthorhombic with a non-integral super lattice, stable orthorhombic and hexagonal, which exist in silica are explained based on on-line high-temperature ultrasonic velocity and attenuation measurements.     

Structural studies on urethane elastomers

A number of techniques have been used to study structural organization and transitions in polyether- and polyester-based urethane elastomers. Scanning thermal methods reveal three characteristic transitions common to samples of varying composition, namely, the major glass transition and two higher transitions believed due to disruption of different types of hydrogen bonding. Other transitions are assigned to melting of ester and of urethane crystallites. Light-scattering measurements on turbid polyether-based samples indicate both isotropic and anisotropic contributions to scattered intensity, the latter reflecting structural ordering on a small scale. X-Ray diffraction studies of stress-induced crystallization in the polyetherbased elastomer show reflections attributed to crystallites of polyether segments. It is concluded that separation into a domain structure occurs in both types of elastomers, but to a higher degree in the polyether-based polymers than in the polyester-based polymers, possibly due to restrictions in the latter imposed by interaction of the ester and urethane groups.     

Structural studies of crystalline MDI-based polyurethanes

Electron microscopy reveals a characteristic structural organization consisting of poorly organized spherulites with an open fibrous texture in which the strands appear to be built up from microfibils. X-ray and birefringence results are also discussed.     

Magnetic studies on mass-selected iron particles

We have investigated the magnetic properties of mass-selected iron clusters using the Magneto-Optical Kerr effect (MOKE) in longitudinal geometry. For the production of these clusters, a newly developed continuous arc cluster ion source (ACIS) was applied. The source is based on cathodic arc erosion in inert gas environment and subsequent expansion into high vacuum. Its intensity and stability allows mass selection within a wide size range. The source efficiency is demonstrated in deposition experiments and transmission electron microscopy. For mass-selected iron particles deposited into a silver matrix we could observe a change in the magnetic behaviour from ferromagnetism to superparamagnetism around a size of 10 nm at room temperature. Received 1st December 2000     

Structural studies of supported tin catalysts

Tin oxide was supported on aluminium oxide, titanium oxide, magnesium oxide and silicon oxide, and the resulting interactions between the components in the prepared samples and after reduction were characterized by Mössbauer spectroscopy. It was observed that in the oxide state, tin is present as SnO₂ on alumina, magnesia and silica, but on titania tin occupies Ti sites in the structure. After hydrogen treatment at high temperatures, tin is reduced from Sn(4) to Sn(2) on alumina and titania; it is reduced from Sn(4) to Sn(0) on silica, and is practically not reduced on magnesia. These results reveal the degree of interaction between tin and the different supports studied.     

Magnetooptical studies on thin iron films

Magnetooptical measurements on Fe-films in the layer system ZnS-Fe-MgF₂-Ag showed a decrease of the Kerr-intensity values with Fe-film thicknesses below 15nm, not predicted by theoretical calculations. Magnetization curves obtained magnetooptically indicate a significant influence of the layer system on the magnetic properties of Fe-films.     

Structural and electronic properties of laser irradiated pure iron

The surface of a pure iron sample irradiated with a high power pulsed laser has been studied by UHV electron spectroscopies in the reflection mode (AES, EELS and EELFS) in order to verify the experimental hint of a laser induced non-crystalline surface layer.Our results point out that the local arrangement of atoms in the laser irradiated surface is not disorder-like: no significant differences in structural and electronic properties between irradiated and non-irradiated samples have been detected in the short-range limit (up to the fourth coordination shell).This result agrees with a theoretical computer calculation which predicts, for pure metals, a cooling rate of the order of 10¹⁰ K sec^?1 after the laser pulse, not sufficient to avoid recrystallization.     

Structural and electrical properties of iron molybdenum phosphate glasses

Summary  Iron molybdenum phosphate glasses [xMoO₃ · (0.6 -x)P₂O₅ · 0.4Li₂O] :yFe₂O₃ with 0 ≤x ≤ 0.6 andy = 0.03 (mol%) prepared in ambient atmosphere using the melt quenching technique were studied by using DC electrical conductivity,⁵⁷Fe M?ssbauer and infrared spectroscopies. The DC conductivity depends on the MoO₃ concentrationx. It was observed that, with increasingx, the ratio Fe²⁺ /(Fe³⁺ + Fe²⁺) and the DC conductivity increase. Infrared spectroscopy and X-ray powder diffraction indicate that a Li₂ MoO₄ crystalline phase is present for high MoO₃ content samples (x = 0.5, 0.6). This work was partly sponsored by FINEP, CNPq (Brazilian agencies) and UECE (Universidade Estadual do Cearà).     

TDPAC studies of the iron–molybdenum cofactor in nitrogenase

The uptake and the processing of molybdenum in nitrogen fixing bacteria was studied applying the TDPAC-technique with radioactive ⁹⁹Mo. The observation of the nuclear quadrupole frequencies allowed a ‘finger-printing’ of the characteristic binding states in three bacteria species. The measurements have been carried out with intact cells and with the isolated proteins. In one bacterium a molybdenum storage protein was identified. This revised version was published online in August 2006 with corrections to the Cover Date.     

一维应变加载下单晶铁结构转变的微观研究

采用嵌入原子势和分子动力学方法,模拟了单晶铁在一维应变条件下由体心立方（bcc）转变为六角密排（hcp）结构的微观过程. 当应变加载至相变临界值时,hcp相开始均匀形核并沿{011}晶面长大为薄片状体系.弹性常数C₃₁和C₃₂在相变前被逐渐硬化,C₃₃则在相变前出现软化行为;当体系完全相变后,上述各弹性常数显示开始随体积压缩而迅速硬化,温度效应对晶格具有软化作用,可削弱C₃₃的硬化和软化过程;样品在压缩过程可出现孪晶结构,孪晶结构使晶格发生剪切变形.混合相中,hcp相势能比bcc相高,最大剪应力方向与bcc相反向;系统的偏应力与hcp相质量分数近似呈线性关系. 关键词： 结构转变 分子动力学 一维应变     

Structural and vibrational studies of anilinium nitrate

The crystals of anilinium nitrate, , were obtained by slow evaporation of an aqueous solution. The crystals belong to the Pbca (no. 61) space group of orthorhombic system, Z=8, a=10.158(2), b=9.277(2), c=16.177(3) Å. Positively charged anilinium cations and anions are present in the structure. Powder FT IR and FT Raman spectra for normal and deuterated samples are discussed with respect to the crystal structure. DSC measurements do not indicate clearly on the occurrence of phase transition in the temperature region 113-293 K.