P. Galinetto

Raman spectroscopy of AMn2O4(A = Mn, Mg and Zn) spinels

First order Raman spectra in the region 200–800 cm−1 have been collected for AMn2O4 (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO4 and MnO6 unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg1−xMnxMn2O4 solid solution; the effect of the spinel inversion has been also studied on MgMn2O4 quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Thermal stability and structural transition of metastable Mn5O8: in situ micro-Raman study

The Raman spectrum of the metastable Mn5O8 phase, obtained from slow oxidation of Mn3O4 at low temperature, is presented and analysed for the first time and the thermal stability is monitored by the changes in Raman spectra due to laser-induced thermal treatments. A structural transformation toward the spinel phase Mn 3O4 is observed at temperature higher than 1000 K. Other Mn oxides, characterised by intermediate Mn oxidation states, are not detected below or during the transition. A compositional model of the sample grains is also proposed by comparing Raman data with X-ray diffraction and scanning electron microscopy measurements. q 1999 Elsevier Science Ltd. All rights reserved.

Enhancement of room temperature ferromagnetism in N-doped TiO2−x rutile: Correlation with the local electronic properties

The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.

Light-induced defects in KTaO3

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation betw...

Optical properties of cubic stabilized zirconia

Optical absorption across the fundamental edge and the associated photoluminescence, have been studied in single crystals of yttrium stabilized zirconia (YSZ) of varying yttrium concentration. The results allow discrimination of a region of intrinsic transitions (above 5.2 eV) from a low energy region, where disorder-induced transitions are dominating. The size of extra absorption in this range shows an inverse correlation to the stoichiometric ratio, to be interpreted in terms of progressive ordering of the vacancy system.

Micro-Raman study of the role of sterilization on carbon nanotubes for biomedical applications

AIM We investigate the effect of four different types of sterilization procedures on the structural properties and morphological features of single-wall carbon nanotube samples approachable by micro-Raman spectroscopy. Sterilization procedures (treatment in humid heat autoclave or ethylene oxide and irradiation with gamma-rays or UV light) are necessary in view of the use of carbon nanotube sterile samples in in vivo toxicity tests on laboratory rats. Micro-Raman spectroscopy allows us to estimate several details about the morphology of the single-wall carbon nanotube mixture (mainly the presence of disorder and diameter distribution) before and after the sterilization treatment. RESULTS The best of these treatments, in other words, the one that least affected the morphology and structural properties of carbon nanotubes, was found to be UV irradiation and has thus been selected for future in vivo tests on rats.

Radiation-induced grafting of carbon nanotubes on HPLC silica microspheres: theoretical and practical aspects

Multi-walled carbon nanotubes (MWCNTs) were grafted for the first time by γ-radiation onto silica microspheres in the presence of polybutadiene (PB) as the linking agent, obtaining a novel hybrid material with chromatographic properties, with an alternative approach to the existing procedures. The synthesis involves the one-pot γ-radiation-induced grafting of MWCNTs onto silica microspheres in the presence of PB as a linking agent. PB also serves as a coating layer of the silica particles, to which MWCNTs are anchored through stable chemical bonds formed via radical chain reaction with the polymer. The product (MWCNT-PB-modified silica) resulted in MWCNT bundles interlaying the silica particles which acted as a support and as a spacer. This new material highlights the unquestionable properties of CNTs also when grafted in a composite, thus allowing the disposition of a more robust material whose properties are still related to the nanotube structure. The grafting was confirmed by Raman spectroscopy. The surface area, determined by BET isotherms, resulted in 132 m(2) g(-1), about 34% lower than that of pure silica, pointing to the cross-linking effect of PB in the silica matrix. The evaluation of MWCNT-PB-modified silica as a LC stationary phase was performed by separation of aromatics, with satisfactory resolution and reproducibility, while structural selectivity was proved by isomer separation. A good resolution was obtained also for acid/basic compounds as barbiturates. A comparison with a commercial C18 sorbent highlighted the advantage in using the CNT column for separating aromatic hydrocarbons. Control experiments on the PB-coated silica column proved the key role of MWCNTs in the chromatographic performance.

Manganese oxide nanoparticles in SrTiO3:Mn

Weakly concentrated “as-grown” and reduced SrTiO3:Mn crystals and ceramics are investigated by means of electron paramagnetic resonance (EPR) to get new insights concerning the Mn substitution in the strontium titanate structure. The temperature dependence of Mn2+ and Mn4+ EPR spectra strongly indicates the formation of MnO2 or MnO small particles to be the favored configuration for the Mn insertion in SrTiO3. Only a part of manganese ions incorporates into the crystal lattice in B position as Mn4+ ions.

Electron paramagnetic resonance of copper centres in potassium tantalate single crystals

Electron paramagnetic resonance (EPR) of exchange-coupled pairs of C2+ ions was observed in KTaO3:Cu, K1−xLixTaO3:Cu and K1−xLixTa1−yNbyO3:Cu single crystals. The spectra are typical of centres with S = 1 and tetragonal symmetry with the pair axis parallel to the [100], [010] or [001] directions. The angular and temperature dependences of the signals have been studied. The experimental results are explained in terms of a pair centre model, which consists of a chain of two equivalent copper ions and three oxygen vacancies. The two copper ions occupy neighbouring tantalum sites and one of the oxygen vacancies lies between them, the chain running in the 100 direction. Ferromagnetic exchange coupling takes place between the copper ions, as confirmed by the thermal behaviour of the corresponding EPR spectra.

Labeling interacting configurations through an analysis of excitation dynamics in a resonant photoemission experiment: the case of rutile TiO2

A detailed study of resonant photoemission at Ti L(2,3) edges of insulating rutile TiO(2-x) thin film is presented. Pure TiO(2) resonating structures, defect-related resonances, resonant Raman-Auger and normal LVV Auger emissions are tracked, including an unpredicted two-hole correlated satellite below the non-bonding part of the valence band. The analysis of excitation dynamics unambiguously addresses the origin of these features and, in particular, the extent of charge transfer effects on the Ti-O bonding in the valence band of rutile, disclosing further applications to the more general case of, formally, d(0) oxides.