René Berger

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Abstract Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x )(0.63B 2 O 3 · 0.37Li 2 O) · x Fe 2 O 3 , with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe 3+ concentrations (Fe 2 O 3 molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g ef = 4.3 peak accompanied by a shoulder continuing down to g ef = 9.7 , are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe 2 O 3 contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe 3+ ions. It is concluded that the spectrum is due to diluted Fe 3+ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe 3+ ions is estimated as 4.9 A. A diluted state of Fe 3+ ions in the glass network in this range is also confirmed by the temperature dependence of the g ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe 3+ ions (Fe 2 O 3 molar contents from 0.01 to 0.1), the width of the g ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g ef ≈ 2 line, which coexists at these concentrations with the g ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe 3+ ions. The temperature dependence of the g ef ≈ 2 line intensity in 0.1 mol Fe 2 O 3 glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe 2 O 3 glass, which is attributed to an appearance, at higher Fe 2 O 3 contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g ef ≈ 2 line in the low-temperature EPR spectra.

Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in t...

Mo5+ ions as EPR structural probes in molybdenum phosphate glasses

Abstract Glasses of P 2 O 5 -Li 2 MoO 4 -Li 2 O system with constant Li 2 MoO 4 molar contents (10%), containing different amounts of the alkaline oxide (Li 2 O) ranging from 0 to 30 mol%, have been studied by electron paramagnetic resonance (EPR) spectroscopy. It is found that less than 1% of molybdenum ions are reduced to Mo 5+ , this percentage decreasing with the Li 2 O content. The components of g - and A -tensors are determined by computer simulations and related to structural properties using molecular orbital method. The distorted octahedral environment of Mo 5+ sites is confirmed and minimal Mo-Mo distance is evaluated from the EPR linewidth as ≈0.38 nm. At Li 2 O contents ≥20 mol%, the coordination polyhedra of Mo 5+ become better ordered though more distorted. The Mo-O bond covalency increases with increasing Li 2 O content.

Lineshapes in magnetic resonance spectra

In magnetic resonance, and in particular, in superparamagnetic resonance studies at variable temperatures, a correlation between the apparent resonance magnetic field and the apparent linewidth is often observed. In order to account for this correlation, we consider the resonance lineshapes resulting from different phenomenological equations of damped motion of the magnetic moments in the cases of a linear paramagnet and of a perfect soft ferromagnet. The Bloch-Bloembergen, modified Bloch, Gilbert, Landau-Lifshitz and Callen equations are analysed. In most cases we obtain analytical expressions for the apparent resonance-field shift. Finally, we report an experimental variable-temperature study of the superparamagnetic resonance of ultrafine Fe2O3 particles in sol-gel glass. Computer simulations using the Landau-Lifshitz lineshape provide good fits of the resonance spectra at different temperatures for the same magnetic and morphological parameters of the particles.

Superparamagnetic resonance of annealed iron-containing borate glass

A lithium borate glass containing a small amount of iron oxide is studied by electron magnetic resonance at room temperature after repeated annealing steps between 460 and . As the anneal temperature increases, the sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow line emerges at , superposed with a broader one, the narrow and the broader components predominating respectively after annealing at lower and at higher temperatures. Computer simulations of spectra have been carried out, based on a model of resonance of ferromagnetic single-domain nanoparticles randomly dispersed in the devitrified glass (superparamagnetic resonance). As the anneal temperature increases, the most probable particle diameter obtained assuming a log-normal distribution of diameters increases from 2.9 to 4.7 nm showing a saturation at higher anneal temperatures, whereas the relative number of larger particles grows continuously.

Magnetic and optical properties and electron paramagnetic resonance of gadolinium-containing oxide glasses

Magnetic susceptibility, electron paramagnetic resonance (EPR) and optical absorption have been studied in a glass system 20La2O3–22Al2O3–23B2O3– 35(SiO2+GeO2) with a part of La2O3 substituted by Gd2O3 in different concentrations. Positive Weiss constants have been found in more heavily doped glasses and ascribed to clustering of Gd3+ ions. Computer simulations of the EPR spectra show that the short-range ordering in the environment of the Gd3+ ions is well preserved. The relative distribution widths of the ligand coordinates are less than 2%. In the more heavily doped glasses the EPR spectra are superpositions of signals arising from isolated ions and ferromagnetic clusters. The increase of Gd3+ concentration is shown to change substantially the strong optical absorption edge while only small changes of f–f absorption band characteristics are observed. This difference is associated with the different effect of the Gd ion clustering on the mechanisms of the strong absorption in the ultraviolet region and the f–f absorption.

Electron paramagnetic resonance and Mössbauer effect studies in iron-doped 57Fe isotope enriched phosphate glasses

Abstract Electron paramagnetic resonance and Mossbauer spectroscopy measurements of phosphate glasses doped with 57Fe isotope enriched Fe2O3 have been carried out. The electron paramagnetic resonance spectra have been computer simulated using an approach based on the eigenfield method applied to the ‘rhombic’ spin Hamiltonian, which contains only the Zeeman and quadrupole fine structure terms. In order to account for the structural disorder in glass, different distribution densities of fine structure parameters D and E have been tried: a two-dimensional Gaussian function of D and λ = | E D | and ‘Czjzeks and related functions’. An agreement between the experimental and computer simulated spectra found with the Gaussian distribution density suggests the presence of a large fraction of Fe3+ sites with axial or feebly rhombic distortions (λ ≤ 0.08). The mean value of the axial fine structure parameter D is consistent with a highly distorted environment of Fe3+ ions in the phosphate glasses (but somewhat less in comparison with the borate glasses). It is shown that the gef = 2.0 absorption in the phosphate glasses consists of two different features: a narrow one results from Fe3+ ions isolated in the glass matrix, while the broad one is due to crystallite inclusions.

X and Q band EPR studies of Cu0.5Zr2(PO4)3 phosphates

Abstract Solid reaction and sol–gel varieties of Cu0.5Zr2(PO4)3 phosphates have been investigated by magnetic susceptibility and electron paramagnetic resonance (EPR). As expected from the formulation, they contain an important amount of paramagnetic Cu2+ ions. Room-temperature EPR spectra in the X and Q bands exhibit relatively different local information about paramagnetic environments related to structural properties. Analysis of the spectra reveals that Cu2+ ions are distributed in two types of site. Computer simulations of EPR spectra have been realized: EPR parameters suggest that Cu2+ environments are less distorted for the first type of site and more distorted for the second one. In the sol–gel variety the copper complex exhibits a larger distortion than in the solid reaction variety. The Cu2+ ions are distributed in ratios of about 2/3 to 1/3 for the solid reaction variety and of about 60 to 40% for the sol–gel variety. From EPR parameters, it is found that the covalency of copper ion bonding with the surrounding oxygen atoms is appreciable.

Differentiated g-values and exchange interaction between dissimilar copper ions as studied by EPR in the linear chain system CuSO4.5H2O

Abstract We have studied the room temperature EPR of a single crystal CuSO 4 .5H 2 O, when rotated around its β and γ magnetic axes at X- and Q-band. A careful analysis of experimental data permits one to obtain precise values of the differentiated principal g factors of the dissimilar copper ions ( g ⊥ = 2.080, g ∥1 = 2.4286, g ∥2 = 2.4046). When the magnetic field is parallel to magnetic axes α, β and γ, the two Cu 2+ ions are magnetically equivalent and the EPR spectra reduces to a Lorentzian singlet at X- and Q-band. When the magnetic field is parallel to a tetragonal axis, the EPR spectra are respectively a broadened singlet and a resolved doublet at X- and Q-band, due to the two dissimilar copper ions. For the compound under study, the simulation of these spectra with a method recently proposed by Hoffmann, in good agreement with the experimental results, permits one to determine an accurate value of the exchange interaction between dissimilar Cu 2+ ions: J ′ = (0.0374±0.0003)cm −1 .

ESR of X-ray irradiated vitreous and crystalline sodium metaphosphate

Abstract Electron spin resonance of X-ray irradiated vitreous and crystalline sodium metaphosphate is presented at X- and Q-bands, the latter operating frequency giving new results. The ESR parameters are determined by computer simulations of the spectra. The results support a model of a defect where the unpaired spin is shared by two oxygen atoms on one PO4 unit.