B. Deroide

Nature and thermal stability of paramagnetic defects in natural clay: a study by electron spin resonance

Abstract A natural clay of South of France is studied by electron spin resonance (ESR). Two radiation-induced defects are revealed, certainly localized in the quartz contained in clay. The first is the peroxy-center, associated with oxygen, formed by the action of an O 2 − ion on a silicon atom to lead to Si–O–O . The second is the E′-center, where an electron was trapped in oxygen vacancy to give Si . The spectroscopic parameters were determined. The thermal stability of these defects is studied by isothermal annealing experiments. Second order kinetics best explain the results, although more complex mechanisms must occur. The two defects have very different behaviors: the peroxy-center seems more stable at high temperature than the E′-center, although its activation energy is much weaker. They could be both used for the ESR dating of old clay.

Electron Spin Resonance and Dilatometric Studies of Ancient Ceramics Applied to the Determination of Firing Temperature

Electron spin resonance (ESR) and dilatometric studies have been performed on clay fired to various temperatures. ESR spectra are very sensitive to the heat treatment and the variations of spectroscopic parameters are discussed as functions of the firing temperatures. The study shows that these ESR parameters can be used for a quantitative determination of the firing temperature. In the dilatometric analysis, the change of the length as well as the parameters related to the quartz transition, are sensitive to the heat treatment. The most significant parameters are selected for this study. The dilatometric determination of the firing temperature is in good agreement with the ESR results. After a control experiment performed by both methods on a known clay sample, we finally study an archaeological sample as an example. Its firing temperature was determined with satisfying accuracy.

Lineshapes of ESR signals and the nature of paramagnetic species in amorphous molybdenum sulfides

Abstract Amorphous and poorly crystallized molybdenum sulfides were studied by ESR. Qualitative analysis of the spectra suggests the presence of three paramagnetic species. A simulation of the ESR spectra was attempted on this basis. ESR lines were calculated over the stoichiometry range MoS3→ MoS2. A good representation of the main part of the experimental lines was obtained. There is no significant variation of the g values of the various components of the spectra during the transformation from MoS3 to MoS2. The first signal is attributed to sulfur centers. The two others are assigned to metal centers. One is due to Mov sites in a chain-like structure, observed in the amorphous phase of MoS3. The other is due to Mov defects in a layered MoS2 microstructure. The variation of the site concentration was calculated for all compounds, from MoS3 to MoS2.

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Abstract Chalcogenides MeS 3− x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature ( TT ) increases. At 1000°C a perfect hexagonal arrangement can be obtained ( x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT . The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities ( g -value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS 3 and MoS 2 . The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.

Mn2+ electron paramagnetic resonance study of a sodium borosilicate glass prepared by the sol-gel method

Abstract Sodium borosilicate glasses were prepared by the sol-gel method and doped with a paramagnetic manganese probe. X-band electron paramagnetic resonance (EPR) experiments, carried out on powdered samples, showed that even for molar ratios higher than 10%, manganese can be inserted in the glass. EPR results obtained on different doped silica glasses showed that the ability of Mn 2+ to be inserted in the glassy network is connected with the presence of alkali species. The bulk location of the probe was confirmed by 29 Si nuclear magnetic resonance investigations. EPR results indicated that manganese is located in distorted sites in an octahedral silica environment.

29Si and 129Xe NMR of Mn2+ doped silica xerogels

Abstract Paramagnetic Mn2+ ions were incorporated in silica gel during the preparation of the sol. Doped silica xerogels were obtained by heat treatment at different temperatures and studied by 29Si and 129Xe nuclear magnetic resonance (NMR) experiments. Comparison between 29Si NMR magic angle spinning (MAS) spectra of doped and undoped xerogels showed that there is a significant effect of the paramagnetic probe on Q2, Q3 and Q4 peaks. Reduction of the 29Si-T1 relaxation time in the presence of the Mn2+ probe was observed. 129Xe NMR spectra were recorded on the xerogels at variable Xe pressures. Results were consistent with 29Si NMR data and confirmed the localization of the paramagnetic probe in surface sites as shown by electron paramagnetic resonance (EPR) of Mn2+.

Transport properties of tungsten sulfides WS2 + x

Abstract The temperature dependence of DC conductivity of tungsten sulfides WS2 + x (0 Analyses of the results show that the conductivity is correctly explained by a mechanism of electronic hops between localized states. Within such a model the number of sites, independent of measuring temperature, is calculated; the value of the parameter WM, the energy required to move two electrons from infinity to the sites, is also evaluated. A quantitative approach to explain the experimental results to the complex permittivity, is proposed.

Paramagnetic defects in solid sulphur and glasses of the system Ge–S

Abstract Ge–S glasses are synthesised by melting pure S and Ge elements. Electron paramagnetic resonance (EPR) spectroscopy is used to analyse the paramagnetic defects in these glasses. Paramagnetic defects are also observed in amorphous and polycrystalline S samples after UV irradiation at T=77 K. The simulation of the EPR spectra leads to spectroscopic parameters of the sulphur centres which are similar in S samples and in S-rich Ge–S glasses, at the liquid nitrogen temperature. In Ge–S glasses, the paramagnetic sites remain stable at room temperature and the spectroscopic parameters are modified. The nature of the sites involved in these absorptions is discussed.

Electron paramagnetic resonance study of Mn2+ and Cu2+ spin probes in (Ag2S)x(GeS2)1 - x glasses

Mn2+ and Cu2+ spin probes have been introduced in glassy (Ag2S)x(GeS2)1 - x (0 ≤ x ≤ 0.8). The electron paramagnetic resonance spectra related to Mn2+ show a poorly resolved hyperfine structure at g = 2 and a characteristic line at g = 4.3 ascribed to the presence of Mn2+ in the center of GeS4 tetrahedra. For small contents of Mn2+, hyperfine structure is resolved as x increases to the limit x = 0.55 and not resolved when x ≥ 0.6. The Cu2+ probe leads to a well resolved signal when x = 0.4. No signal is observed for x < 0.3. These results seem to confirm the presence of a phase separation domain for x < 0.3 and the changes of the local structure of the probes with the appearance of micro-crystallinity due to the Ag8GeS6 phase when x ≥ 0.55.

Characterisation of calcined silica-pillared zirconium phosphate: a Cu2+ e.p.r. study

Abstract Silica-pillared zirconium phosphate shows both micro- and mesoporous character, with an average pore dimension of 23 A after calcination at 480°C. Cu2+ coordinates in solution to the molecular precursor of the silica pillars, 2-aminoethyl-3-aminopropylsiloxane oligomers, and is carried into the interlayer region on contacting this solution with α-zirconium phosphate, where it serves as a probe for the evolution of the intercalated phase to a porous pillared solid on calcination. Electron paramagnetic resonance spectroscopy shows that the environment of Cu2+ changes from (4 + 2) nitrogen + oxygen to one containing oxygen only as the organic functions are eliminated. Above 400°C, (the temperature at which the material starts to exhibit appreciable surface area) and up to 600°C, the e.p.r. spectrum remains unchanged, and these limits define the temperature region in which the silica-pillared phosphate is stable. Two copper sites are identified in the pillared phase. These are considered to correspond to one in which copper is surrounded by oxygen atoms of the silica pillars, suggesting that part of the Cu2+ forms an integral part of the pillars, and a second related to Cu2+ occluded in the pores and accessible to incoming species such as water.