Y. Bensimon

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.

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.

ESR study of influence of oxygen on molybdenum sulfides

Abstract Electron spin resonance (ESR) measurements of amorphous molybdenum sulfides MoS 2+ x (0 ≤ x ≤ 1) and their derived compounds MoS y O z , which are obtained by oxygen exposure at 130°C, are performed in order to determined the nature and the number of the active sites. The ESR spectrum is always the sum of three elementary signals, the responsible species of which are analyzed by means of a simulation program. These species can be related either to unsaturated sulfur atoms or to Mo V atoms within two different surroundings. The strong modification of the ESR spectra, around the composition MoS 2.6 , can be considered as a “pseudo-transition”. The influence of oxygen exposure confirms this assumption and leads to an increasing number of paramagnetic defects in accordance with the catalytic properties of these solids.

Influence of oxygen on electrical conductivity of amorphous molybdenum sulfides

Abstract The electronic properties of hydrocracking catalysts such as molybdenum sulfides, obtained by sulfurization of oxides, are little known. Similar compounds can be obtained from amorphous molybdenum sulfides after contact with oxygen. The resulting by-products, corresponding to the general formula MoS y O z , are still amorphous. The study of the polarization conductivity shows that in the sulfur-rich solids the action of oxygen creates a decrease of the potential energy, W M , of the charge carriers and of the number of the sites, N , involved in the transport phenomenon. By contrast, for lower sulfur ratios, these parameters remain unchanged. The results show that the mechanism of conductivity can be ascribed to non-bonding orbitals due to sulfur and molybdenum atoms.

Eyidence of microstructural modification in amorphous molybdenum sulphides obtained from d.c. and a.c. conductivities and electron spin resonance measurements

Abstract Amorphous molybdenum sulphides (MoS2 +x 0  x  1) have been prepared by thermal decomposition of synthetic ammonium thiomolybdate. Heat treatment of this compound leads to amorphous sulphides, the composition of which varies from MoS3 to MoS2. This evolution is followed by means of d.c. and a.c. conductivities as well as by electron spin resonance (ESR). The polarization conductivity is interpreted using a theoretical model, based on the transport of the electrical current by means of single-electron hops between localized sites. This approach leads to the determination of the potential energy W M of the electron trapped in its site, and the number N of sites involved in the conduction process. The evolution of W M and N as a function of the heat treatment temperature (TT) shows a pseudotransition corresponding to the values of composition near MoS2.6.(TT = 315°C). The ESR measurements confirm this behaviour, and the observed signals show sharp modifications at the same composition and at the sa...