X-RAY ABSORPTION STUDY OF FREE AND BOUND SULPHUR IN MoS2+x (0 ≤ x ≤ 1)

Le spectre de photoabsorption X près du seuil K du soufre (XANES), présente des structures indiquant la présence de plusieurs degrés de charge. Le spectre obtenu en retranchant la contribution de MoS2 du spectre de MoS3 est comparé à celui du soufre natif. Cette comparaison indique que le soufre additionnel dans MoS3 présente une coordination proche de celle du soufre natif et qu'il est probablement faiblement lié.The X-ray near K edge structure of S in MoS2 and MoS3 show features indicating different charge states. The MoS3-MoS2 difference spectrum is compared to the K edge spectrum of native sulphur. From this cornparison it is concluded that the additional S atom in MoS3 has similar coordination as in native sulphur and is probably weakly bound

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

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.</p

Dc and ac Conductivities of (V₂O₅)_x(B₂O₃) _1-X oxide glasses

Glasses of composition (V2O5)x(B2O3) 1-x with x = 0.1, 0.3, 0.5, 0.7 and 0.9 were obtained by a quenching technique. Conductivity measurements using σac were performed on compact pellets of these materials. The conductivity behavior in direct regime, σdc, is described by the "small polaron" model. The room temperature variation of log σdc with the activation energy of each composition, suggests an adiabatic hopping character to the conduction. Application of Hunt's model to describe the ac conductivity behavior shows that the relaxation process has a local character and can be described by hops between each pair of sites.</p

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

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 Mn2+ to be inserted in the glassy network is connected with the presence of alkali species. The bulk location of the probe was confirmed by 29Si nuclear magnetic resonance investigations. EPR results indicated that manganese is located in distorted sites in an octahedral silica environment.</p

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 &lt; 0.3. These results seem to confirm the presence of a phase separation domain for x &lt; 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.</p

ESR study and dc conductivity of binary glasses of the system (V₂O₅)_x(B₂O₃) _1-x

Glasses of the system (V2O5)x(B2O3) 1-x were prepared by melting and quenching in the domain 0&lt;x&lt;1. Electron spin resonance of V4+ species was investigated at room temperature. Two different VO2+ sites were identified and spin Hamiltonian parameters have been determined for these two sites. Variations of these parameters with composition were studied and dc conductivity measurements were performed on the same samples. DC transport properties were ascribed to thermally activated hopping between localised sites.</p