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Ionic glasses obtained combining silver iodide and silver molybdate are characterized by quite low values of the glass transition temperature Tg around 320–350 K, by high values of the dc ionic conductivity even at room temperature and by a peculiar behavior of the mechanical response at ultrasonic frequencies. In fact, at temperatures well below their glass transition temperature, these glasses exhibit an intense peak of acoustic attenuation well described by two different and almost overlapping relaxational contributions. Considering also that negative thermal expansion has been reported for some molybdate crystalline compounds, we have investigated in this work the thermal expansion of two silver iodomolybdate glasses (AgI)1-x(Ag2MoO4)x for x=0.25 and x=0.33 in a wide temperature range (4.2–300 K) from cryogenic temperatures up to some 20 K below Tg using a precision capacitance dilatometer aiming to understand whether the expansivity shows some possible fingerprint corresponding to the above-mentioned mechanical response. Two different measuring methods, a quasiadiabatic and a continuous one, have been used for the thermal expansion measurements. The results are discussed in comparison with the information obtained from previous investigations based on the extended x-ray absorption fine structure (EXAFS) technique and with the behavior of other ionic glasses. [ABSTRACT FROM AUTHOR]

The evaluation of uncertainty in temperature-dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K-edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest-neighbours parameters: bond thermal expansion, parallel and perpendicular mean-square relative displacements and the third cumulant. Different causes of uncertainty, a comparison of experimental results with theoretical models, the difference between EXAFS and crystallographic thermal expansions and the meaning of the third cumulant are discussed. [ABSTRACT FROM AUTHOR]

Extended x-ray absorption fine structure (EXAFS) measurements have been performed at the Kedge of iodine in (AgI)1-y(Ag2MoO4)y glasses, with y = 0.25 and 0.33, as a function of temperature from 25 K to the glass transition temperature Tg ≃ 323 K. EXAFS monitors the short-range component of the I-Ag distance distribution. At 25 K, iodine coordinates to about four silver ions, at an average distance about 0.06 Å larger than in β-AgI. As temperature increases, EXAFS monitors a progressive depletion of the short-range distribution, due to transitions of Ag ions into a long-range component. Coordination number and mean value of the short-range I-Ag distribution decrease while temperature is increasing. EXAFS results give information on the local modifications around iodine due to the ionic diffusivity. [ABSTRACT FROM AUTHOR]

Abstract: EXAFS investigation about Metal Induced Crystallization (MIC) of a-Si thin films doped with Ni, has been carried out at the K edge of Ni. Several a-Si films deposited on quartz and annealed at different temperatures and a non-annealed sample have been analyzed in order to study the variation of the nickel surroundings as a function of temperature. Nickel particles were co-sputtered together with silicon to obtain a metal percentage of about at. 0.5%. In all the annealed samples it was found that nickel, in its first shell, is 8-fold coordinated to silicon while a weak signal corresponding to the second shell appears in the Fourier transform of the spectra as in crystalline nickel di-silicide (c- NiSi2) used as reference compound. No presence of Ni clustering has been ascertained. In the non-annealed sample, where the NiSi2 formation has never been observed, EXAFS shows a deformed first shell environment on Ni similar to that of NiSi2. [Copyright &y& Elsevier]

Abstract: EXAFS can represent a powerful probe of the local behaviour of negative thermal expansion (NTE) materials, thanks to the possibility of measuring the expansion of selected inter-atomic bonds and the perpendicular relative atomic displacements. The effectiveness of EXAFS for NTE studies is illustrated by a comparison of results recently obtained on germanium, CuCl and the cuprites Cu2O and Ag2O. [Copyright &y& Elsevier]

Temperature dependent extended X-ray absorption fine structure (EXAFS) measurements give original information on local structure and dynamics of materials with framework structure, exhibiting negative thermal expansion. By proper cumulant data analyses, the temperature dependence of the nearest neighbours distance can be directly measured and the degree of rigidity of basic structural units monitored. Recent results obtained for Ag2O show that the contraction of the lattice parameter is accompanied by the expansion of the 1st-shell Ag–O distance, and that the deformation of the Ag4O tetrahedral units is not negligible. [Copyright &y& Elsevier]