G. Saffarini

Crystallization of AsxSe1−x from the glassy state (0.005<x<0.03)

Abstract The crystallization of As x Se 1− x from the glassy state (0.005 x x , the apparent activation energy decreases. It is also found that the dependence of the inverse of the apparent activation energy for crystallization falls linearly with the reciprocal of the mean temperature used in the experiment. The latter result unambiguously indicates that the crystallization process for As x Se 1− x , in the composition range investigated, is mainly controlled by the viscosity of the medium. Therefore, the crystallization reaction totally disappears when the amount of As atoms in the alloys reaches such a value capable of connecting enough Se chains to block their movement.

Strong-fragile glass forming liquid concept applied to GeTe chalcogenide glasses

The values of the thermal heat-capacity changes at the glass transition temperature Tg (DCp ¼ CplCpg ) andthe values of the apparent activation energy Dhof the sub Tg relaxation process are determined for GexTe1� x (0.154x40.20) vitreous alloys by means of differential scanning calorimetry investigations. The values of Dhare then used to determine the values of the fragility index, m. The comparison of these new data to those already obtained on the GexSe1� x vitreous system leads to conclude that both systems exhibit strong thermodynamic character. The stronger thermodynamic character is reached at the Phillips-Thorpes floppy-to-rigid transition. On the other hand, we findthat Ge xTe1� x exhibits a kinetically fragile character which decreases as the amount of Ge increases in the alloy. # 2001 Elsevier Science B.V. All rights reserved.

About the coordination number of indium in GexSe1 − x − y Iny glasses

Abstract The results obtained by many authors on the GeSeIn vitreous system, in particular, those concerning the variations of the glass transition temperature, T g , with the average coordination number, Z , are investigated from a new perspective. From the assumption that In atoms participate in the formation of In 2 Se 3 microclusters, we show that the variations of T g with Z of the alloy are consistent with Sreerams relationship and with Tanakas structural transition model. We conclude that In is threefold coordinated with Se atoms to form In 2 Se 3 microclusters and that these microclusters do not contribute to the variations of T g with the alloy composition.

Glass transition temperature and molar volume versus average coordination number in Ge100−xSx bulk glasses

Glass transition temperatures (Tg) and molar volumes (V) of 13 glassy compositions of the Ge100−xSx system were determined using Differential Scanning Calorimetry (DSC) and density measurements, respectively. The variation of Tg and V with the average coordination number m are examined in light of the models proposed for the structure of these covalently bonded solids.

An EXAFS investigation of the local order around indium in Ge–Se–In glasses

Abstract EXAFS measurements were performed, at the In K edge, on devitrified Ge 5 Se 80 In 15 and on a series of vitreous Ge–Se–In alloys. The results indicate that the In atoms have essentially the same local surroundings in all of the samples investigated. We found that an In atom is surrounded by three Se atoms at a distance of about 0.259 nm and forms In 2 Se 3 microclusters.

On topological transitions and chemical ordering in network glasses of the Ge-Ga-S system

Abstract Chalcogenide glasses of the Ge-Ga-S system were prepared using high purity (5 N) Ge, Ga and S. Molar volume, V, and glass transition temperature, Tg, were obtained from the measured densities and the differential scanning calorimetry (DSC) thermograms. The property-composition dependence is examined in the light of the chemically ordered covalent network (COCN) model and the topological models which are proposed for the atomic arrangement in these covalently bonded glasses.

The influence of mechanical and chemical thresholds on the free volume percentage in Ge–Se–(Fe,In) chalcogenide glasses

Abstract Assuming that the increase in the volume during melting of a solid is inherent in the amorphous state (supercooled liquid), then the structure of amorphous solids should contain free volume. In the present paper, we determine the free volume percentage (FVP) in Ge–Se–(Fe,In) chalcogenide glasses and examine its relationship with the mean coordination number.

X-ray photoelectron spectroscopic measurements on glassy Ge20S80−xBix (x=0,16)

Abstract X-ray photoelectron spectroscopy measurements have been performed on n-type Bi-modified Ge 20 S 64 Bi 16 glass. The observed chemical shifts show that Bi is incorporated as a positive charged center into the matrix of Ge 20 S 80 parent glass.

Local environment of indium in Ge23.33Se64.67In12 chalcogenide glass

Abstract EXAFS measurements, from the In K edge, on Ge 23.33 Se 64.67 In 12 glass, have been performed. For this glass, it is found that In is only coordinated with Se atoms. By complementing the EXAFS results obtained from the fitting procedure followed in this work with the earlier reported DSC results obtained on this system, it is concluded that in this glass, the coordination number of In is equal to three. Moreover, this approach of coupling EXAFS with DSC results allows one to calibrate the Γ factor for systems which lack a reference material, or systems for which an ab initio calculation cannot be performed, as in our case.

Fragmentation of Ge1−xSnxSe2 glasses

Abstract Density, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements of Ge1−xSnxSe2 within 0.00 ⪕ x ⪕ 0.6 have been obtained. Upon increasing the Sn content, the density measurements show a non-monotonic decrease, while the differential scanning calorimetry measurements show a monotonic decrease of the glass transition temperature. The X-ray diffraction measurements on these glasses give interference functions which have first sharp diffraction peaks (FSDP) indicating the presence of intermediate range order (IRO). The X-ray photoelectron spectroscopy measurements were used to determine the plasmon energies from the L3M4,5M4,5 selenium Auger peaks which may be looked upon as being determined by the local electron density about the selenium atoms. It is found that the plasmon energies deduced from the Auger peaks change markedly with the Sn content in these glasses. These results are consistent with the fragmentation of the molecular cluster network model proposed by Mikrut and McNeil.