M.A. Ureña

Crystallization processes of Ag-Ge-Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge25Se75)100−yAgy with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag8GeSe6 and the secondary phase GeSe2 was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag8GeSe6 phase in all the (Ge25Se75)100−yAgy samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.

Characterization of Ag-Ge-Se bulk glasses by means of Mössbauer effect on 57Fe and 119Sn atomic nuclei

In this work, the structure of Fe and Sn doped Agx(Ge0.25Se0.75)100−x (x=0 to 25 at.%) intrinsically inhomogeneous glasses is analyzed employing 119mSn and 57Fe Mossbauer spectroscopy, X-ray diffractometry and scanning electron microscopy. 119mSn enters in the glass as a substitutional impurity for Ge whereas 57Fe enters as an interstitial impurity. Mossbauer spectra obtained with 119mSnO3Ca source, from samples containing about 1% 119Sn for Ge, reveal that the local order of Ge in both amorphous phases is basically the same whereas Mossbauer spectra obtained with 57Fe(Rh) source, from samples containing about 0.5% 57Fe, evidence the differences between both phases.