EXAFS and Mössbauer spectroscopy analyses to explore the short range structure in Agx(Ge0.25Se0.75)1-x glasses with 0≤x≤0.25

J.M. CONDE-GARRIDO; J.ROCCA; M.A. UREÑA; M. FONTANA; B. ARCONDO

Valparaiso, Chile

Workshop; Solidos 2009; 2009

(GeySe1-y)1-x  glasses have caused great impact for at least two decades due to their remarkable transport properties. Glasses with y~0.25at. are semiconductors for x<0.08at. whereas for x≥0.08at. they are fast ionic conductors. This abrupt change of transport regime (seven orders of magnitude for conductivity) takes place in a very narrow composition range and is a consequence of the inhomogeneous morphology of these glasses (see Fig. 1) controlled by a large miscibility gap in the liquid phase of the Se-Ag2Se-GeSe2 system. Spinodal decomposition is observed upon solidification in a narrow composition range (0.08-0.10at.) outflanked by zones that present binodal decompositions. With the aim to correlate conductivity and morphology with structure, amorphous samples of compositions Agx(Ge0.25Se0.75)1-x  with 0≤x≤0.25 were analyzed by means of extended X-ray absorption fine structure (EXAFS) and Mössbauer effect (ME) spectroscopies. EXAFS analyses (Ge and Se K-edges) were performed at different temperatures and Mössbauer spectroscopy in transmission geometry with 119mSnO3Ca source was performed at 300°K on 119Sn atoms introduced as substitution impurity for Ge.  Ge K-edge EXAFS experiments show that Ge–Se distance in amorphous samples is very similar to that of the crystalline phase Ag8(Ge, Sn)Se6 except for sample with x=0.25. On the other hand Se K-edge experiments show similarities between the crystalline phase and glass with x=0.25 due to a common Ag-Se correlation. These features are corroborated by ME. Mössbauer spectra of glasses with x<0.25 are very similar and similar to the spectrum of Ag8(Ge, Sn)Se6. However, samples with x≥0.10 present evidences of Ag diffusion about the tetrahedral cage that encloses Sn (or Ge) atoms. On the other hand, Mössbauer spectrum of sample with x=0.25 diverts from this trend. Results are discussed and compared to results of structure analyses reported by other authors.