Local atomic structure and electrical properties of Ge20Se80-xTex (x=0, 5, 10, and 15) glasses doped with Ho

Measurements of ac and dc conductivities, complex electrical modulus, static permittivity, dielectric relaxation, and X-ray diffraction of the glassy system Ge20Se80 Te-x(x) (x = 0, 5, 10, and 15), "pure'' and doped with 1000-2000 wt.-ppm Ho (added as metal or oxide), are presented and discussed. Influence of crystallization and/or phase separation on these properties is described. Temperature dependences of the dc conductivity are Arrhenius-like; their conduction activation energy increases (0.73-0.84 eV) and the dc conductivity decreases (1.5 x 10 (6) - 8 x 10 (8) S/m, at 60 degrees C) with decreasing concentration of Te. Doping with Ho3+ ions in a metallic form decreases the conduction activation energy. Relative static permittivity of glasses ranges from 9.6 to 12.8. The highest values are found in heavily doped, partly crystallized glasses. The lowest values are found in "pure'' glasses with a high content of Te. At 1000 wt.-ppm Ho, modular diagrams are almost semicircular, modular spectra are Debye-like, and their shape is independent on temperature. At 2000 wt.-ppm Ho, two relaxation processes appear and the shape of both modular diagrams and modular spectra depend on temperature. A partial crystallization takes place in these glasses. For heavily doped glasses (1500 and 2000 wt.-ppm Ho), XRD experiments, using high-energy photons, show distinct Bragg peaks stemming from a tiny fraction (about 0.25%) of crystalline phases. Crystalline component is rather homogeneously distributed within the sample. Changing level of Ho doping affects the short-distance arrangement in glasses. A higher level of Ho doping implies shortening of the inter-atomic distances, higher mean atomic density, and higher coordination numbers what suggests better atomic packing. (C) 2013 Elsevier B.V. All rights reserved.