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Abstract
Tellurite glasses have recently become of interest as one of the leading candidate materials for optical fiber telecommunication: TeO2 network provides wide band infrared transmittance, large refractive index, large third-order non-linear optical susceptibility, etc. Their origin physical propertiesis still unclear understaniding due to a lack data on their electronic structural. So, we used IR, X-ray photelctron spectroscopy (XPS) and NMR to investigate the electronic structure of binary xV2O5-(1-x)TeO2; x= 0.1-0.6 and ternary 0.5[xAg2O-(1-x)V2O5]-0.5TeO2; x=0.1-0.8 because these glasses have interesting electrical and optical properties and are easy to synthesize over a large composition. These classes of glasses were prepared by quenhing the melt. IR result does not give us the detail information about the structure because the cmplex electronic structure of these glasses. NMR result indicates it is very difficult to investigate the structure of these glasses due to the V-ions. While XPS is power full tool used to determines the qualitative and quantitative electronic structures of glasses through the bindind energy and distinguishes oxygen atoms in different structural units. On gradual replacement of TeO2 by V2O5 in binary vanadium tellurite the electron transferred from the vanadium to TeO4 trigonal bipyramids weakens the Te-Oax bond, and leads to the transformation of TeO4 through TeO3+1 polyhedra to TeO3 trigonal pyramids. The results based on the model of Sekiya and electronegativity indicate that the coordination number of tellurium changes from four to three with the addition V2O5. Oxygen (1s) spectrum indicates bridging oxygen atoms (Te-O-Te) and non-bridging oxygen atoms (Te=O, Te-Oax-V and Te-Oeq-V). The Te (3d5/2) spectrum shows the presence of tellurium atoms TeO4, TeO3+1, (TeO3)- and (Te2O5)2- structural units. The fraction of TeO4 decreases with increasing V2O5 content. At the same time the fraction of TeO3 increases. The fraction of TeO3+1 shows a maximum at 40 mol of V2O5. In the glasses 50 mol % V2O5, isolated (Te2O5)2- structural units start to form. The Te (3d5/2) XPS spectrum in ternary silver vanadium tellurite glasses shows the presence of various structural units of tellurium atoms like, TeO4, (TeO3)-, (TeO3)2-, TeO3+1, (Te2O5)2- and Te-Ag bond. A multivalent state for the V ions is indicated by an asymmetry and broadening in the V (2P3/2) spectra in vanadium tellurite and silver vanadium tellurite glasses.
In the second part of this thesis the ac conductivity and the capacity for xV2O5-(1-x)TeO2, 0.5[xAg2O-(1-x)V2O5]-0.5TeO2 and 0.5[xTiO2-(1-x)V2O5]-0.5TeO2 glasses with x=0.1-0.6, 0.1-0.8 and 0.1-0.6, repectively were measured at 10Hz to 100kHz from 4K to 425K. The dc data indicate that the conduction is electronic in vanadium tellurite and titanium vanadium tellurite glasses. While in the case of silver vandium tellurite glasses the conduction changes over from predominantly electronic to ionic mechanism for x=0.1-0.55 mol %. This change of conduction mechanism in silver vanadium tellurite glasses is due to the change in glass structure, which affects both electronic and ionic transport properties. The electronic dc conductivity of these three classes tellurite glasses has been analyzed in terms of non-adiabatic small polaron hopping of electrons between vanadium ions within the range of temperature RT-425K. The polaron band width 0.01 eV < WH / 3 ( 0.09) in the non-adiabatic region. While in the temperature rang 70K to RT, the dc conductivity data interpreted in terms of variable-range hopping model. Mott parameters analysis T1/4 gave the density states at the Fermi level, N(EF), 1020-1021 in vanadium tellurite glasses. While in the case of silver vanadium tellurite and titanium vanadium tellurite glasses, Mott parameters analysis T1/4 and also Greaves T1/4 gave the density states at the Fermi level, N(EF), 1022-1023 eV-1 cm-3. The values of the density of states at the Fermi level, N(EF) are normal values in vanadium tellurite but unusally large values in silver vanadium tellurite and titanium vanadium tellurite glasses. Thus the variable range-hopping model appears to be not valid for dc conductivity of silver vanadium tellurite and titanium vanadium tellurite glasses. So, we analysied dc conductivity in terms of Triberies and Friedman’s T1/4 formula at low temperature. The estimation of N(EF), 1019-1020 eV-1 cm-3, in silver vanadium tellurite and titanium vanadium tellurite glasses. While their values of hopping carrier mobility in order 10-7 to 10-9 cm2 V-1 s-1 and the carrier density in the order of 1019 cm-3 at 300K. The generalized polaron-hopping model of Schnakenberg can not be fitted dc conductivity of the silver vanadium tellurite and titanium vanadium tellurite glasses but the model of Triberies and Friedman’s T1/4 is the best fit model.