Electrical and optical properties of vanadium tellurite glasses
Flynn, Brian William
Glasses in the system v₂o₅-Te0₂ were prepared at intervals throughout the glass-forming composition range. The vanadium valence state was varied for each composition by the addition of elemental tellurium to reduce the melt. A chemical analysis method was devised to determine the v⁴⁺:vtotal ratio. Conductivity and thermopower measurements were made as functions of composition, valence ratio and temperature. The conductivity was found to increase as the total vanadium content increased and to be 21/2 to 3 orders of magnitude greater than that of a v₂0₅-P₂0₅ glass of similar v₂o₅ content. A maximum in conductivity was observed as the vanadium valence ratio was varied. This maximum was shifted to values around v⁴⁺:vtotal = 0.2 from the theoretically predicted value of 0.5. At temperatures above 250 K the conductivity varied exponentially with temperature with activation energies in the range 0.25 to 0.40 eV. Below this temperature there was a continuous decrease in slope of the conductivity down to the lowest temperature at which measurements were made. Thermopower was found to be negative and temperature independent above 250 K, while below this temperature it decreased in an activated manner with an activation energy of 0.02 eV. As a function of valence ratio the thermopower followed the behaviour predicted by Heikes and Ure, with the exception that the change of sign occurred at v⁴⁺:vtot ratios of around 0.2, instead of the theoretically predicted value of 0.5. (iii) Measurements were made of the optical absorption coefficient as functions of photon energy, composition, valence ratio and temperature. Temperature and composition did not affect the absorption spectrum significantly except that an absorption band appeared below the fundamental absorption edge as the glasses were reduced. The high electric field and a.c. properties of the glasses were also measured. These are consistent with hopping conduction of carriers between v⁴⁺ and v⁵⁺ sites. This data gives indirect indications of an inhomogeneous glass structure.