AC Conductivity of Lead-Tungsten Phosphate Glasses

Abstract

AC conductivity of six samples of lead-tungsten phosphate glasses having compositions x mol% PbO-(60-x) mol% WO₃–40 mol% P₂O₅ has been studied at low frequencies in the temperature range 77 K to 300 K. Measured ac conductivity, σ_t(ω) at 1 kHz or at 10 kHz has been observed to decrease with the increase of tungsten ion concentration whereas for 60 mol% WO₃–40mol% P₂O₅ glass values of σ_t(ω) are larger than those for lead-tungsten phosphate glasses. The ac conductivity obeys σ_ac (ω) = A ω^s relation over a considerable range of low temperatures. At higher temperatures the ac conductivity starts deviating from linearity and the temperature at which the deviation from linearity starts, increases with increasing frequency. Values of s are slightly greater than unity for glass samples having lead ions concentration larger or equal to the tungsten ion concentration whereas for other samples the values of s are less than unity and the values decrease with the increase of tungsten ion concentration. Theoretical values of ‘s’ are evaluated by applying the hopping over barrier model (HOB). Values of density of states at the Fermi level for different compositions of glasses have the order 10¹⁹ (assuming α to 0.5A^-1), which suggest that the states are localized near the Fermi level.