Dielectric relaxation and ac conductivity of sodium tungsten phosphate glasses

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77-420 K. Complex relative permittivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (sigma(m)(omega)) has been found to be higher than ad, at low temperatures whereas at high temperature sigma(m)(omega) becomes equal to sigma(dc) at all frequencies. The ac conductivity obeys the relation sigma(ac)(omega) = Aomega(S) over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(E-F)) have been evaluated at 80 K assuming values of electron wave function decay constant a to be 0.5 (Angstrom)(-1). Values of N(EF) have the order 1020 which are well within the range suggested for localized states. Present values of N(EF) are smaller than those for tungsten phosphate glasses.