Influence of cation constriction on the ac conductivity dispersion in metaphosphate glasses

The ac conductivity resulting from ion motion in glasses displays a power-law frequency dependence characterized by an exponent n<1. Recently, it was suggested that this exponent depends upon the dimensionality of the local cation conduction space, such that n decreases with decreasing dimensionality. Here, I report measurements of the ac conductivity of two metaphosphate glass systems. The first are the superionic glasses formed by doping AgI into AgPO3. The second are the alkali-metal metaphosphate glasses, MPO3, where M = Li, Na, K, Rb, or Cs. In both glass systems, the conductivity exponent varies with expansion of the phosphate chains which comprise the glass network. In the AgI-doped glasses, n increases with increasing expansion of the network, whereas in the alkali-med series, n decreases with the expansion. However, when n is considered as a function of the "constriction" of the cation (i.e., the cation size relative to the chain separation), this exponent behaves similarly for both glass systems, decreasing with increasing constriction of the cation. This decrease is proposed to result from a reduction in the coordination of the cation's local conduction space caused by increased constriction.