Hopping conduction at high electric fields in transition metal ion glasses

The high-field electrical properties of several groups of transition-metal phosphate glasses have been examined, especially those based on V, Fe, Cu and Mo ions, and pure and doped tungstates. Conductivity measurements have been carried out under DC and pulse conditions at fields up to approximately 10 6 V cm -1 but below the fields required for switching. In contrast to the chalcogenide glasses, these materials show ohmic behaviour up to approximately 10 5 V cm -1 . At higher fields the conductivity increases by an order of magnitude. Small, but systematic deviations are observed at the highest fields and in some compositions. An expression is derived which corresponds to a simple field-dependent hopping mobility, but the experimental jump parameters are always some 4-6 times larger than the average transition metal ion spacing. Several models are discussed and it is shown that when a formula of this type is applied to a disordered system, the effective field ( epsilon ) may be enhanced, due to fluctuations in local site barriers.