Most of the semiconducting oxide glasses described in the literature a r e based on the V 0 -P 0 system (1). Comparatively less information is obtainable about 2 5 2 5 the iron phosphate glasses.The conductivity process in such glasses is due t o the presence of metal ions of more than one valency state. According to Mott (2,3) the process is similar to the hopping conductivity in doped and compensated semiconductors.

The results of electrical surface conductivity measurements of the glass systems SiO2-PbO-Bi2O3 after hydrogen treatment are presented. The conductivity-temperature as well as conductivity-pressure characteristics proved that conduction is due to tunnelling of the electrons between lead crystallites. The comparison of the mean size of the lead crystallites studied by X-ray diffraction with the surface conductivity shows that bismuth ions play the role of resonant tunnelling centres.

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