A study of the electrical and optical properties of cobalt-phosphate glasses
Binary series CoO-P205 and ternary series CoO-NiO-P205 glasses are prepared. The density, d.c. conductivity, highfield I-V characteristicsr memory switching actiong a.c. electrical conductivity, infra-red absorption and optical absorption edge of the glasses are presented. The historical background and the formation and theory of the glassy state are reviewed. Previous works on the electrical conductivity of phosphate glasses are reviewed in the framework of Mott's theory. In the binary glasses, the glass acquires a more compact structure with increased CoO content whereas for the ternary series the results show otherwise. A polaronic model is shown to be generally applicable to explaining the results of electrical conduction measurements. The pre-exponential factor containing the term exp (-2aR) arising from electron tunnelling should not be ignored; thus the theory of the small polaron hopping is in the non-adiabatic regime. The high-field ohmic behaviour is observed up to a field of about 3x 104 V-cm-l'. Apparentlyt the field dependence is a property of the bulk material. Thin films of glass with 30 Mol % CoO show memory switching phenomena. The results of a.c. measurements characterize hopping conduction as a dominant process in cobalt-phosphate glasses. Results of the infra-red spectra measured in the range 400 cm-1 to 4000 cm-1 show that the phosphate polyhedra dominate the structure of the glasses. Measurements of the optical gap in the binary series show decreasing values with increased CoO content. It is believed that the fundamental absorption arises from indirect transitions.