Properties and structure of semiconducting sodium iron germanoborate glasses

Several properties of semiconducting Na₂B₄O₇–GeO₂–Fe₂O₃ glasses have been measured. These properties include Mössbauer, density, d.c. conductivity, thermal expansion coefficient, glass transition temperature (T _g), and softening temperature (T _s). The d.c. conductivity was found to decrease as the iron content increases while the activation energy increases with increasing iron content in the glasses. From the conductivity temperature relation, it was found that the small polaron hopping model was applicable at temperature above _D/2 (_D: the Debye temperature); and the electrical conduction at T>_D/2 was due to non-adiabatic small polaron hopping of electrons between iron ions. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. Dilatometric measurements indicated that the thermal expansion coefficient decreases with the increase of iron content whereas both of the (T _g) and the (T _s) increase with increasing iron content. Different behaviors were observed in the Fourier transform infrared (FTIR) absorption spectra as a result of the progressive replacement of GeO₂ by Fe₂O₃. In the case of 20 and 25 mol % GeO₂, simultaneous existence of GeO₄] and GeO₆] polyhedra can be observed in the glass network. Below 20 mol % GeO₂, all germanium participating into the glass network is tetrahedrally coordinated, that is GeO₄]. It is also indicated that, with the increase of iron content, boron attains an enhanced state for its coordination number to increase from 3 to 4, and consequently the tetrahedral BO₄] units increase at the expense of the trigonal BO₃] units, indicating the increase of the bridging character. The results of the measured properties were correlated with those from the FTIR spectroscopy.