Effect of processing method on physical properties of Nb2O5

Samples of Nb2O5 were prepared by laser floating zone (LFZ) technique and by solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. Single crystals fibres were obtained by LFZ, while a structural orthorhombic to monoclinic phase transition was observed in samples sintered at temperature higher than 800 °C. Transmission optical spectroscopy and photoconductivity measurements allowed identifying a ∼3.2 eV bandgap energy for the H-Nb2O5 monoclinic crystalline phase. Band gap shrinkage of ∼100 meV was observed from 14 K to RT. For the orthorhombic phase (T-Nb2O5), the photoconductivity measurements evidence a higher energy bandgap. The sintered samples have shown a broad recombination luminescence band at the orange/red spectral region while no luminescence was detected from the LFZ grown fibres. A dielectric constant of ∼40 was found for the 800 °C and 1200 °C sintered pellets while that of 1000 °C reached four times that value.