A study on GE₅O(PO₄)₆ : an oxide ion conductor

Abstract

The work in this thesis concerns the synthesis, structural, and electrical analysis of the oxide ion conductor Ge₅O(PO₄)₆ and related materials. The syntheses of the materials were performed using the traditional solid state method.

Variable temperature NPD was performed on Ge₅O(PO₄)₆ and the isostructural material, Si₅O(PO₄)₆. For Ge₅O(PO₄)₆ this gave an insight into the oxygen conduction mechanism that occurs within the material. NPD provided experimental evidence of the origin of the mobile oxide ion within the ‘excess’ oxygen structure and the local lattice distortions that mobilise and stabilise migrating oxide ions. Fourier difference calculations were also performed to determine the location of interstitial oxide ions between the temperature range of 300-1073 K. In addition to this, the data was compared and contrasted to the Si₅O(PO₄)₆ NPD data to determine structural nuances between the two materials. A comparison of bond lengths and angles also showed there were local differences in the isostructural materials. The structural studies suggest that there was the formation of highly distorted SiO₆ when compared to the more ideal octahedral geometry of the GeO₆ subunit within Ge₅O(PO₄)₆.

Related materials are synthesised by the doping of cations such as Si, Sn, Ga, Al and Ti onto the Ge site. A solid solution was established between the Ge and Si end members. This was evaluated by the changes in unit cell parameters with varying Si:Ge ratios. Sn-doped materials were also evaluated. An increase in unit cell size for the Sn-doped materials suggested that tin was successfully entering the structure. Electrical measurements were also performed. It showed that Ge₅O(PO₄)₆ has a low activation energy for oxide ion conduction. Whilst the dopants of the Ga, Al and Ti doped compositions did not significantly improve oxide ion conducting properties, they gave an insight into the structural changes that effect oxide ion conductivity.