Defect chemical investigations on the LaNbO4-LaTaO4 system are, except for investigations on the parent phases, LaNbO4 and LaTaO4, non-existent. Research on this system is required to understand the impact of tantalum-doping on the conductivity of LaNbO4. If Ta-doping is found to increase the conductivity of LaNbO4, it could, in turn, open interesting, new avenues relating to doping and solid solubility in solid oxide fuel cell materials research.
In the work presented in this thesis, defect situations and conductivities for Ta-doped LaNbO4 were investigated as a function of Ta-content. Electrical characterization of several samples, containing concentrations of between 38 and 48 atomic-percent Tantalum, were carried through. Measurements were done in reducing and oxidizing atmospheres in a temperature range from 500°C to 1200°C.
The phase transition between low-temperature monoclinic and high-temperature tetragonal LaNbO4 was seen to increase, with increasing tantalum content, to a limit of ~850°C with 48 atomic-percent Ta. This was also shown to be the highest Ta-doping attainable under present experimental conditions.
Results showed that the defect model of pure LaNbO4 fit that of Ta-doped LaNbO4 rather well, with only a few alterations: Under wet, reducing conditions, proton conductivity dominated at any temperature. Under oxidizing conditions the low-temperature phase displayed decent proton conductivity. This changed over to oxygen ionic conductivity and later p-type electronic conductivity as temperature increased and the high-temperature phase was reached. It is possible that an observed increase in conductivity under reducing conditions, compared to oxidizing conditions, was due to a reduction of the samples.
It was seen that none of the Ta-containing LaNbO4-samples had conductivities exceeding that of its parent pure Ca-doped LaNbO4-phase. The highest proton conductivity measured was 4.5•10-4 S•cm-1 for 48 atomic-percent Ta-doped LaNbO4 at 1100°C.
XRD measurements on the sample containing 48 atomic-percent Ta showed three new phases in addition to LaNbO4; La0.33TaO3 (tetragonal), La(Ta3O9) (orthorhombic) and LaTaO4. The new phases did not appear to impact conductivity much, if anything.