Use this URL to cite or link to this record in EThOS:
Title: Structural, surface and electrochemical studies of LaNbO4 based ionic conductors
Author: Li, Cheng
ISNI:       0000 0004 6496 2604
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
Availability of Full Text:
Access from EThOS:
Access from Institution:
In the search for novel solid oxide fuel cell (SOFC) electrolytes, research focus has shifted from conventional materials with high symmetry fluorite and perovskite structures, to those structures with anisotropic conducting paths and defective local environment. Amongst the novel fast ion conductors are the hyperstoichiometric modulated CeNbO4+d phases, which have displayed remarkably oxygen diffusivity in the intermediate temperature region (500 °C to 750 °C). The oxidation of Ce3+ to Ce4+, however, leads to undesirable electronic conduction. In addition, the structure and conducting mechanism of the modulated fergusonite phases has been left unexplored due to the structural modulation. This work thus aims to investigate the crystal structure, phase evolution and transport properties of LaNb1-xWxO4+d, a structural analogue to CeNbO4.25. The hyperstoichiometric LaNb1-xWxO4+d phases (x = 0.08–0.16) adopt a (3 + 2)D modulated structure, with cation ordering at the Nb/W site. Using the superspace group I2/c(α1,0,γ1)00(α2,0,γ2)00, the room temperature crystal structure of LaNb0.88W0.12O4+d has been refined using the Rietveld method with powder synchrotron diffraction data. The refined structure model suggests extended B-site coordination numbers, ranging from 6 to 8 with a 3Å cut-off. The additional B-O bonds accommodate the variations in local environment and provide linkages between the NbOx slabs, forming a 3D network similar to that in CeNbO4.25. In situ neutron diffraction has revealed a sequence of structural transformations which has been overlooked in the literature. Compared with the undoped LaNbO4, the conductivity of LaNb1-xWxO4+d phases is much improved, reaching 8 × 10-3 S cm-1 at 863 °C for LaNb0.88W0.12O4+d. 18O tracer diffusivity measurements have confirmed oxygen ions being the major charge carriers, and an interstitial conducting mechanism has been proposed to explain the improved conductivity. Despite the huge interest of utilising LaNbO4 based materials for SOFC electrolyte application, their surface chemistry, which plays an important role in determining electrochemical performance, has been left unexplored. Surface chemistry of acceptor doped LaNbO4 is investigated using LEIS and SIMS. A Nb and Sr enriched surface layer, approximately 6–7 nm thick, has been identified after annealing La0.99Sr0.01NbO4-d at 1000 °C. Isotopic exchange experiments reveal a systematic decrease in isotopic surface concentration after the Sr enriched layer has developed at the surface, highlighting the impact of surface chemistry on the ion transport process.
Supervisor: Skinner, Stephen ; Vandeperre, Luc Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral