Oxide ion conductors have received much attention in recent years due to their application as solid oxide fuel cells (SOFC) electrolytes. A strong correlation exists between the oxide ion conductivity and the crystal structure of an oxide ion conductor. Consequently, to develop novel electrolytes it is important to discover new structural families of oxide ion conducting materials. In the present study, the electronic properties and crystal structure of the hexagonal perovskite derivative Ba3MoNbO8.5 are reported. Ionic transport number and variable oxygen partial pressure conductivity measurements evidenced that Ba3MoNbO8.5 presents solid oxide ion conduction. A bulk conductivity of 2.2 x 10-3 S cm-1 at 600 C was observed, which is comparable to other leading oxide ion conductors. Ba3MoNbO8.5 is the first hexagonal perovskite derivative to exhibit fast solid oxide ion conductivity. The Ba3MoNbO8.5 structure was described by a hybrid structural model composed by a superimposition of the 9R hexagonal perovskite and palmierite structures. (Mo/Nb)O4 units coexist with (Mo/Nb)O6 units within the structure, forming a disordered arrangement of Mo/Nb tetrahedra and octahedra. Variable temperature neutron diffraction experiments allowed determination of the structural factors at the basis of the oxide ion conduction. In particular, the flexible coordination of the Mo/Nb cations and the distortion of the Mo(1)/Nb(1) polyhedra are thought to enhance the electrical properties so that a conductivity comparable to other leading solid oxide ion conductors is observed. Study of the electrical and structural features of the Ba3Mo1 xNb1+xO8.5-x/2 (x = 0.10, 0.20, 0.30) series also evidenced that the relative ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra and the disorder of the oxygen sub lattice are important for the conduction of the Ba3MoNbO8.5 system. Impedance spectroscopy measurements on the hexagonal perovskite derivatives Ba7MoNb4O20 and Ba3WNbO8.5 showed evidence of ionic contributions in these systems. In addition, neutron diffraction experiments revealed that both Ba7MoNb4O20 and Ba3WNbO8.5 exhibit structural characteristics analogues to Ba3MoNbO8.5. The results of the present study indicate the prospect of designing new oxide ion conductors with mixed tetrahedral and octahedral d-metal units in the hexagonal perovskite family.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:701786 |
| Date | January 2016 |
| Creators | Fop, Sacha |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=231087 |
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