New fast-ion conductors from uranium halides - the UCl6/Na2UBr6 structures

A fast-ion conductor might be constructed from a stable regular lattice of large atoms in which holes can be partially filled by small ions without destroying the original structure. For example, large halide ions form a suitable basis, and a hexagonal close packed array of such ions can be stabilised by a six-coordinated ion such as U6+. In UCl6, the chlorines are in almost perfect close packing, with the small uranium atoms occupying only one sixth of the available octahedral holes. Some of these remaining holes can be filled with a monovalent cation such as Na+ without changing the structure; the uranium valence then changes from 6+ to 4+. These Na+ ions should then become available for ion transport, especially since the UCl6 octahedra are expected to rotate at higher temperature, thus dynamically unblocking the channels between empty octahedral holes. It is shown that the room temperature structure of Na2UBr6 conforms to these principles, which might explain the 652 and 663 K phase transitions at which the conductivity of the solid state exceeds that of the liquid state. (author)