Ionic size limits for a ions in brannerite (ATi{sub}O{sub}6) and pyrochlore (CaATi{sub}2O{sub}7) Titanate structures (A=tetravalent rare earths and actinides)

摘要

The lower limit of the size of the octahedral A{sup}(4+) ion in the ATi{sub}2O{sub}6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U{sup}(5+) ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi{sub}2O{sub}6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi{sub}2O{sub}6 showed that the average B site size could tolerate deviations of < 1%. Ce{sup}(4+) is unusually stabilised in air atmospheres at temperatures close to the melting point of 140°C in the A site of brannerite. Lattice parameter data on different endmember ATi2O brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi{sub}2O{sub}7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO{sub}6, whose structure is still uncertain, plus brannerite and rutile.