Rare‐Earth Niobates and TantaIates of Defect Fluorite‐ and Weberite‐Type Structures

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Abstract
By firing 3/1 mixtures of rare‐earth oxide (A23+O3) and niobia or tantala (B26+O5) at high temperatures, compounds of composition A33+‐B5+O7 may be prepared. The crystal structures of such compounds have been examined and two related types have been found. These are identified as a cubic defect fluorite or pyrochlore‐like type and one analogous to orthorhombic weberite. Lanthanum and neodymium oxides form compounds with the orthorhombic structure, whereas with samarium, gadolinium, dysprosium, yttrium, and scandium oxides the compounds are cubic. For a given rare earth the lattice parameters of compounds prepared with either niobia or tantala are substantially the same. The division between the weberite‐type and defect fluorite‐type structures appears to depend primarily on the radius of the rare‐earth cation, the cations of largest radii producing the deformed orthorhombic structure. The results are consistent with suggestions made by Aleshin and Roy in a recent discussion of the crystal chemistry of pyrochlore.

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