A detailed transmission electron microscope and X-ray powder diffraction study has been made of the so-called 'defect fluorite' phase field in the ZrO2-PrO1.5 system and of its close relationship to the pyrochlore solid solution field in the same system. Even for the lowest possible PrO1.5 content within the 'defect fluorite' phase field, it is clear that the sharp Bragg reflections characteristic of the underlying fluorite average structure are accompanied by some of the 'satellite reflections' characteristic of the pyrochlore solid solution field. As the PrO1.5 content increases, these satellite reflections increase systematically in intensity as well as sharpening very considerably. It is shown that this 'defect fluorite' phase field cannot be adequately described either in terms of random point defects within an average fluorite-type matrix or in terms of a diphasic texture of pyrochlore domains embedded coherently into a fluorite matrix, but must be regarded as enuinely intermediate between these two end-member structures and of commensurately modulated fluorite type. A group theoretical approach is used to propose a model for the structural deviation from the underlying fluorite average structure.

The preparation, chemical characterization and structural investigation by X-ray and electron diffraction of two uranates (VI), namely Sr5U3O14 and of Pb11U5O26, are reported. The strontium compound, the unit cell of which is metrically tetragonal with a = 11.248(5), c = 11.086(5) Ǻ, Z = 4, was shown by electron diffraction to be orthorhombic, space group Pbca with a = b. Crystals of Sr5U3O14 are invariably submicroscopically twinned and, because of the identity of a and b, mimic tetragonal symmetry. The lead uranate , Pb11U5O26, was shown to be a long-period superstructure with unit cell dimensions a = 44.54, b = 15.586, c = 8.241 Ǻ, Z = 8.

Zr108N98F138 is an x = 2.185 member of a wide range non- stoichiometric solid solution Zr (N,O,F)x, 2.12 ≤ x ≤ 2.25, in the zirconium nitride oxide fluoride system. The structure is rerefined as a composite modulated structure composed of two mutually incommensurable Q and H substructures with overall superspace group symmetry Acmm (0,0,1.185..)0s0. The Q substructure has lattice parameters a = 5.186, b = 5.368 and cQ = 5.374 Ǻ, and substructure superspace group symmetry Acmm (0,0,1.185..)0s0. The H substructure has lattice parameters a = 5.186, bH = 1/2b = 2.684 and CH = 4.534 Ǻ, and substructure superspace group symmetry Pmcm (0,±,0.844...)s00. While employing almost an order of magnitude fewer positional parameters than the previous study, refinement on 956 unique reflections converged to R = 0.057 instead of R = 0.113.