Preparation and Partial Characterization of Two New Hexavalent Uranium Metal Uranates

Abstract

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.

The Structure of BaSr4U3O14 and a Structural and Thermodynamic Characterization of Sr5U3O14

“…With the subgroup Pbca a successful model could be derived where the octahedron at 000 has a small additional tilt around the c axis. The subgroup Pbca is in agreement with the electron di!raction results of Sterns et al (8). Details of the re"nements are given in Table 1.…”

“…In the compounds above, the octahedra of the parent structure are rotated along a two-fold axis in the layer plane, destroying two symmetry planes, resulting in space group Cmca for opposite rotations in the alternate layer and an a ( parallel to the c axis of about 2.33. It is possible that this compound has the Cmca structure at high temperatures and that at a lower temperature a second-order transformation to Pbca occurs; twinning is then most likely, as has been reported by Sterns et al (8).…”

“…The perovskite-related compound Sr UO (2), in which uranium is in the pentavalent state, has been investigated by us previously, whereas the structures of the compounds with hexavalent uranium Sr UO (3,4), Sr UO (3,5), and both modi"cations of SrUO (6) and Sr U O (7) had been published before. Recently, Sterns et al, (8) reported on the existence of a new strontium uranate phase with the composition Sr U O with a chiolite-like structure, but the structural details could not be determined. In this paper the structure and thermodynamic properties of Sr U O are reported.…”

“…O in an alumina crucible in air at 1473 K with intermediate grindings for 3 days; according to the X-ray powder di!raction pattern, this sample was without any detectable impurities. Because the single-crystal structure determination of Sterns et al (8) was unsuccessful through microtwinning, neutron powder di!raction measurements were taken on the powder di!ractometer at the HFR at ECN, in Petten for structure determination. The experimental conditions were reported earlier (9).…”