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The thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH 4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the B O 4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO 4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH 4 TcO 4 and NH 4 ReO 4 is a consequence of differences in the H B O 4 interactions due to changes in the B O bond lengths within the B O 4 groups. Both ammonium salts decompose when heated above ∼500 K. Variable temperature synchrotron X-ray diffraction revels that the thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K is highly anisotropic, including negative thermal expansion along the c -axis. Whereas NH 4 ReO 4 exhibits negative volume thermal expansion over a narrow temperature range NH 4 TcO 4 does not exhibit negative volume thermal expansion. Both oxides have the tetragonal Scheelite type structure and the observed anomalous thermal expansion is associated with disorder in the orientation of the NH 4 groups and the presence of both equatorial and axial H-bonding to the B O 4 tetrahedral. Image 1 • Structures of NH 4 TcO 4 and NH 4 ReO 4 determined using synchrotron X-ray powder diffraction. • Thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K determined. • Anisotropic thermal expansion, including NTE, related to disorder of NH 4 groups. [ABSTRACT FROM AUTHOR]

The crystal structures of seven samples of orthorhombic ( Pnma ) Ln 2 TiO 5 compounds with Ln =La, Pr, Nd, Sm, Gd, Tb and Dy were refined by Rietveld analysis of synchrotron X-ray powder diffraction (S-XRD) data. With increasing size of the lanthanide cation, the lattice parameters increase systematically: c by only ~1.5% whereas both a and b by ~6% from Dy 2 TiO 5 to La 2 TiO 5 . The mean Ti–O bond length only increases by ~1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti–O 3 bonds to opposite corners of the Ti–O 5 square based pyramid polyhedra. For Dy 2 TiO 5 and Tb 2 TiO 5 , a significant variation in Ti–O 1 and Ti–O 4 bond lengths results in an increased deformation of the Ti–O 5 base. The particular configuration consists of large rhombic shaped tunnels and smaller triangular tunnels along the b axis, which have implications for defect formation and migration caused by radiation damage or the ionic conductivity. [ABSTRACT FROM AUTHOR]

A combination of S-XRD and NPD demonstrate the structure of Ba 2 Y 0.879 UO 6+x to be monoclinic in space group I 2/ m . That the U is hexavalent is evident from the U L 2 -edge XANES measurements. This appears to be a rare example of a double perovskite containing vacancies at the octahedral B -sites and interstitial oxygen defects, which combine to stabilise hexavalent U and appears to be a consequence of the preparation of the sample in air. The Y vacancies, coupled with anion disorder, results in a distortion of the B O 6 octahedra. [ABSTRACT FROM AUTHOR]

The structures of some A UO 4 ( A =Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L 3 -edge. The smaller Ca cation favours a rhombohedral A UO 4 structure with 8-coordinate UO 8 moieties whilst an orthorhombic structure based on UO 6 groups is found for BaUO 4 . Both the rhombohedral and orthorhombic structures can be stabilised for SrUO 4 . The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO 4 , is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. [ABSTRACT FROM AUTHOR]