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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]

Abstract: A series of single phase compounds with nominal stoichiometry Sm(x)Yb(2−x)TiO5 (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln 2TiO5 series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P63 /mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. [Copyright &y& Elsevier]

Abstract: Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ionscm−2 at a temperature of 50K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (T c ) of 647K for LaAlO3, 840K for La2Ti2O7, 865K for La2/3TiO3, and 1027K for La2TiO5. The T c values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between T c and melting temperature (T m ) in the two systems. More complex relationships exist between T c and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation. [ABSTRACT FROM AUTHOR]

Abstract: The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation–anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation–anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature. [Copyright &y& Elsevier]

Abstract: High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1−x)CaTiO3−xLa2/3TiO3. The structures at room temperature were determined according to composition: in Pbnm for 0⩽x⩽0.5, Ibmm for 0.50.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition–temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm→I4/mcm, Ibmm→I4/mcm, I4/mcm→Pm3¯m and Cmmm→P4/mmm. [Copyright &y& Elsevier]

Abstract: Garnets, A 3 B 2C3O12, are an important group of minerals and have potential uses in the safe immobilisation of high-level nuclear waste. They have been found naturally to incorporate Zr, Ti and Fe, three elements of interest in the safe storage of nuclear waste. Kimzeyite, Ca3(Zr,Ti)2(Si,Al,Fe)3O12, is a naturally occurring garnet that contains Zr in a high percentage∼30wt%. For such a material to be of potential immobilisation for nuclear waste the structure needs to be completely understood. Electron channelling studies have shown that the Zr/Ti cations are located on the Y-site, with the Al/Fe cations located on the Z-site. This work has investigated synthetic analogues of kimzeyite, Ca3(Zr,Hf)2(Al,Fe,Si)3O12, by neutron powder diffraction, using the C2 spectrometer at the Chalk River nuclear facility, coupled with 57Fe Mössbauer spectroscopy. Such work has allowed the structure of the synthetic material to be determined along with the distribution of cations across the X (CN=8), Y (CN=6), and Z (CN=4) sites. Results have shown that it is possible to synthesise Ca3(Zr,Hf)2(Al,Fe,Si)3O12 with a range of Al/Fe ratios containing Zr and Hf. The Mössbauer data has indicated the Fe is located on the Z site. The structural analyses show that the unit cell changes linearly as a function of composition, and analysis of the disorder indicates that the Zr, Hf reside on the Y site and the Al, Fe, and Si reside on the Z site. [Copyright &y& Elsevier]

The distribution of A-site cations in the perovskite system LaxSr1-3x/2TiO3 depends on the concentration of La3+ ions and associated vacancies. For small x (x⪝0.2), the substitutions are expected to be random. For x⩾0.55, the cations are ordered in such a way that successive layers of A-sites are occupied to greater and lesser degree, and this ordering drives a tetragonal distortion. For x from about 0.3 to 0.5, the X-ray patterns show diffuse peaks indicative of similar ordering, but this is not long-range order and no tetragonal distortion results. The lower temperature structures also exhibit out-of-phase tilting of the TiO6 octahedra, setting in at temperatures varying linearly with composition from 105 K for x=0, to about 650 K at x=2/3. [Copyright &y& Elsevier]

Abstract: Time-of-flight powder neutron diffraction has been performed on oxides with composition (La1− x Nd x )2Zr2O7 and Nd2(Zr1− x Ti x )2O7, where , 0.2, 0.4,…1.0, in order to determine the solid solution behaviour across each series. Between La2Zr2O7 and Nd2Zr2O7, a cubic pyrochlore phase is observed (, ). A linear decrease in the lattice parameter from 10.8047 to 10.6758Å indicates complete miscibility of the two end-members. For the same series, the oxygen x-parameter increases from 0.3313 to 0.3348, suggesting increased distortion of the 6 coordinate B sites and reduced distortion of the 8 coordinate sites. There is limited solubility of Nd2Ti2O7 in Nd2Zr2O7. Exsolution of a monoclinic phase (, ) rich in Nd2Ti2O7 is observed at approximately . The compositional range over which a solid solution exists is more extensive than that which has been previously reported. The solubility of Nd2Zr2O7 in Nd2Ti2O7 is very low. [Copyright &y& Elsevier]

With the coordination of dimethylformamide (DMF), two new uranium(VI) complexes with either 4-hydroxybenzoic acid (H 2 phb) or terephthalic acid (H 2 tph) have been synthesized under solvothermal conditions and structurally characterized. [(UO 2 ) 2 (Hphb) 2 (phb)(DMF)(H 2 O) 3 ]·4H 2 O ( 1 ) has a dinuclear structure constructed with both pentagonal and hexagonal bipyramidal uranium polyhedra linked through a µ 2 -bridging ligand via both chelating carboxylate arm and alcohol oxygen bonding, first observation of such a coordination mode of 4-hydroxybenzoate for 5 f ions. [(UO 2 )(tph)(DMF)] ( 2 ) has a three-dimensional (3D) framework built with pentagonal bipyramidal uranium polyhedra linked with µ 4 -terephthalate ligands. The 3D channeled structure is facilitated by the unique carboxylate bonding with nearly linear C–O–U angles and the coordination of DMF molecules. The presence of phb ligands in different coordination modes, uranyl ions in diverse environments and DMF in complex 1 , and tph ligand, DMF and uranyl ion in complex 2 has been confirmed by Raman spectroscopy. In addition, their thermal stability and photoluminescence properties have been investigated. [ABSTRACT FROM AUTHOR]

Three new coordination polymers of uranium(VI) with pyromellitic acid (H 4 btca) have been synthesized and structurally characterized. (ED)[(UO 2 )(btca)]·(DMSO)·3H 2 O ( 1 ) (ED=ethylenediammonium; DMSO=dimethylsulfoxide) has a lamellar structure with intercalation of ED and DMSO. (NH 4 ) 2 [(UO 2 ) 6 O 2 (OH) 6 (btca)]·~6H 2 O ( 2 ) has a 3D framework built from 7-fold coordinated uranyl trinuclear units and btca ligands with 1D diamond-shaped channels (~8.5 Å×~8.6 Å). [(UO 2 ) 2 (H 2 O)(btca)]·4H 2 O ( 3 ) has a 3D network constructed by two types of 7-fold coordinated uranium polyhedron. The unique μ 5 -coordination mode of btca in 3 enables the formation of 1D olive-shaped large channels (~4.5 Å×~19 Å). Vibrational modes, thermal stabilities and fluorescence properties have been investigated. [ABSTRACT FROM AUTHOR]

Four dysprosium (Dy) single carboxylates, a formate, a propionate, a butyrate and an oxalate have been synthesized and structurally characterized. The structure of Dy(HCO 2 ) 3 ( 1 ) contains nine-fold coordinated Dy polyhedra in perfect tricapped trigonal prisms. They are linked through trigonal O atoms forming 1D pillars which are further linked together through tricapped O atoms into a 3D pillared metal organic framework. The network structure is stable up to 360 °C. The structure of [Dy 2 (C 2 O 4 ) 3 (H 2 O) 6 ]·2.5H 2 O ( 2 ) contains nine-fold coordinated Dy polyhedra linking together through μ 2 -bridging oxalate anions into a 2D hexagonal layered structure. Both [Dy 2 (Pr) 6 (H 2 O) 4 ]·(HPr) 0.5 ( 3 ) [Pr=(C 2 H 5 CO 2 ) −1 ] and [Dy 2 (Bu) 6 (H 2 O) 4 ] ( 4 ) [Bu=(C 3 H 7 CO 2 ) −1 ] have similar di-nuclear structures. The Raman vibration modes of the complexes have been investigated. [ABSTRACT FROM AUTHOR]

Abstract: The crystal structures of the compounds La2− x Y x Zr2O7 and La2− x Y x Hf2O7 with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La0.8Y1.2Zr2O7 and La0.4Y1.6Hf2O7, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases. [Copyright &y& Elsevier]