Showing papers by "Brendan J. Kennedy published in 2006"
TL;DR: As the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphization is more easily attained, consistent with an increasingly exothermic trend in the enthalpies of formation of pyroChlores from the oxides.
Abstract: The lanthanide stannates, Ln2Sn2O7, Ln = La−Lu and Y, have the isometric pyrochlore structure, A2B2O7, and their structural properties have been refined by Rietveld analysis of powder neutron and s...
169 citations
TL;DR: High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba(2)Bi(3+)Bi(5+)O(6) (dibarium dibarium bismuth antimony hexaoxide) and Ba(1)/n to determine the low-temperature monoclinic structure, which is closer to second order in nature than in nature.
Abstract: High-resolution neutron powder diffraction has been used to investigate the structures and phase transitions in the double perovskites Ba2Bi3+Bi5+O6 (dibarium dibismuth hexaoxide) and Ba2BiSbO6 (dibarium bismuth antimony hexaoxide) in the temperature ranges 42–973 and 42–625 K, respectively The charge-ordered bismuthate adopts four structures in the temperature range – monoclinic in P21/n, monoclinic in I2/m, rhombohedral in R\bar 3, and finally cubic in Fm\bar 3 m The low-temperature monoclinic structure has been determined for the first time The transitions from P21/n to I2/m, at 132 K, and R\bar 3 to Fm\bar 3m, at 820 K, are tricritical in nature; the transition from I2/m to R\bar 3 at ca 430 K is discontinuous The behaviour of Ba2BiSbO6 is very similar, except that the transition temperatures are lower – 250 K for I2/m to R\bar 3 and 515 K for R\bar 3 to Fm\bar 3m – and the low-temperature structure is not formed at all The R\bar 3 to Fm\bar 3 m transition in this compound is closer to second order in nature, although there is evidence for some contribution from higher-order terms
100 citations
TL;DR: A methodology where the spatial distribution of lead in the dentine of human primary teeth may be used to obtain temporal information of environmental lead exposure during the pre- and neonatal periods is demonstrated.
73 citations
TL;DR: In this article, the structure of CaMnO3 has been studied from room temperature to 800°C by high-resolution synchrotron X-ray powder diffraction.
53 citations
TL;DR: The perovskite Sr2MnSbO6 has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction as mentioned in this paper.
35 citations
TL;DR: In this paper, the authors used electron diffraction to investigate the large amplitude displacive disorder characteristic of Bi 2 (M III Nb V )O 7 Bi-pyrochlores, Bi 2 InNbO 7 and Bi 2 FeNb O 7, as well as of their A site substituted Bi 1.5 Y 0.5
29 citations
TL;DR: In this paper, high-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB2O4 (A=Ca2+, Sr2+ and Ba2+, and B=Al3+ and Ga3+).
27 citations
TL;DR: The orthorhombic-tetragonal phase transition in the perovskite series Sr1−xCaxMnO3 0.6 has been studied by synchrotron X-ray powder diffraction.
23 citations
TL;DR: The crystal structures of the end members of the series were refined using powder neutron diffraction data and are cubic in Fm 3 ¯ m and monoclinic in P 2 1 / n, respectively, in both cases there being considerable anti-site Cr-Nb mixing as mentioned in this paper.
11 citations
5 citations
TL;DR: The structure of the non-stoichiometric pyrochlore NaW2O6·nH2O has been refined from powder neutron diffraction data using the Rietveld method as mentioned in this paper.
Abstract: The structure of the non-stoichiometric pyrochlore NaW2O6·nH2O has been refined from powder neutron diffraction data using the Rietveld method. The stoichiometry found from the structural refinements is Na0.85(2)W2.02(2)O6·0.92(5)H2O in good agreement with other analytical studies. The Na cation is seven-coordinated bonding to six framework oxygen atoms and one oxygen atom from the water. The refinements show a disordered H2O molecule to be present with the O(2) on a 32e site and the H(1) 48f site.
TL;DR: In this article, the symmetry increases with the Sr content as a consequence of the gradual reduction, and ultimately removal, of the in-phase tilting of the BO 6 octahedra.
Abstract: Neutron powder diffraction data has been used to determine precise structures for four oxides in the series Ca 2− x Sr x CoWO 6 ( x =1, 1.5, 1.7, 2). The studies demonstrated that the symmetry increases with Sr content as a consequence of the gradual reduction, and ultimately removal, of the in-phase tilting of the BO 6 octahedra. The sample of Sr 2 CoWO 6 was a mixture of monoclinic (P2 1 /n) and tetragonal ( I 4/ m ) phases at room temperature but was purely monoclinic at 20 K.
TL;DR: In this article, the structure of two ordered double perovskites A 2 CrTaO 6, A=Sr, Ca was studied using Neutron powder diffraction and it was shown that replacing Sr as the A-type cation introduced both in-phase and out-of-phase tilting of the octahedra and the structure changed from cubic in Fm3m with a =7.8910(2)
Abstract: Neutron powder diffraction was used to study the structure of the two ordered double perovskites A 2 CrTaO 6 , A=Sr, Ca. It was shown that replacing Sr as the A-type cation introduced both in-phase and out-of-phase tilting of the octahedra and the structure changed from cubic in Fm3m with a =7.8910(2) A to monoclinic in P2 1 /n with a =5.4283(1) A, b =5.4904(10) A, c =7.7137(2) A and β =89.99(1)°.
TL;DR: In this paper, the technique of standard addition in combination with powder X-ray diffraction was used to identify and quantify the amount of Ln3+ segregating into secondary phases from Ln 3+ doped alkaline earth aluminates.
Abstract: The technique of standard addition in combination with powder X-ray diffraction was used to identify and quantify the amount of Ln3+ segregating into secondary phases from Ln3+ doped alkaline earth aluminates. Results indicate that Ln3+ ions are more soluble in Ca Al2 O4 than Sr Al2 O4 and Ba Al2 O4, with this being rationalized by the structural details of the A sites. These results indicate that the enhancement of the luminescence afterglow obtained by doping A Al2 O4: Eu2+ samples with Ln3+ ions is a result of much lower doping levels than previously thought. © International Centre for Diffraction Data.
TL;DR: In this paper, powder neutron diffraction measurements were carried out for the pyrochlore-type oxides Bi 2− x Nd x Ru 2 O 7-d, x = 0.625 and 1.375, from T = 6 K to RT in order to probe the structural changes that occur on going from the metallic oxides.
Abstract: Powder neutron diffraction measurements were carried out for the pyrochlore-type oxides Bi 2− x Nd x Ru 2 O 7-d , x =0.625 and 1.375, from T =6 K to RT in order to probe the structural changes that occur on going from the metallic oxides, x x >0.75. There is no evidence of any structural phase transition in either sample. The Rietveld refinements for Bi 0.625 Nd 1.375 Ru 2 O 7-d and Bi 1.375 Nd 0.625 Ru 2 O 7-d reveal that both the Ru–O bond length and Ru–O–Ru bond angle vary continuously with temperature. In both oxides there was evidence for static disorder of the Bi cations. The study gave further evidence on the requirements of a metallic state in ruthenate pyrochlore, i.e. Ru–O–Ru angle is greater than 132.5° and that there is an appreciable amount of vacancies on the oxygen site of the Bi 2 O sublattice.
TL;DR: Several double perovskite compounds of the type Ba 2 NdB′O 6− δ (B′=Sn 4+, Nb 5+ and Sb 5+) were synthesized in this paper.
Abstract: Several double perovskite compounds of the type Ba 2 NdB′O 6− δ (B′=Sn 4+ , Nb 5+ and Sb 5+ ) were synthesised. Room temperature neutron diffraction patterns were obtained in order to determine the precise structure of these compounds. It was found that monoclinic Ba 2 NdSnO 5.5 and Ba 2 NdNbO 6 and rhombohedral Ba 2 NdSbO 6 all featured only out-of-phase tilting. The structures of each of these three compounds revealed consistent overbonding of the Nd 3+ cations most likely as a result of the significant size difference between the Nd 3+ ions and B′ cations.
TL;DR: The perovskite Sr2MnSbO6 has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction as discussed by the authors.
Abstract: The perovskite Sr2MnSbO6 has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I 4 / m . The octahedra were found to feature Jahn–Teller (JT) distortion due to the presence of Mn3+, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn3+ and Sb5+ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb5+ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.