Browsing by Author "Saines, PJ"
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- ItemCrystal structures and phase transitions in a-site deficient perovskites Ln1/3TaO3(American Chemical Society, 2008-11-11) Zhou, QD; Saines, PJ; Sharma, N; Ting, J; Kennedy, BJ; Zhang, Z; Withers, RL; Wallwork, KSThe synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating. © 2008, American Chemical Society
- ItemDetailed investigations of phase transitions and magnetic structure in Fe(iii), Mn(ii), Co(ii) and Ni(ii) 3,4,5-trihydroxybenzoate (gallate) dihydrates by neutron and x-ray diffraction(Royal Society of Chemistry, 2011-06-28) Saines, PJ; Yeung, HHM; Hester, JR; Lennie, AR; Cheetham, AKThe effect of cation valency on the complex structures of divalent and trivalent transition metal gallates has been examined using a combination of neutron and synchrotron X-ray powder diffraction, single-crystal X-ray diffraction and XANES spectroscopy. In the divalent frameworks, M(C7H4O5)[middle dot]2H2O (M = Mn, Co and Ni), it was found that charge balance was achieved via the presence of protons on the meta-hydroxyl groups. It was also established that these compounds undergo a discontinuous phase transition at lower temperatures, which is driven by the position of the extra-framework water molecules in these materials. By contrast, in the trivalent Fe gallate, Fe(C7H3O5)[middle dot]2H2O, it was found that the stronger bonding between the meta-hydroxy oxygen and the cations leads to a weakening of the bond between this oxygen and its proton. This is turn is thought to lead to stronger hydrogen bonding with the extra-framework water. The lattice water is disordered in the Fe(iii) case, which prevents the phase transition found in the M(ii) gallates. Refinement against the neutron diffraction patterns also revealed that the relatively mild microwave synthesis of gallate frameworks in D2O led to an extensive deuteration of the ortho-hydrogen sites on the aromatic ring, which may suggest a more versatile method of deuterating aromatic organics. The antiferromagnetic structure of Co gallate has also been determined. © 2011, Royal Society of Chemistry
- ItemDiffuse scattering in the cesium pyrochlore CsTi0.5W1.5O6(Elsevier, 2008-04-01) Thorogood, GJ; Saines, PJ; Kennedy, BJ; Withers, RL; Elcombe, MMThe structure of the defect pyrochlore CsTi0.5W1.5O6 has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 angstrom with displacive disorder of the Cs cations along the < 1 1 1 > direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the < 1 1 0 > directions. The thermal expansion of the material is also described. © 2008, Elsevier Ltd.
- ItemNeutron diffraction study of the magnetic structures of manganese succinate Mn(C4H4O4): a complex inorganic-organic framework(American Physical Society, 2010-10-25) Saines, PJ; Hester, JR; Cheetham, AKThe antiferromagnetic structures of the Mn succinate framework, Mn(C4H4O4), have been determined using neutron diffraction. The structure comprises alternating layers containing chains of edge-sharing Mn(II)O6 octahedra and sheets of corner-sharing Mn(II)O6 octahedra, respectively, with a layer separation of ∼7.5 Å. At 10 K the edge-sharing MnO6 octahedral chains order antiferromagnetically into a collinear sinusoidal spin structure with a propagation vector k2=(0,−0.5225,0), in which individual edge-sharing MnO6 chains are ferromagnetically ordered. The sheets of corner-sharing MnO6 octahedra order magnetically at 6 K, adopting the antiferromagnetic structure expected for a square arrangement of cations with a propagation vector k8=(−1,0,1). The ordering of these sheets at a lower temperature than the chains is consistent with their longer nearest-neighbor superexchange pathway. The magnetic structure of the edge-sharing layers is unaffected by the 6 K phase transition, indicating that the orderings of the two different layers are essentially independent of each other. © 2010, American Physical Society
- ItemNew quenched-in fluorite-type materials in the Bi2O3-La2O3-PbO system: synthesis and complex phase behaviour up to 750 degrees C(Pergamon-Elsevier Science Ltd, 2011-04-01) Webster, NAS; Hartlieb, KJ; Saines, PJ; Ling, CD; Lincoln, FJNew quenched-in fluorite-type materials with composition (BiO1.5)(0.94-x)(LaO1.5)(0.06)(PbO)(x), x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 degrees C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO1.5)(0.92)(LaO1.5)(0.06)(PbO)(0.02) revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a beta-Bi2O3-type tetragonal phase around 400 degrees C. In the range 450-700 degrees C, alpha-Bi2O3-type monoclinic, Bi12PbO19-type bcc and beta(1)/beta(2)-type rhombohedral phases, and what appeared to be a epsilon-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 degrees C. (C) 2010 Elsevier Ltd.
- ItemPhase and valence transitions in Ba(2)LnSn(x)Sb(1-x)O(6-delta) (Ln = Pr and Tb)(Elsevier, 2008-11) Saines, PJ; Kennedy, BJ; Elcombe, MM; Harris, HH; Jang, LY; Zhang, ZCompounds in the double perovskites series Ba(2)LnSn(x)Sb(1-x)O(6-delta) (Ln = Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba2PrSnxSb1-xO6-delta series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr3+ to Pr4+ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba2TbSbO6 and Ba2TbSnO6-delta are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb3+ to Tb4+. Multiple linear regression analysis of the Pr and Tb L-III-edge XANES indicates that the rate of Ln(3+) transforming to Ln(4+) is such that there are no oxygen vacancies in Ba2PrSnxSb1-xO6-delta but in Ba2TbSnxSb1-xO6-delta there is a small amount of oxygen vacancies, with a maximum of delta approximate to 0.05 present. © 2008, Elsevier Ltd.
- ItemStructural and electronic phase transitions in Sr1-xCexMnO3 perovskites(Elsevier, 2008-10-01) Kennedy, BJ; Saines, PJ; Zhou, QD; Zhang, Z; Matsuda, M; Miyake, MThe structures of eight members of the series Sr1-xCexMnO3 with 0.075≤x≤0.4 have been established using synchrotron X-ray powder diffraction. Unusual peak broadening due to domainwall scattering is observed near the composition-induced first-order cubic–tetragonal transition. The temperature dependence of the structures for three examples x = 0.1, 0.2 and 0.35 is described. For the x = 0.1 and 0.35 samples evidence for a first-order loss of the orbital ordering associated with a cooperative Jahn–Teller distortion is presented. The onset of the orbitally disordered state is accompanied by a discontinuous reduction in cell volume. © 2008, Elsevier Ltd.
- ItemStructures and crystal chemistry of the double perovskites Ba(2)LnB'O6 (Ln = lanthanide B'=Nb5+ and Ta5+), Part I - investigation of Ba(2)LnTaO(6) using synchrotron x-ray and neutron powder diffraction(Elsevier, 2007-11) Saines, PJ; Spencer, JR; Kennedy, BJ; Avdeev, MThe structure of 14 compounds in the series Ba(2)LnTaO(6) have been examined using synchrotron X-ray diffraction and found to undergo a sequence of phase transitions from I2/m monoclinic to I4/m tetragonal to Fm (3) over barm cubic symmetry with decreasing ionic radii of the lanthanides. Ba2LaTaO6 is an exception to this with variable temperature neutron diffraction being used to establish that the full series of phases adopted over the range of 15-500 K is P2(1)/n monoclinic to I2/m monoclinic to R (3) over bar rhombohedral. The chemical environments of these compounds have also been investigated and the overbonding to the lanthanide cations is due to the unusually large size for the B-site in these perovskites. © 2007, Elsevier Ltd.