Browsing by Author "Knight, KS"
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- Item(Ca0.37Sr0.63)TiO3 perovskite - an example of an unusual class of tilted perovskites(Institute of Physics, 2008-04-02) Howard, CJ; Withers, RL; Knight, KS; Zhang, ZThe structure of Ca0.37Sr0.63TiO3 perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]∗) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]∗). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the T line of symmetry (k = [0, 0, ξ]∗). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous k = (1/2, 1/2, ξ)∗ modulation wavevectors for a value of ξ different from 0 and 1/2. © 2008, Institute of Physics
- ItemCrystal structure of Ln(1/3)NbO(3) (Ln = Nd, Pr) and phase transition in Nd1/3NbO3(Elsevier, 2007-06) Zhang, Z; Howard, CJ; Kennedy, BJ; Knight, KS; Zhou, QDThe crystal structure of the A-site deficient perovskite Ln1/3NbO3 (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd1/3NbO3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650°C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature. © 2007, Elsevier Ltd.
- ItemGiant magnetoelastic effect at the opening of a spin-gap in Ba3BiIr2O9(American Chemical Society, 2012-01-26) Miiller, W; Avdeev, M; Zhou, Q; Kennedy, BJ; Sharma, N; Kutteh, R; Kearley, GJ; Schmid, S; Knight, KS; Blanchard, PER; Ling, CDAs compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba3BiIr2O9, which contains 5d Ir4+ (S = 1/2) dimerized into isolated face-sharing Ir2O9 bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at T* = 74 K. The resulting first-order transition is characterized by a remarkable 4% increase in Ir–Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature). © 2012 American Chemical Society
- ItemImpact of Jahn-Teller active Mn3+ on strain effects and phase transitions in Sr0.65Pr0.35MnO3(American Physical Society, 2012-03-19) Tan, TY; Kennedy, BJ; Zhou, QD; Ling, CD; Miiller, W; Howard, CJ; Carpenter, MA; Knight, KSThe mixed-valence manganite Sr(0.65)Pr(0.35)MnO(3) has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO(6) octahedra about the c axis and a large Jahn-Teller-type distortion due to the presence of Mn(3+). The sample exhibits a reversible phase transition from the cubic Pm3m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm (3) over barm <-> I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn-Teller-type distortion of the MnO(6) octahedra, exhibits an unusual e(0.5)(t)alpha(T(c)-T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature T(N) of 250 K. TheMnmagnetic moment at 7 K is 2.99(2) mu(B)/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting. © 2012, American Physical Society
- ItemInfluence of cation ordering on octahedral tilting in perovskites(The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Zhang, Z; Howard, CJ; Carpenter, MA; Knight, KSThe compound LaO.6SrO.1Ti03, taken to represent the ideal Lag/3TiO3, adopts a structure in which the A-site layers are alternately fully and partly occupied by the La(Sr)ions. At room temperature the structure also shows a tilting of the TiO5 octahedra around a single axis perpendicular to the layering direction. Quenching the material during preparation can produce a variant in which the cations are disordered on the A-site, offering the opportunity to study the effect of cation ordering on octahedral tilting. Slow-cooled and quenched specimens were studied over a range of temperatures using high-resolution neutron powder diffraction. Inspection of the relevant diffraction peaks confirmed the expected difference in cation ordering. At room temperature, the cell volume was 0.1% less in the ordered sample than in the disordered one. The extent of octahedral tilting was much the same in the two samples, with similar transition temperatures for removal of the tilting. The orthorhombic distortion that accompanied this tilting in the ordered sample was, surprisingly, not evident in the disordered sample——that is the effect of disordering the cations seems to switch off the strain (tilt) order parameter coupling in this perovskite. © 2005 The Authors
- ItemStructure, crystal chemistry and thermal evolution of the δ-Bi2O3-related phase Bi9ReO17(Elsevier, 2009-09) Sharma, N; Withers, RL; Knight, KS; Ling, CDThe thermal evolution and structural properties of fluorite-related δ-Bi2O3-type Bi9ReO17 were studied with variable temperature neutron powder diffraction, synchrotron X-ray powder diffraction and electron diffraction. The thermodynamically stable room-temperature crystal structure is monoclinic P21/c, a=9.89917(5), b=19.70356(10), c=11.61597(6) Å, β=125.302(2)° (Rp=3.51%, wRp=3.60%) and features clusters of ReO4 tetrahedra embedded in a distorted Bi–O fluorite-like network. This phase is stable up to 725°C whereupon it transforms to a disordered δ-Bi2O3-like phase, which was modeled with δ-Bi2O3 in cubic Fm3¯m with a=5.7809(1) Å (Rp=2.49%, wRp=2.44%) at 750°C. Quenching from above 725°C leads to a different phase, the structure of which has not been solved but appears on the basis of spectroscopic evidence to contain both ReO4 tetrahedra and ReO6 octahedra. © 2009, Elsevier Ltd.
- ItemStructures and phase diagram for the system CaTiO3-La2/3TiO3(Elsevier, 2007-03) Zhang, ZM; Lumpkin, GR; Howard, CJ; Knight, KS; Whittle, KR; Osaka, KHigh-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. © 2007, Elsevier Ltd.
- ItemSuppression of strain coupling in perovskite La0.6Sr0.1TiO3 by cation disorder(American Physical Society, 2007-08) Howard, CJ; Zhang, Z; Carpenter, MA; Knight, KSAn octahedral tilting transition has been investigated in two samples of La0.6Sr0.1TiO3 by powder neutron diffraction. One (slowly cooled) sample had a degree of cation/vacancy ordering on the A sites and was tetragonal, P4/mmm, at high temperatures. The second (quenched) sample had disordered cations and cubic, Pm3¯m, symmetry at high temperatures. On cooling, both underwent the same R-point tilting transition at ~560–570 K, to give symmetry changes P4/mmm↔Cmmm and Pm3¯m↔I4/mcm, respectively. From the evolution of the tilt angles, the transition appears to be close to tricritical in both cases. As expected from a Landau expansion in the two order parameters, the transition temperature was found to be only weakly dependent on the degree of cation order. In contrast with the expectation of standard patterns of strain to order parameter coupling, however, the tilted (tetragonal) form of the disordered sample remained metrically cubic, implying that coupling of the tetragonal shear strain with the octahedral tilting was suppressed. It is proposed that cation disordering causes the development of local strain heterogeneities in crystals of La0.6Sr0.1TiO3 which prevent a coherent, long-range shear strain from developing. Manipulation of the degree of cation order in perovskites with carefully selected compositions might therefore provide a means by which the magnitudes of ferroelastic strains could be engineered according to the requirement of particular applications. © 2007, American Physical Society
- ItemThermal expansion of deuterated monoclinic natrojarosite; a combined neutron–synchrotron powder diffraction study(John Wiley & Sons, Inc, 2017-04) Brand, HEA; Scarlett, NVY; Knight, KSA combination of time-of-flight neutron diffraction and synchrotron X-ray powder diffraction has been used to investigate the thermal expansion of a synthetic deuterated natrojarosite from 80 to 440 K under ambient-pressure conditions. The variation in unit-cell volume for monoclinic jarosite over this temperature range can be well represented by an Einstein expression of the form V = 515.308 (5) + 8.5 (4)/{exp[319 (4)/T] − 1}. Analysis of the behaviour of the polyhedra and hydrogen-bond network suggests that the strength of the hydrogen bonds connected to the sulfate tetrahedra is instrumental in determining the expansion of the structure, which manifests primarily in the c-axis direction. © International Union of Crystallography
- ItemTuning the giant magnetoelastic transition in Ba3BiIr2O9 and Ba3BiRu2O9(IOP Science, 2014-06-17) Huang, Z; Avdeev, M; Kennedy, BJ; Knight, KS; Zhou, Q; Ling, CDWe have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba3BiIr2O9 and Ba3BiRu2O9. For both compounds, reducing the unit cell volume by either external physical and internal chemical pressure was found to reduce the temperature T* of the transition and, to a lesser extent, the magnitude of the associated negative thermal volume expansion. The results yield the latent heat associated with the transitions, −3.34(3) × 102 J mol−1 for Ba3BiIr2O9 and −7.1(5) × 102 J mol−1 for Ba3BiRu2O9. The transition in Ba3BiRu2O9 is significantly more robust than in Ba3BiIr2O9, requiring an order of magnitude higher pressures to achieve the same reduction in T*. The differing responses of the two compounds points to differences between the 4d and 5d metals and hence to the importance of spin-orbit coupling, which is expected to be much stronger in the Ir compound. © 2014, IOP Publishing Ltd.