Browsing by Author "Carpenter, MA"

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    Impact 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, KS
    The 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
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    Influence 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, KS
    The 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
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    Suppression of strain coupling in perovskite La0.6Sr0.1TiO3 by cation disorder
    (American Physical Society, 2007-08) Howard, CJ; Zhang, Z; Carpenter, MA; Knight, KS
    An 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
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    Temperature-dependent electrical, elastic and magnetic properties of sol-gel synthesized Bi(0.9)Ln(0.1)FeO(3) (Ln = Nd, Sm)
    (IOP Publishing Ltd, 2012-03-28) Schiemer, J; Withers, RL; Carpenter, MA; Liu, Y; Wang, JL; Norén, L; Li, Q; Hutchison, WD
    This report details correlated electrical, mechanical and magnetic behaviour in BiFeO3 ceramics doped with 10% Ln (Ln = Sm, Nd) ions on the Bi, or perovskite A, site and synthesized by a sol-gel method. The ceramics exhibit bulk piezoelectric and ferroelectric properties and clear ferroelectric domain patterns through piezoresponse force microscopy. Resonant ultrasound spectroscopy, dielectric spectroscopy and magnetometry studies show correlated magnetoelectromechanical behaviour and the existence of weak ferromagnetism for both compositions. An anomaly with simultaneous mechanical and magnetic signatures is discovered in both materials near room temperature, while previously reported transitions and anomalies are found to exhibit electro- and/or magnetomechanical coupling. Magnetism is significantly enhanced in the Sm doped sample, which is a promising multiferroic material. © 2012, IOP Publishing LTD.