Browsing by Author "Tan, TY"

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    Crystal structures and phase transitions in Sr doped Ba2InTaO6 perovskites
    (Academic Press INC Elsevier Science, 2013-10-01) Zhou, QD; Tan, TY; Kennedy, BJ; Hester, JR
    The crystal structures of the double perovskites of the type Ba2-xSrxInTaO6 have been studied by synchrotron X-ray and neutron powder diffraction methods. The structures are based on ordered array of the tilted InO6 and TaO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm (3) over barm(x similar to 0) (6)-> I4/m(x similar to 1.2)-> I2/m(x similar to 1.6)-> P2(1)/n. The changes in the symmetry induced by replacing the Ba with Sr have been identified by a symmetry-mode analysis. Variable temperature S-XRD studies of selected examples have shown that heating the samples induces the same sequence of transitions observed in the composition dependent studies. © 2013, Elsevier Ltd.
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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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    Valence changes of manganese and structural phase transitions in Sr1-xPrxMnO3 (0.1 <= x <= 0.6)
    (Elsevier, 2013-05-01) Tan, TY; Martin, N; Zhou, Q; Kennedy, BJ; Gu, QF; Kimpton, JA; Zhang, Z; Jang, LY
    Fifteen perovskite manganites Sr1−xPrxMnO3, x=0.1−0.6 in steps as fine as 0.025, have been synthesized by solid state methods and their crystal structures determined using synchrotron X-ray powder diffraction. At room temperature two first order phase transitions associated with changes in the orbital ordering and the tilting of the corner shared MnO6 octahedra are evident cubic/Pm3¯m⟹x≈0.15tetragonal/(I4/mcm)⟹x≈0.45orthorhombic/Imma, with two phase regions associated with both transitions. Heating the Pr rich oxide Sr0.5Pr0.5MnO3 provides a rare example where chemical pressure and temperature result in a different sequence of transitions Imma –R3¯c–Pm3¯m, reflecting the importance of the Pr 4f electrons. X-ray absorption near-edge structure (XANES) measurements at the Mn K and Pr L3 edges show the valence state of the Mn linearly decreases with increasing Pr content whereas the Pr remains +3 across the entire series. Magnetic susceptibility measurements for selected samples are presented. © 2013, Elsevier Inc.