Browsing by Author "Pandey, D"

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    Ground state of (Pb0.94Sr0.06)(Zr0.530Ti0.470)O3 in the morphotropic phase boundary region: evidence for a monoclinic Cc space group
    (American Physical Society, 2011-10-24) Solanki, RS; Singh, AK; Mishra, SK; Kennedy, SJ; Suzuki, T; Kuroiwa, Y; Moriyoshi, C; Pandey, D
    The antiferrodistortive (AFD) phase transition for a pseudotetragonal composition of Pb(Zr(0.530)Ti(0.470))O(3) doped with 6% Sr has been investigated using sound velocity (4-320 K), high-resolution synchrotron x-ray powder diffraction (100-800 K), and high-resolution as well as high-flux neutron powder diffraction measurements (4 K) to settle the existing controversies about the true ground state of Pb(Zr(x)Ti(1-x))O(3) (PZT) in the morphotropic phase boundary (MPB) region. The multiplet character of the neutron diffraction profiles of (3/2 1/2 1/2)(pc) (pseudocubic or pc indices) and (3/2 3/2 1/2)(pc) superlattice peaks, appearing below the AFD transition temperature, rules out the rhombohedral R3c space group. The true ground state is confirmed to be monoclinic in the Cc space group, in agreement with the predictions of the first-principles calculations and earlier findings for pure PZT in the MPB region. Here, 6% Sr(2+) substitution and the use of high-wavelength (lambda = 2.44 angstrom) neutrons have played a key role in settling the existing controversies about the true ground state of PZT in the MPB region. © 2011, American Physical Society
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    Magnetic transitions and site-disordered induced weak ferromagnetism in (1-x)BiFeO3-xBaTiO3
    (American Physical Society, 2014-01-31) Singh, A; Senyshyn, A; Fuess, H; Kennedy, SJ; Pandey, D
    We present evidence for weak ferromagnetism in both the rhombohedral and cubic compositions of BF-x BT solid solutions for x  < 0.55. Rietveld refinement of nuclear and magnetic structures reveals that the G -type antiferromagnetic ordering of the Fe 3+ magnetic sublattice survives up to x  ∼ 0.50. We address the issue of weak ferromagnetism due to spin canting, which is allowed by the symmetry in the R 3c space group but not in the cubic Pm3 ¯ m space group. It is shown that the local symmetry of the average cubic compositions of BF-x BT for 0.35 < x  < 0.55 is broken due to off-centering of Bi 3+ in the (1-10) plane and O 2− along the ⟨110⟩ direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O 2− disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe 3+ -O 2− -Fe 3+ bond angle from 180° that allows spin canting due to Dzyaloshinskii-Moriya interaction D ⃗ i,j ⋅(S ⃗ i ×S ⃗ j ) , which is otherwise irreconcilable with the ideal cubic symmetry. The magnetization and neutron powder diffraction measurements confirm the absence of magnetic ordering at room temperature for x  ≳ 0.55. ©2014 American Physical Society