Browsing by Author "Hiraka, Haruhiro"

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    Damped soft phonons and diffuse scattering in 40%Pb(Mg1/3Nb2/3)O-3-60%PbTiO3
    Stock, Chris; Ellis, D.; Swainson, I. P.; Xu, Guangyong; Hiraka, Haruhiro; Zhong, Z.; Luo, Hongjie; Zhao, X.; Viehland, Dwight D.; Birgeneau, R. J.; Shirane, Gen (American Physical Society, 2006-02-13)
    Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg1/3Nb2/3)O-3-60% PbTiO3 (PMN-60PT) with pure Pb(Mg1/3Nb2/3)O-3 (PMN) and PbTiO3 (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and Pb(Zn1/3Nb2/3)O-3 (PZN). PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unit cell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortion is long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present. Even though polar nanoregions are absent, the soft optic mode is short-lived for wave vectors near the zone center. Therefore PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder which is the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zone center and not due to the formation of short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of the PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates with increasing doping of PbTiO3.
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    Evidence for anisotropic polar nanoregions in relaxor Pb(Mg1/3Nb2/3)O-3: A neutron study of the elastic constants and anomalous TA phonon damping in PMN
    Stock, Chris; Gehring, Peter M.; Hiraka, Haruhiro; Swainson, I. P.; Xu, Guangyong; Ye, Z. G.; Luo, Hongjie; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2012-09-18)
    We use neutron inelastic scattering to characterize the acoustic phonons in the relaxor Pb(Mg1/3Nb2/3)O-3 (PMN) and demonstrate the presence of a highly anisotropic damping mechanism that is directly related to short-range polar correlations. For a large range of temperatures above T-c similar to 210 K, where dynamic, short-range polar correlations are present, acoustic phonons propagating along [1 (1) over bar0] and polarized along [110] (TA(2) phonons) are overdamped and softened across most of the Brillouin zone. By contrast, acoustic phonons propagating along [100] and polarized along [001] (TA(1) phonons) are overdamped and softened for a more limited range of wave vectors q. The anisotropy and temperature dependence of the acoustic phonon energy linewidth Gamma are directly correlated with neutron diffuse scattering cross section, indicating that polar nanoregions are the cause of the anomalous behavior. The damping and softening vanish for q -> 0, i.e., for long-wavelength acoustic phonons near the zone center, which supports the notion that the anomalous damping is a result of the coupling between the relaxational component of the diffuse scattering and the harmonic TA phonons. Therefore, these effects are not due to large changes in the elastic constants with temperature because the elastic constants correspond to the long-wavelength limit. We compare the elastic constants we measure to those from Brillouin scattering experiments and to values reported for pure PbTiO3. We show that while the values of C-44 are quite similar, those for C-11 and C-12 are significantly less in PMN and result in a softening of (C-11 - C-12) over PbTiO3. The elastic constants also show an increased elastic anisotropy [2C(44)/(C-11 - C-12)] in PMN versus that in PbTiO3. These results are suggestive of an instability to TA(2) acoustic fluctuations in PMN and other relaxor ferroelectrics. We discuss our results in the context of the current debate over the "waterfall" effect and show that they are inconsistent with acoustic-optic phonon coupling or other models that invoke the presence of a second, low-energy optic mode.
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    Low symmetry phase in (001) BiFeO3 epitaxial constrained thin films
    Xu, Guangyong; Hiraka, Haruhiro; Shirane, Gen; Li, Jiefang; Wang, J. L.; Viehland, Dwight D. (AIP Publishing, 2005-05-01)
    The lattice of (001)-oriented BiFeO3 epitaxial thin film has been identified by synchrotron x-ray diffraction. By choosing proper scattering zones containing the fixed (001) reflection, we have shown that low-symmetry phases similar to a M-A phase exist in the thin film at room temperature. These results demonstrate a change in phase stability from rhombohedral in bulk single crystals, to a modified monoclinic structure in epitaxial thin films. 2005 American Institute of Physics.
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    Structural phase transformation and phase boundary/stability studies of field-cooled Pb(Mg1/3Nb2/3O3)-32%PbTiO3 crystals
    Cao, Hu; Bai, Feiming; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Hiraka, Haruhiro; Shirane, Gen (American Institute of Physics, 2005-05-01)
    Structural phase transformations in (001)-oriented (1-x)Pb(Mg1/3Nb2/3O3)-32%PbTiO3 crystals have been investigated by x-ray diffraction. A C -> T -> M-C sequence was observed in both the field-cooled and zero-field-cooled conditions. Most interestingly, an anomalous increase in the C -> T phase boundary with increasing field has been observed, which is seemingly a common characteristic of crystals whose compositions are in the vicinity of the morphotropic phase boundary, irrespective of the width of the T and M-C phase regions. (C) 2005 American Institute of Physics.