Browsing by Author "Luo, Hongjie"

Now showing 1 - 6 of 6
  • Thumbnail Image
    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.
  • Thumbnail Image
    Direct observation of the near-surface layer in Pb(Mg1/3Nb2/3)O-3 using neutron diffraction
    Conlon, K. H.; Luo, Hongjie; Viehland, Dwight D.; Li, Jiefang; Whan, T.; Fox, J. H.; Stock, Chris; Shirane, Gen (American Physical Society, 2004-11-19)
    Spatially resolved neutron diffraction as a function of crystal depth in Pb(Mg1/3Nb2/3)O-3 reveals the presence of a distinct near-surface region where a strong distortion in the lattice exists. A dramatic change in both the lattice constant and the Bragg peak intensity as a function of crystal depth is observed to occur in this region over a length scale similar to100 mum. This confirms a previous assertion, based on a comparison between high-energy x rays and neutrons, that such a near surface region exists in the relaxors. Consequences to both single crystal and powder diffraction measurements and previous bulk neutron diffraction measurements on large single crystals are discussed.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Interplay between static and dynamic polar correlations in relaxor Pb(Mg1/3Nb2/3)O-3
    Stock, Chris; Van Eijck, L.; Fouquet, P.; Maccarini, M.; Gehring, Peter M.; Xu, Guangyong; Luo, Hongjie; Zhao, X.; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2010-04-01)
    We have characterized the dynamics of the polar nanoregions in Pb(Mg1/3Nb2/3)O-3 through high-resolution neutron-backscattering and spin-echo measurements of the diffuse-scattering cross section. We find that the diffuse-scattering intensity consists of both static and dynamic components. The static component first appears at the Curie temperature Theta similar to 400 K while the dynamic component freezes completely at the temperature T-f similar to 200 K; together, these components account for all of the observed spectral weight contributing to the diffuse-scattering cross section. The integrated intensity of the dynamic component peaks near the temperature at which the frequency-dependent dielectric constant reaches a maximum (T-max) when measured at 1 GHz, i.e., on a time scale of similar to 1 ns. Our neutron-scattering results can thus be directly related to dielectric and infrared measurements of the polar nanoregions. Finally, the global temperature dependence of the diffuse scattering can be understood in terms of just two temperature scales, which is consistent with random-field models.
  • Thumbnail Image
    Neutron and x-ray diffraction study of cubic [111] field-cooled Pb(Mg1/3Nb2/3)O-3
    Stock, Chris; Xu, Guangyong; Gehring, Peter M.; Luo, Hongjie; Zhao, X.; Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Shirane, Gen (American Physical Society, 2007-08-01)
    Neutron and x-ray diffraction techniques have been used to study the competing long- and short-range polar order in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O-3 (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E>1.7 kV/cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90 degrees-alpha=0.03 degrees) for fields up to 8 kV/cm. Furthermore, we observe a structural transition confined to the near surface volume or "skin" of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T-c=210 K for E>4 kV/cm, for which 90 degrees-alpha=0.08 +/- 0.03 degrees below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T-c=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T-c, however, below T-c the diffuse scattering is reduced in intensity and adopts an asymmetric line shape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin may be necessary for a relaxor ground state.
  • Thumbnail Image
    Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg1/3Nb2/3O3
    Swainson, I. P.; Stock, Chris; Gehring, Peter M.; Xu, Guangyong; Hirota, K.; Qiu, Y.; Luo, Hongjie; Zhao, X.; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2009-06-10)
    We report lattice-dynamical measurements, made using neutron inelastic-scattering methods, of the relaxor perovskite PbMg1/3Nb2/3O3 (PMN) at momentum transfers near the edge of the Brillouin zone. Unusual "columns" of phonon scattering that are localized in momentum, but extended in energy, are seen at both high-symmetry points along the zone edge: (Q) over right arrowR= {1/2,1/2,1/2} and (Q) over right arrowM={1/2,1/2,0}. These columns soften at similar to 400 K which is similar to the onset temperature of the zone-center diffuse scattering, indicating a competition between ferroelectric and antiferroelectric distortions. We propose a model for the atomic displacements associated with these phonon modes that is based on a combination of structure factors and group theoretical analysis. This analysis suggests that the scattering is not from tilt modes (rotational modes of oxygen octahedra), but from zone-boundary optic modes that are associated with the displacement of Pb2+ and O2- ions. Whereas similar columns of scattering have been reported in metallic and (less commonly) molecular systems, they are unusual in insulating materials, particularly in ferroelectrics; therefore, the physical origin of this inelastic feature in PMN is unknown. We speculate that the underlying disorder contributes to this unique anomaly.