Browsing by Author "Xu, Guangyong"

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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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    Dynamic origin of the morphotropic phase boundary: Soft modes and phase instability in 0.68Pb(Mg(1/3)Nb(2/3)O(3))-0.32PbTiO(3)
    Cao, Hu; Stock, Chris; Xu, Guangyong; Gehring, Peter M.; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2008-09-04)
    We report neutron inelastic scattering and high-resolution x-ray diffraction measurements on single crystal 0.68Pb(Mg(1/3)Nb(2/3)O(3))-0.32PbTiO(3)) (PMN-0.32PT), a relaxor ferroelectric material that lies within the compositional range of the morphotropic phase boundary (MPB) where the piezoelectric properties of PMN-xPT compounds are close to maximal. Data were obtained between 100 and 600 K under zero and nonzero electric field applied along the cubic [001] direction. The lowest-energy, zone-center, transverse optic phonon is strongly damped and softens slowly at high temperature; however, the square of the soft-mode energy begins to increase linearly with temperature as in a conventional ferroelectric, which we term the soft-mode "recovery," upon cooling into the tetragonal phase at T(C). Our data show that the soft mode in PMN-0.32PT behaves almost identically to that in pure PMN, exhibiting the same temperature dependence and recovery temperature even though PMN exhibits no well-defined structural transition (no T(C)). The temperature dependence of the soft mode in PMN-0.32PT is also similar to that in PMN-0.60PT; however, in PMN-0.60PT the recovery temperature equals T(C). These results suggest that the temperature dependence and the energy scale of the soft-mode dynamics in PMN-xPT are independent of concentration on the Ti-poor side of the MPB, but scale with T(C) for Ti-rich compositions. Thus the MPB may be defined in lattice dynamical terms as the concentration where T(C) first matches the recovery temperature of the soft mode. High-resolution x-ray studies show that the cubic-to-ferroelectric phase boundary shifts to higher temperatures by an abnormal amount within the MPB region in the presence of an electric field. This suggests that an unusual instability exists within the apparently cubic phase at the MPB.
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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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    Evidence of decoupled lattice distortion and ferroelectric polarization in the relaxor system PMN-xPT
    Xu, Guangyong; Viehland, Dwight D.; Li, Jiefang; Gehring, Peter M.; Shirane, Gen (American Physical Society, 2003-12-30)
    We report high q-resolution neutron scattering data on PMN-xPT single crystals with x=20% and 27%. No rhombohedral distortion occurs in the 20PT sample for temperatures as low as 50 K. On the other hand, the 27PT sample transforms into a rhombohedral phase below T(C)similar to375 K. Our data provide conclusive evidence that a phase with an average cubic lattice is present in the bulk of this relaxor system at low PT concentration, in which the ferroelectric polarization and lattice distortion are decoupled. The rhombohedral distortion is limited to the outermost tens of microns of the crystal.
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    Fragile phase stability in (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO(3) crystals: A comparison of 001 and 110 field-cooled phase diagrams
    Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong (American Physical Society, 2006-05-09)
    Phase diagrams of [001] and [110] field-cooled (FC) (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO(3) or PMN-xPT (0.15 <= x <= 0.38) crystals have been constructed, based on high-resolution x-ray diffraction data. Comparisons reveal several interesting findings. First, a region of abnormal thermal expansion (c not equal a) above the dielectric maximum was found, whose stability range extended to higher temperatures on application of electric field (E). Second, the rhombohedral (R) phase of the zero-field-cooled (ZFC) state was replaced by monoclinic M-A in the [001] FC diagram, but with monoclinic M-B in the [110] FC. Third, the monoclinic M-C phase in the ZFC and [001] FC diagram was replaced by an orthorhombic (O) phase in the [110] FC diagram. Finally, in the [001] FC diagram, the phase boundary between tetragonal (T) and M-A phases was extended to lower PT content (x=0.25); whereas in the [110] FC diagram, this extended region was entirely replaced by the O phase. These results clearly demonstrate that the phase stability of PMN-xPT crystals is quite fragile-depending not only on modest changes in E, but also on the direction along which E is applied.
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    Ground state monoclinic (M-b) phase in (110)(c) BiFeO3 epitaxial thin films
    Xu, Guangyong; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2006-11-01)
    The lattice structure of (110)-oriented BiFeO3 epitaxial thin layers has been identified by synchrotron x-ray diffraction. By using (221) and (22 (1) over bar) peaks in the (HHL) zone, a ground state monoclinic M-b phase has been observed with lattice parameters of (beta;a(m)/root 2 and c(m))=(89.35 degrees;3.985 and 3.888 angstrom). These results demonstrate a change in phase stability from rhombohedral in bulk single crystals, to monoclinic in epitaxial thin films with two domain states whose polarization is slightly tilted away from [110] towards [111]. (c) 2006 American Institute of Physics.
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    Intermediate ferroelectric orthorhombic and monoclinic M-B phases in [110] electric-field-cooled Pb(Mg1/3Nb2/3)O-3-30%PbTiO3 crystals
    Cao, Hu; Bai, Feiming; Wang, Naigang; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Shirane, Gen (American Physical Society, 2005-08-01)
    Structural phase transformations of [110] electric-field-cooled Pb(Mg1/3Nb2/3)O-3-30%PbTiO3 (PMN-30%PT) crystals have been performed by x-ray diffraction. A phase sequence of cubic (C)-> tetragonal (T)-> orthorhombic (O)-> monoclinic (M-B) was found on field cooling (FC); and a rhombohedral (R)-> M-B-> O sequence was observed with increasing field, beginning from the zero-field-cooled condition at room temperature. The application of the [110] electric field induced a dramatic change in the phase sequence in the FC condition, compared to the corresponding data for PMN-30%PT crystals in a [001] field, which shows that the phase sequence in the FC condition is altered by the crystallographic direction along which a modest electric field (E) is applied. Only when E is applied along [110] are intermediate O and M-B phases observed.
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    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.
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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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    Monoclinic M-C vs orthorhombic in 001 and 110 electric-field-cooled Pb(Mg1/3Nb2/3O3)-35%PbTiO3 crystals
    Cao, Hu; Li, Jiefang; Viehland, Dwight D.; Xu, Guangyong; Shirane, Gen (AIP Publishing, 2006-02-01)
    Structural phase transformations in Pb(Mg1/3Nb2/3O3)-35%PbTiO3 crystals with an electric field (E) applied along [110] and [001] directions have been performed by x-ray diffraction. In the field-cooling process, a phase transformation sequence of Cubic(C)-> Tetragonal(T)-> Orthorhombic(O) was found for E//[110]; whereas a sequence of C -> T -> monoclinic(M-C) was found for E//[001]. Our results establish that the stability of M-C relative to O (or limiting M-C) is altered by the direction along which E is applied.
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    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.
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    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.
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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.
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    X-ray and neutron diffraction investigations of the structural phase transformation sequence under electric field in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO(3) crystal
    Bai, Feiming; Wang, Naigang; Li, Jiefang; Viehland, Dwight D.; Gehring, Peter M.; Xu, Guangyong; Shirane, Gen (American Institute of Physics, 2004-08-01)
    The structural phase transformations of 0.7Pb(Mg1/3Nb2/3)O-3-0.3PbTiO(3) (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)--> tetragonal (T)--> rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C-->T--> monoclinic (M-C)--> monoclinic (M-A) in the field-cooled (FC) condition; and (iii) R-->M-A-->M-C-->T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the M-A phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the M-A phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data. (C) 2004 American Institute of Physics.