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首页> 外文期刊>Physical review >Evidence for anisotropic polar nanoregions in relaxor Pb(Mg_(1/3)Nb_(2/3))O_3: A neutron study of the elastic constants and anomalous TA phonon damping in PMN
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Evidence for anisotropic polar nanoregions in relaxor Pb(Mg_(1/3)Nb_(2/3))O_3: A neutron study of the elastic constants and anomalous TA phonon damping in PMN

机译：Pb（Mg_（1/3）Nb_（2/3））O_3中各向异性极性纳米区域的证据：中子研究PMN中弹性常数和TA声子反常阻尼的异常

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摘要

We use neutron inelastic scattering to characterize the acoustic phonons in the relaxor Pb(Mg_(1/3)Nb_(2/3))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 ～ 210 K, where dynamic, short-range polar correlations are present, acoustic phonons propagating along [llO] 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 Γ 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 PbTiO_3. We show that while the values of C44 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 PbTiO_3. The elastic constants also show an increased elastic anisotropy [2C_(44)/(C_(11)n - C_(12))] in PMN versus that in PbTiO_3 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.
机译：我们使用中子非弹性散射来表征弛豫器Pb（Mg_（1/3）Nb_（2/3））O_3（PMN）中的声子，并证明存在与短程直接相关的高度各向异性阻尼机制极相关。对于T_c〜210 K以上的大范围温度，在其中存在动态的短程极性相关，在整个布里渊区中，沿[llO]传播并沿[110]极化的声子（TA_2声子）被过度阻尼和软化。 。相比之下，沿波[q]传播并沿[001]极化的声子（TA_1声子）被过度阻尼和软化。声子能量线宽度Γ的各向异性和温度依赖性与中子扩散散射截面直接相关，表明极性纳米区域是异常行为的原因。对于q→0，即对于区域中心附近的长波声子，阻尼和软化消失了，这支持了异常阻尼是扩散散射的弛豫分量与谐波TA声子之间耦合的结果。 。因此，这些影响不是由于弹性常数随温度的大变化，因为弹性常数对应于长波长极限。我们将测得的弹性常数与布里渊散射实验测得的弹性常数以及纯PbTiO_3报道的数值进行比较。我们显示，虽然C44的值非常相似，但PMN中C_（11）和C_（12）的值明显较小，并导致PbTiO_3上的（C_（11）-C_（12））软化。与PbTiO_3相比，PMN中的弹性常数还显示出更大的弹性各向异性[2C_（44）/（C_（11）n-C_（12））]。这些结果表明，PMN和其他弛豫器中TA_2声波波动不稳定。铁电体。我们在当前关于“瀑布”效应的辩论中讨论了我们的结果，并表明它们与声光声子耦合或调用第二种低能量光学模式存在的其他模型不一致。

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  • 来源
    《Physical review》 |2012年第10期|p.104108.1-104108.18|共18页
  • 作者

    C. Stock; P. M. Gehring; H. Hiraka; I. Swainson; Guangyong Xu; Z.-G. Ye; H. Luo; J.-F. Li; D. Viehland;

  • 作者单位

    NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6100, USA,Indiana University, 2401 Milo B. Sampson Lane, Bloomington, Indiana 47404, USA;

    NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6100, USA;

    Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan;

    National Research Council, Chalk River, Ontario, Canada KOJI JO;

    Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA;

    Department of Chemistry and4D Labs, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6;

    Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China;

    Department of Materials Science, Virginia Tech., Blacksburg, Virginia 24061, USA;

    Department of Materials Science, Virginia Tech., Blacksburg, Virginia 24061, USA;

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  • 关键词

    dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.);

    机译：动态特性（动态磁化率;自旋波;自旋扩散;动态缩放等）;

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