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arXiv 0712.0174cond-mat.mtrl-sci
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立方[111]场冷Pb(Mg1/3Nb2/3)O3的中子和X射线衍射研究

Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3

C. Stock, Guangyong Xu, P. M. Gehring, H. Luo, X. Zhao, H. Cao, J. F. Li, D. Viehland, G. Shirane

摘要:

利用中子和X射线衍射技术研究了弛豫铁电体Pb(Mg $_{1/3}$ Nb $_{2/3}$)O $_{3}$(PMN)在[111]电场作用下的长程和短程竞争极性序.尽管报道了从立方相到菱面体相的结构转变,对于电场E $>$ 1.7 kV/cm,我们发现,对于高达8 kV/cm的电场,大体积晶胞保持立方(在90 $^{\circ}$ - $α$ =0.03 $^{\circ}$的灵敏度内)。此外,我们观察到一个结构转变局限于近表面体积或“皮肤”的晶体,其中立方晶胞转化为菱面体晶胞在T $_{c}$ =210 K的E $>$ 4 kV/cm,其中90 $^{\circ}$ - $α$ =0.08 $\pm$ 0.03 $^{\circ}$低于50 K。虽然散装单位电池保持立方,抑制的漫散射和随之而来的布拉格峰强度的增强被观察到低于T $_{c}$ =210 K,表明一个更有序的结构与增加的电场,但没有一个长程铁电基态的散装。在T $_{c}$以上,电场强度对漫散射的影响很小,而在T $_{c}$以下,漫散射强度降低,在倒易空间中呈现非对称线形。在我们的中子测量(散装)和两个不同的温度尺度的存在下,滞后的情况下,表明PMN的基态是不是一个冻结的玻璃相所建议的一些理论,但更好地理解通过结构紊乱的存在下引入的随机场。基于这些结果,我们还建议,PMN代表了一个极端的例子的两个长度尺度的问题,以及一个独特的皮肤的存在下可能需要一个松弛基态。

英文摘要:

Neutron and x-ray diffraction techniques have been used to study the competing long and short-range polar order in the relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)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$^{\circ}$-$α$ =0.03$^{\circ}$)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$^{\circ}$-$α$=0.08 $\pm$ 0.03$^{\circ}$ 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 lineshape 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 maybe necessary for a relaxor ground state.

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