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~(23)Na NMR study of sodium order in Na_xCoO_2 with 22 K Neel temperature

机译：Na_xCoO_2中Na（22）Neel温度下钠阶的〜（23）Na NMR研究

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摘要

We report a systematic study of the olattice parameter in the Na_xCoO_2 phases versus Na content x > 0.5, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an antiferromagnetic order at a Neel temperature T_N = 22 K, which is found to occur for x ≈ 0.77(1). Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical ~(23)Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display T dependencies of their NMR shifts, which scale with each other. This allows us to establish that the T variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3~+. The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of ~(23)Na lines permits us to reveal that the two-dimensional structure of the Na order corresponds to ten Na sites on top of a thirteen-Co-sites unit cell, that is with x = 10/13 ≈ 0.77. This structure fits with that determined from local density calculations and involves triangles of three Na sites located on top of Co sites [so-called Nal sites]. The associated ordering of the Na vacancies is quite distinct from that found for x < 0.75.
机译：我们报告了Na_xCoO_2相相对于Na含量x> 0.5的晶格参数的系统研究，其中钠始终显示有序排列。这使我们能够选出在尼尔温度T_N = 22 K时表现出反铁磁有序的第一相，发现发生在x≈0.77（1）处。已经研究了该相的纯样品，既是取向粉末又是单晶。它们表现出相同的〜（23）Na NMR光谱，其中三组Na位点可被完全分辨，并且发现它们的NMR位移显示T依赖性，它们相互成比例。这使我们可以确定，位移的T变化是由于Co电荷的正电荷大于3〜+的Co位的顺磁性引起的。存在具有轴向电荷对称性的钠位点和〜（23）Na线组之间的强度比，这使我们能够揭示Na阶的二维结构对应于13 Co顶部的10个Na位点。个位元单元，即x = 10/13≈0.77。这种结构与通过局部密度计算确定的结构相吻合，涉及位于Co站点顶部的三个Na站点的三角形（所谓的Nal站点）。 Na空位的相关排序与x <0.75的发现完全不同。

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  • 来源
    《Physical review》 |2012年第13期|p.134433.1-134433.11|共11页
  • 作者

    H. Alloul; I. R. Mukhamedshin; A. V. Dooglav; Ya. V. Dmitriev; V.-C. Ciomaga; L. Pinsard-Gaudart; G. Collin;

  • 作者单位

    Laboratoire de Physique des Solides, CNRS UMR 8502, Universite Paris-Sud, 91405 Orsay, France;

    Laboratoire de Physique des Solides, CNRS UMR 8502, Universite Paris-Sud, 91405 Orsay, France,Institute of Physics, Kazan Federal University, 420008 Kazan, Russia;

    Laboratoire de Physique des Solides, CNRS UMR 8502, Universite Paris-Sud, 91405 Orsay, France,Institute of Physics, Kazan Federal University, 420008 Kazan, Russia;

    Laboratoire de Physique des Solides, CNRS UMR 8502, Universite Paris-Sud, 91405 Orsay, France,Institute of Physics, Kazan Federal University, 420008 Kazan, Russia;

    lnstitut de Chimie Moleculaire et des Materiaux d 'Orsay, UMR 8182, Universite Paris-Sud, 91405 Orsay, France;

    lnstitut de Chimie Moleculaire et des Materiaux d 'Orsay, UMR 8182, Universite Paris-Sud, 91405 Orsay, France;

    Laboratoire de Physique des Solides, CNRS UMR 8502, Universite Paris-Sud, 91405 Orsay, France;

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  • 正文语种 eng
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  • 关键词

    nuclear magnetic resonance and relaxation; structure of specific crystalline solids; strongly correlated electron systems; heavy fermions;

    机译：核磁共振和弛豫;特定结晶固体的结构;强相关电子系统;重费米子;

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