~（23）Na NMR研究22 K Neel温度下NaxCoO2中钠的有序性
23Na NMR study of sodium order in NaxCoO2 with 22K Neel temperature
摘要：
我们报告了一个系统的研究$c$晶格参数在Na $_{x}$ CoO $_{2}$阶段与钠含量$x>0.5$，其中钠总是显示有序排列。这使我们能够挑选出的第一阶段，表现出AF磁序在Néel温度$T_{N}=$ 22 K，这是发现发生$x\approx 0.77(1)$。该相的纯样品已被研究为取向粉末和单晶。它们具有相同的$^{23}$ Na NMR谱，其中三组Na位点可以完全分辨，并且发现它们的NMR位移彼此成比例地显示$T$依赖性。这使我们能够建立的$T$变化的位移是由于顺磁性的正式电荷状态大于3 $^{+}$的Co网站。一个钠网站的存在与轴向电荷对称性和之间的强度比的$^{23}$ Na线的集合允许我们揭示，2D结构的Na顺序对应于10个Na网站上的13个Co网站的晶胞，即与$x=10/13\approx 0.77$。这种结构与从局部密度计算确定的结构相吻合，并且涉及位于Co位点顶部的3个Na位点的三角形（所谓的Na 1位点）。Na空位的相关顺序与$x<0.75$的顺序完全不同。
英文摘要：
We report a systematic study of the $c$ lattice parameter in the Na$_{x}$CoO$_{2}$ 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 AF magnetic order at a Néel temperature $T_{N}=$22 K which is found to occur for $x\approx 0.77(1)$. 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 2D structure of the Na order corresponds to 10 Na sites on top of a 13 Co sites unit cell, that is with $x=10/13\approx 0.77$. This structure fits with that determined from local density calculations and involves triangles of 3 Na sites located on top of Co sites (so called Na1 sites). The associated ordering of the Na vacancies is quite distinct from that found for $x<0.75$.
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