Whangbo M-H, Dai D, Kremer R K
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA.
Inorg Chem. 2006 Jul 24;45(15):5989-95. doi: 10.1021/ic060691l.
Nonstoichiometric Na(x)CoO2 (0.5 < x < 1) consists of CoO2 layers made up of edge-sharing CoO6 octahedra and exhibits strongly anisotropic magnetic susceptibilities as well as metallic properties. A modified Curie-Weiss law was proposed for systems containing anisotropic magnetic ions to analyze the magnetic susceptibilities of Na(x)CoO2 (x approximately 0.75), and implications of this analysis were explored. Our study shows that the low-spin Co4+ (S = 1/2) ions of Na(x)CoO2 generated by the Na vacancies cause the anisotropic magnetic properties of Na(x)CoO2 and suggests that the six nearest-neighbor Co3+ ions of each Co4+ ion adopt the intermediate-spin electron configuration, thereby behaving magnetically like low-spin Co4+ ions. The Weiss temperature of Na(x)CoO2 is more negative along the direction of the lower g factor (i.e., theta|| < theta(perpendicular) < 0 and g|| < g(perpendicular)). The occurrence of intermediate-spin Co3+ ions surrounding each Co4+ ion accounts for the apparently puzzling magnetic properties of Na(x)CoO2 (x approximately 0.75), i.e., the large negative Weiss temperature, the three-dimensional antiferromagnetic ordering below approximately 22 K, and the metallic properties. The picture of the magnetic structure derived from neutron scattering studies below approximately 22 K is in apparent conflict with that deduced from magnetic susceptibility measurements between approximately 50 and 300 K. These conflicting pictures are resolved by noting that the spin exchange between Co3+ ions is more strongly antiferromagnetic than that between Co4+ and Co3+ ions.
非化学计量比的Na(x)CoO2（0.5 < x < 1）由通过共边的CoO6八面体构成的CoO2层组成，具有强烈的各向异性磁化率以及金属特性。针对包含各向异性磁性离子的体系提出了修正的居里 - 外斯定律，以分析Na(x)CoO2（x约为0.75）的磁化率，并探讨了该分析的意义。我们的研究表明，由钠空位产生的Na(x)CoO2中的低自旋Co4 +（S = 1/2）离子导致了Na(x)CoO2的各向异性磁性特性，并表明每个Co4 +离子的六个最近邻Co3 +离子采用中间自旋电子构型，从而在磁性上表现得像低自旋Co4 +离子。Na(x)CoO2的外斯温度沿较低g因子方向更负（即，θ|| < θ⊥ < 0且g|| < g⊥）。每个Co4 +离子周围存在中间自旋Co3 +离子解释了Na(x)CoO2（x约为0.75）明显令人困惑的磁性特性，即大的负外斯温度、低于约22 K时的三维反铁磁有序以及金属特性。在约22 K以下通过中子散射研究得出的磁结构图像与在约50至300 K之间通过磁化率测量推断出的图像明显冲突。通过注意到Co3 +离子之间的自旋交换比Co4 +和Co3 +离子之间的自旋交换更强的反铁磁性，可以解决这些相互冲突的图像。
