Zhang Xiuyun, Ng Man-Fai, Wang Yanbiao, Wang Jinlan, Yang Shuo-Wang
Department of Physics, Southeast University, Nanjing 211189, PR China.
ACS Nano. 2009 Sep 22;3(9):2515-22. doi: 10.1021/nn900401b.
Europium (Eu)-cyclootetatrene (COT = C(8)H(8)) multidecker clusters (Eu(n)COT(n+1), n = 1-4) are studied by relativistic density functional theory calculations. These clusters are found to be thermodynamically stable with freely rotatable COT rings, and their total magnetic moments (MMs) increase linearly along with the number of Eu atoms. Each Eu atom contributes about 7 mu(B) to the cluster. Meanwhile, the internal COT rings have little MM contribution while the external COT rings have about 1 mu(B) MM aligned in opposite direction to that of the Eu atoms. The total MM of the Eu(n)COT(n+1) clusters can thus be generalized as 7n - 2 mu(B) where n is the number of Eu atoms. Besides, the ground states of these clusters are ferromagnetic and energetically competitive with the antiferromagnetic states, meaning that their spin states are very unstable, especially for larger clusters. More importantly, we uncover an interesting bonding characteristic of these clusters in which the interior ionic structure is capped by two hybrid covalent-ionic terminals. We suggest that such a characteristic makes the Eu(n)COT(n+1) clusters extremely stable. Finally, we reveal that for the positively charged clusters, the hybrid covalent-ionic terminals will tip further toward the interior part of the clusters to form deeper covalent-ionic caps. In contrast, the negatively charged clusters turn to pure ionic structures.
通过相对论密度泛函理论计算研究了铕（Eu）-环辛四烯（COT = C₈H₈）多层簇（EuₙCOTₙ₊₁，n = 1 - 4）。发现这些簇在热力学上是稳定的，COT环可自由旋转，并且它们的总磁矩（MM）随Eu原子数线性增加。每个Eu原子对簇的贡献约为7 μB。同时，内部COT环对磁矩的贡献很小，而外部COT环具有约1 μB的磁矩，其方向与Eu原子的磁矩方向相反。因此，EuₙCOTₙ₊₁簇的总磁矩可以概括为7n - 2 μB，其中n是Eu原子的数量。此外，这些簇的基态是铁磁性的，并且在能量上与反铁磁态竞争，这意味着它们的自旋态非常不稳定，特别是对于较大的簇。更重要的是，我们发现了这些簇的一个有趣的键合特征，即内部离子结构由两个混合共价 - 离子末端封端。我们认为这种特征使EuₙCOTₙ₊₁簇极其稳定。最后，我们发现对于带正电的簇，混合共价 - 离子末端将进一步向簇的内部倾斜，形成更深的共价 - 离子封端。相反，带负电的簇则转变为纯离子结构。
