Theory of Nanomaterials Group, Chemistry Department, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium.
Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, Dresden 01069, Germany.
J Chem Phys. 2017 Sep 28;147(12):124305. doi: 10.1063/1.5004183.
The magnetic properties and electronic structure of the ground and excited states of two recently characterized endohedral metallo-fullerenes, [Gd@C] (1) and [Gd@C] (2), have been studied by theoretical methods. The systems can be considered as [Gd] dimers encapsulated in a fullerene cage with the fifteen unpaired electrons ferromagnetically coupled into an S = 15/2 high-spin configuration in the ground state. The microscopic mechanisms governing the Gd-Gd interactions leading to the ferromagnetic ground state are examined by a combination of density functional and ab initio calculations and the full energy spectrum of the ground and lowest excited states is constructed by means of ab initio model Hamiltonians. The ground state is characterized by strong electron delocalization bordering on a σ type one-electron covalent bond and minor zero-field splitting (ZFS) that is successfully described as a second order spin-orbit coupling effect. We have shown that the observed ferromagnetic interaction originates from Hund's rule coupling and not from the conventional double exchange mechanism. The calculated ZFS parameters of 1 and 2 in their optimized geometries are in qualitative agreement with experimental EPR results. The higher excited states display less electron delocalization, but at the same time they possess unquenched first-order angular momentum. This leads to strong spin-orbit coupling and highly anisotropic energy spectrum. The analysis of the excited states presented here constitutes the first detailed study of the effects of spin-dependent delocalization in the presence of first order orbital angular momentum and the obtained results can be applied to other mixed valence lanthanide systems.
理论方法研究了最近被表征的两种内包金属富勒烯[Gd@C]（1）和[Gd@C]（2）的基态和激发态的磁性和电子结构。这些体系可以被视为[Gd]二聚体被富勒烯笼包封，其中 15 个未配对电子在基态中通过铁磁耦合形成 S = 15/2 高自旋构型。通过密度泛函和从头算计算的组合，研究了控制 Gd-Gd 相互作用导致铁磁基态的微观机制，并通过从头算模型哈密顿量构建了基态和最低激发态的全能谱。基态的特征是强电子离域，接近于σ型单电子共价键，并且具有较小的零场分裂（ZFS），这可以成功地描述为二阶自旋轨道耦合效应。我们已经表明，观察到的铁磁相互作用源于 Hund 规则耦合，而不是传统的双交换机制。在优化结构中，1 和 2 的计算 ZFS 参数与实验 EPR 结果定性一致。较高的激发态显示出较少的电子离域，但同时它们具有未淬灭的一阶角动量。这导致强的自旋轨道耦合和高度各向异性的能谱。这里呈现的激发态分析构成了对存在一阶轨道角动量时自旋相关离域影响的首次详细研究，并且获得的结果可以应用于其他混合价镧系元素体系。
