LiFePO4的非常规磁性和带隙形成：聚阴离子诱导的非平面性的结果
Unconventional Magnetism and Band Gap Formation in LiFePO4: Consequence of Polyanion Induced Non-planarity
摘要：
氧在强关联过渡金属氧化物中起着关键的作用，因为晶场效应是决定d/f价态局域化程度的关键因素之一。在此基础上，提出了Mott-Hubbard、Charge-transfer和Slater等一系列经典机制来解释这些关联系统中的反铁磁绝缘（AFI）现象。从LiFePO 4的案例研究，通过密度泛函计算，我们证明，这些机制都不严格适用于解释AFI行为时，过渡金属氧化物具有聚阴离子，如（PO 4）3-。降低金属-氧络合物的结晶度，以稳定聚阴离子，为d/f态产生不对称的晶体场。在LiFePO 4中，该场产生完全非简并的Fe-d态，其具有可忽略的p-d和d-d共价相互作用，变得原子局域化以确保费米能级处的能隙。由于大的交换分裂，高自旋态是有利的，反铁磁结构是稳定的。对于原型LiFePO 4，独立电子近似足以获得AFI基态。包括额外的相关性测量（如Hubbard U）简单地放大了间隙，因此LiFePO 4可以优选地被称为弱耦合莫特绝缘体。
英文摘要：
Oxygen plays a critical role in strongly correlated transition metal oxides as crystal field effect is one of the key factors that determine the degree of localization of the valence d/f states. Based on the localization, a set of conventional mechanisms such as Mott-Hubbard, Charge-transfer and Slater were formulated to explain the antiferromagnetic and insulating (AFI) phenomena in many of these correlated systems. From the case study on LiFePO4, through density-functional calculations, we demonstrate that none of these mechanisms are strictly applicable to explain the AFI behavior when the transition metal oxides have polyanions such as (PO4)3-. The symmetry-lowering of the metal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystal field for d/f states. In LiFePO4 this field creates completely non-degenerate Fe-d states which, with negligible p-d and d-d covalent interactions, become atomically localized to ensure a gap at the Fermi level. Due to large exchange splitting, high spin state is favored and an antiferromagnetic configuration is stabilized. For the prototype LiFePO4, independent electron approximation is good enough to obtain the AFI ground state. Inclusion of additional correlation measures like Hubbard U simply amplifies the gap and therefore LiFePO4 can be preferably called as weakly coupled Mott insulator.
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