Inelastic neutron scattering on an Mn10 supertetrahedron: assessment of exchange coupling constants, ferromagnetic spin waves and an analogy to the Hückel method.
Inelastic neutron scattering on an Mn10 supertetrahedron: assessment of exchange coupling constants, ferromagnetic spin waves and an analogy to the Hückel method.
复制标题
Mn10 超四面体上的非弹性中子散射：交换耦合常数、铁磁自旋波的评估以及与休克尔方法的类比。
作者：
S. Stuiber;Gang Wu;J. Nehrkorn;J. Dreiser;Yanhua Lan;G. Novitchi;C. Anson;T. Unruh;A. Powell;O. Waldmann
混合价 Mn(10) 超四面体聚集体的合成、晶体结构和磁化和非弹性中子散射 (INS) 磁性表征报道了[Mn(III)(6)Mn(II)(4)(μ(4)-O)(4)(μ(3)-N(3))(3)(μ(3)-Br)(Hmpt)(6)(Br)]Br(0.7)(N(3))(0.3)·2MeOH·3MeCN (1) (H(3)mpt=3-甲基戊烷-1,3,5-三醇)。该分子的磁芯可以描述为具有四个面的六个 S=2 Mn(III) 离子的八面体，每个面由一个 S=5/2 Mn(II) 离子覆盖，从而形成超四面体。与大多数相关配合物不同，分子对称性从大约 T(d) 略微降低到 C(3)。磁性数据显示，由于分子内的铁磁交换耦合，基态下的总自旋为 S=22。组合 INS 和磁数据可以准确确定交换耦合常数。发现了两种类型。内部八面体中 Mn(III) 离子之间的耦合由 J(a)=18.4(3) K 表征，而顶端 Mn(II) 离子与相邻 Mn(III) 离子之间的耦合由 J(b)=7.3(2) K 给出。与 J(b) 相比，耦合强度 J(a) 明显更大，并且近 T(d) 对称性对能谱产生深远的影响，对此进行了讨论并仔细分析。特别是，观察到的 INS 光谱可以通过简化模型一致地再现，其中内部八面体被长度为 S(0)=12 的一个大自旋取代。该模型提供了对磁谱结构的直观了解。此外，在铁磁线性自旋波理论的框架内分析了低温下的磁激励，这允许对能级进行分析计算。对于铁磁团簇，可以与电子结构计算的休克尔方法进行密切类比，这使得人们能够分别以化学语言掌握自旋波理论或磁激发谱的结果。
The synthesis, crystal structure and magnetic characterisation by magnetisation and inelastic neutron scattering (INS) of a mixed-valent Mn(10) supertetrahedral aggregate [Mn(III)(6)Mn(II)(4)(μ(4)-O)(4)(μ(3)-N(3))(3)(μ(3)-Br)(Hmpt)(6)(Br)]Br(0.7)(N(3))(0.3)·2MeOH·3MeCN (1) (H(3)mpt=3-methylpentan-1,3,5-triol) is reported. The magnetic core of the molecule can be described as an octahedron of six S=2 Mn(III) ions with four faces, each capped by a S=5/2 Mn(II) ion such as to form the supertetrahedron. Unlike most related complexes, the molecular symmetry is slightly reduced from approximately T(d) to C(3). The magnetic data reveal a total spin of S=22 in the ground state due to ferromagnetic exchange couplings within the molecule. The combined INS and magnetic data permits the accurate determination of the exchange coupling constants. Two types are found. The couplings between the Mn(III) ions in the inner octahedron are characterised by J(a)=18.4(3) K, whereas the couplings between the apical Mn(II) ions to the neighbouring Mn(III) ions are given by J(b)=7.3(2) K. The significantly larger coupling strength J(a) as compared to J(b), and the near-T(d) symmetry have profound consequences on the energy spectrum, which are discussed and carefully analysed. In particular, the observed INS spectra can consistently be reproduced by a simplified model in which the inner octahedron is replaced by one large spin of length S(0)=12. This model provides intuitive insight into the structure of the magnetic spectrum. Additionally, the magnetic excitations at low temperature are analysed within the frame of ferromagnetic linear spin-wave theory, which permits an analytical calculation of the energy levels. For ferromagnetic clusters, a close analogy to the Hückel method of electronic structure calculation can be drawn, which allows one to grasp the results of the spin-wave theory or the magnetic excitation spectrum, respectively, in a chemical language.