Interplay of Magnetism and Bonding in GaV4–xCrxSe8

The solid solution GaV4–xCrxSe8 (x = 0–4) was synthesized; their crystal structures and magnetic properties were determined and compared with results of ab initio density functional theory (DFT) calculations. The compounds crystallize in the GaMo4S8-type structure (F4̅3m), consisting of GaS4 tetrahedra and heterocubane-like (V4–xCrx)Se4 units. Significant metal–metal bonding within tetrahedral metal clusters exists up to GaV2Cr2Se8 (x = 2), while the bonds gradually disperse when x > 2. The dissolving of the metal–metal bonds is associated with large increases in the magnetic moments. This can be explained by metal–metal bonded tetrahedral V4–xCrx clusters with (small) magnetic moments localized in cluster molecular orbitals for x ≤ 2, whereas the magnetism of Cr-rich compounds is better described by a mixture of V2Cr2 clusters and nonbonded Cr atoms with stronger magnetism. GaV4Se8 and hypothetical GaCr4Se8 were investigated as model systems by ab initio DFT calculations. Thus, the ground state of GaV4Se8 contains V4 cluster units with small magnetic moments, while GaCr4Se8 has no tendency to form strong Cr–Cr bonds, but a much higher magnetic moment, both in good agreement with the experimental data.

本研究合成了固溶体GaV₄₋ₓCrₓSe₈（x=0~4），测定了其晶体结构与磁学性能，并与从头算密度泛函理论（ab initio density functional theory, DFT）的计算结果进行了对比。该类化合物结晶于GaMo₄S₈型结构（空间群F4̅3m），由GaS₄四面体与类异立方烷（V₄₋ₓCrₓ）Se₄单元构成。在x≤2的范围内，四面体金属团簇内均存在显著的金属-金属键，直至GaV₂Cr₂Se₈（x=2）；当x>2时，金属-金属键逐渐弥散弱化。金属-金属键的断裂与磁矩的显著提升密切相关，这一现象可通过以下机制阐释：对于x≤2的体系，形成金属-金属键的V₄₋ₓCrₓ四面体簇的小磁矩局域于簇分子轨道中；而富铬化合物的磁学行为则更适合用V₂Cr₂簇与具有更强磁性的非键合Cr原子的混合体系来描述。本研究还以GaV₄Se₈与假想相GaCr₄Se₈作为模型体系，通过DFT计算开展了相关研究。结果显示，GaV₄Se₈的基态包含具有小磁矩的V₄簇单元，而GaCr₄Se₈则无形成强Cr-Cr键的趋势，且磁矩显著更高，上述结果均与实验数据高度吻合。