Hybrid density functional study of organic magnetic crystals: bi-metallic Cr^III cyanides and rhombohedral C₆₀

Abstract

Periodic hybrid-exchange density functional theory calculations have been used to investigate the magnetic properties of two classes of organic magnets, namely the bi-metallic Cr^III cyanides and the polymerized rhombohedral C₆₀ fullerenes (Rh-C₆₀). For the systems KM^II[Cr^III(CN)₆] with M ^II=V, Mn, Ni and Cr^III[Cr^III(CN)₆], the magnetic ordering energies, Mulliken populations, and spin density plots are reported for the optimized geometries. The qualitative nature of the magnetic coupling mechanism is consistent with that observed in previous unrestricted Hartree–Fock calculations, but the coupling energies computed here are significantly higher. The increased coupling is found to be a result of both changes in the geometry and the electronic structure resulting from the more reliable treatment of electronic exchange and correlation effects. The existence of long-range coupling between local spin moments is investigated in three different defective Rh-C₆₀ structures: (i) a previously proposed prototype structure, where an atom is removed from the C₆₀ cage; (ii) a related structure in which vacancies in nearby cages are brought closer together in pairs; and (iii) a structure where the intra-fullerene bond between the two inter-fullerene bonds is broken spontaneously after applying isotropic pressure to one layer of the Rh-C₆₀ structure. All of these structures are characterized by low flat spin polarized bands at the Fermi edge and localized spin moments around the defects, but no evidence of long-range magnetic coupling is found.

Keywords:

• Organic magnetic crystals
• Hybrid density functional study
• Bi-metallic Cr^III cyanides
• Rhombohedral S₆₀

Acknowledgements

The authors would like to thank the EPSRC for provision of computer time under the Materials Chemistry Consortium project, GR/S13422/01, and also Professor J. Gale for useful discussions.