Abstract
We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α−MoCl4 at pressure P ~ 11 GPa.
Highlights
If there are more than one electron per site and tc is large enough, the bonding molecular orbital is fully occupied and corresponding electrons do not contribute to the total magnetic moment of the dimer[17]
Increasing pressure we induce magnetic transition, as it was described above in details. We studied this transition by the total energy (E) generalized gradient approximation (GGA) +U calculations for ferromagnetic and nonmagnetic configurations for several volumes (V)
To sum up in the present paper we considered the dimerised transition metal compounds with degenerate half-filled magnetic molecular orbitals and showed that the pressure-induced magnetic transition is possible in this case
Summary
If there are more than one electron per site and tc is large enough (with respect to Hund’s rule coupling JH), the bonding molecular orbital is fully occupied and corresponding electrons do not contribute to the total magnetic moment of the dimer[17]. Qualitative picture is rather general: having magnetic dimerised system with few degenerate or nearly degenerate half-filled d molecular orbitals one may expect to have a transition to nonmagnetic state under external or due to internal (chemical) pressure. Bonding xy orbitals are fully occupied and this explains experimentally observed partial suppression of the magnetic moment in this system at ambient conditions This strong splitting ~3.2 eV is clearly seen from the nonmagnetic band structure (Fig. 3(a)), obtained in the generalized gradient approximation (GGA). This strongly reminds pressure-induced transition in α−MoCl4 discussed in the present paper
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