Magnetism and charge ordering in high- and low-temperature phases of Nb2O2F3

It is shown using ab initio band structure calculations that Nb2O2F3 is in the orbital-selective regime in the high-temperature phase (T>90K), when two electrons occupy singlet molecular orbital, while the magnetic response comes from the remaining single electron in Nb23.5+ dimer. The charge order occurs at low temperatures, resulting in the formation of Nb3+-Nb3+ and Nb4+-Nb4+ dimers, which makes this system nonmagnetic. The single electron with unpaired spin is transferred to Nb3+-Nb3+ dimer, but due to a strong splitting of the bonding xz/yz molecular orbitals the low-spin state with S=0 is stabilized. We argue that the mechanism of the charge ordering in Nb2O2F3 is the gain in kinetic energy related to the formation of molecular orbitals, which occurs due to a strong nonlinear distance dependence of the hopping parameters.