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

## TL;DR

This paper investigates the magnetic and charge ordering phenomena in Nb2O2F3, revealing an orbital selective regime at high temperatures and a kinetically driven charge disproportionation at low temperatures, with implications for similar dimer-based systems.

## Contribution

It introduces a kinetic energy-driven mechanism for charge ordering in Nb2O2F3, emphasizing the role of molecular orbital formation in structural dimers.

## Key findings

- Magnetic response linked to orbital selectivity above 90 K
- Charge disproportionation occurs at around 90 K with modest electron transfer
- Charge ordering driven by kinetic energy gain in molecular orbital formation

## Abstract

To sum up, we show in the present paper that magnetic response in Nb$_2$O$_2$F$_3$ at the high-temperatures ($T>$90 K) is related to the orbital selective regime, when part of the electrons form molecular orbitals while other electrons have local magnetic moments. The charge disproportionation, which occurs at $T\sim$90 K is seen in the GGA calculations, but its degree ($\delta n \sim 0.1$ electron) is far from what one would expect from naive expectations based on the formal ionic valences. The mechanism of the charge ordering is argued to be related with a sizable kinetic energy gain due to formation of two molecular orbitals in short Nb$^{3+}$-Nb$^{3+}$ dimers caused by a strong nonlinearity of the distance dependence on electron hopping. We think that this mechanism of charge ordering, stabilized not by decrease of interaction energy, but rather by the gain in kinetic energy, may be operative in many other systems, especially consisting of structural dimers.