Charge localization in the Verwey structure of magnetite
Mark S. Senn, Jon P. Wright, James Cumby, and J. Paul Attfield
Phys. Rev. B (2015) doi: PhysRevB.92.02410
Magnetite (Fe3O4) undergoes a complex electronic ordering phase transition at ~125 K known as the Verwey phase transition. The crystal structure – which at high temperatures may be described by only one internal degrees of freedom – is immensely complicated at low temperatures consisting of a monoclinic structure with 168 independent refinable parameters. Our previous results show how some sense can be made of this madness in terms of a physical model involving charge ordering of the Fe2.5+; spinel B-sites at high temperatures to Fe2+/Fe3+ at low temperatures and associated orbital and “molecular-like” ordering. However, a crystal structure is just a single snap shot at a given temperature, to understand what the driving force for a phase transition is, you need a crystal structure determined as a function of temperature across the phase transition. This is exactly what we have done for Magnetite, 20 crystal structures in the temperature range 124 to 20 K, each with 506 refinable parameters distilled down into one paper for your reading pleasure!
Our results show that the Verwey phase transition is strongly first order and is almost in the “frozen phonon” limit with only small additional rearrangements of atomic displacements occurring below the phase transitions temperature. However, these small additional rearrangements corresponds to an increase in the “molecular-like” ordering of the Fe2+/Fe3+ sites as a function of temperature suggesting that this is the true driving force for the Verwey phase transition. And all this is only possible thanks to the ultra-high resolution single crystal diffraction data collected at ID11, ESRF giving us a reproducible real space resolution of the order of a thousandths of an Angstrom!