posted on 2021-02-03, 15:06authored byGalo Jaime Páez Fajardo, Sebastian A. Howard, Egor Evlyukhin, Matthew J. Wahila, Wasim Raja Mondal, Mateusz Zuba, Jos E. Boschker, Hanjong Paik, Darrell G. Schlom, Jerzy T. Sadowski, Samuel A. Tenney, Benjamin Reinhart, Wei-Cheng Lee, Louis F. J. Piper
The
metal to insulator transition of NbO2 has been predicted
to be a result of a structural phase transition (SPT) governed by
Peierls physics. However, direct observation of the SPT using experimental
techniques is still restricted by the extremely high transition temperature
(810 °C) and the proclivity for NbO2 to oxidize into
Nb2O5 above 400 °C when exposed to air.
Here, we address these issues and employ temperature-dependent X-ray
spectroscopy to describe the SPT of NbO2 from the bulk
to surface. Temperature-dependent extended X-ray absorption fine structure
spectroscopy (T-EXAFS) reveals a gradual weakening of the bulk Nb
dimers over a large temperature range, which is indicative of a second-order
Peierls mechanism. From these measurements, we determine the critical
dimer distance to be 2.77 Å. Our T-EXAFS observations are supported
by density functional theory of the phonon dispersion and the electronic
density of states of NbO2, which conclude that the dimerization
is responsible for the insulating phase. The dimerization does not
extend to the topmost layers, where an oxygen rich surface reconstruction
is preferred irrespective of temperature even in extremely reducing
environments; changes in the low-energy electron diffraction patterns
are attributed to oxygen concentration and are independent of the
underlying bulk phase transitions of NbO2.