American Chemical Society
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Structural Phase Transitions of NbO2: Bulk versus Surface

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journal contribution
posted on 2021-02-03, 15:06 authored by Galo 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.