Year: 2021
Type: article
Abstract: The metal to insulator transition of NbO 2 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 NbO 2 to oxidize into Nb 2 O 5 above 400 °C when exposed to air. Here, we address these issues and employ temperature-dependent X-ray spectroscopy to describe the SPT of NbO 2 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 NbO 2, 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 NbO 2 .
Source: Chemistry of Materials
Language: English
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, U.S. Department of Energy, Fulbright Association, Secretaría de Educación Superior, Ciencia, Tecnología e Innovación, Office of Science, Binghamton University, Leibniz-Gemeinschaft, Division of Materials Research, Basic Energy Sciences, Air Force Office of Scientific Research, Argonne National Laboratory, Brookhaven National Laboratory
Awards: G107546177, G1226239018, G1285306620, G1286251239, G1313983767, G1664757510, G171842289, G1734011414, G1847291448, G1941675983, G2287611682, G241279073, G262962599, G3075337988, G3262563802, G357145406, G426311843, G4574155343, G498139845, G5387317844, G5535608740, G5765906131, NNCI: Cornell Nanoscale Science and Technology Facility (CNF), G6848031779, G6918803902, G6965541988, G7463374569, G7583137537, G874797274, G969889393, G993149817
OpenAlex ID: W3127116596