Abstract
Solid state materials with crystalline order have been well-known and characterized for almost a century while the description of disordered materials still bears significant challenges. Among these are the atomic short-range order and electronic properties of amorphous transition metal oxides [aTMOs], that have emerged as novel multifunctional materials due to their optical switching properties and high-capacity to intercalate alkali metal ions at low voltages. For decades, research on aTMOs has dealt with technological optimization. However, it remains challenging to unveil their intricate atomic short-range order. Currently, no systematic and broadly applicable methods exist to assess atomic-size structure, and since electronic localization is structure-dependent, still there are not well-established optical and electronic mechanisms for modelling the properties of aTMOs. We present state-of-the-art systematic procedures involving theory and experiment in a self-consistent computational framework to unveil the atomic short-range order and its role for the electronic properties. The scheme is applied to amorphous tungsten trioxide aWO3, which is the most studied electrochromic aTMO in spite of its unidentified atomic-size structure. Our approach provides a one-to-one matching of experimental data and corresponding model structure from which electronic properties can be directly calculated in agreement with the electronic transitions observed in the XANES spectra.
Highlights
Amorphous transition metal oxides [aTMOs] are key components in optoelectronics, sensors, photoelectrochemical conversion, energy/data storage and emerging water splitting applications, where extensive research on their technological optimization has been performed[1]
From RMC-EXAFS simulations we show that the disordered structure of aWO3 comprises mainly corner-sharing and a small proportion of edge-sharing distorted WO6,5,4-unit-blocks, while the O atoms hold nearly two-fold coordination with W atoms
Since the STF approach does not provide any 3D-structure of aWO3, the experimental k3χ(k) spectrum was compared against the ab-initio MD-EXAFS, k3χ(k)MD function, calculated directly from MD structural trajectories of aWO3
Summary
Spectral fitting from real-space position oRfMthCe-fEirXstA[FWS-rOef]inaenddFsTecko3nχd(kc)oRoMrdCitnoatthioenexshpeelrlism[Wen-tWal F],Tank3dχt(hke) spectra reproduces multiple-scattering contributions observed in the experimental spectra These results confirm that the atomic short-range order of aWO3, can be properly extracted through RMC-EXAFS simulations based on ab-initio FMS approaches [Fig. 1b, Real and Im components at the bottom]. We use the RMC-EXAFS optimized structures of aWO3 to assess the correlations between atomic short-range order and the electronic properties by detailed calculations of the electronic structure by hybrid density functional theory [DFT]29, and electronic transitions associated to the XANES spectra by ab-initio finite difference methods [FDM]30 [see Methods]. The relative intensity, energy position, line shape and the electronic transitions in the experimental spectrum are well reproduced by the FDM-XANES function calculated from the RMC-EXAFS optimized structures of aWO3. The approach provides a one-to-one matching of experimental data and corresponding model structure from which electronic properties can be directly calculated in agreement with the electronic transitions giving rise to the XANES spectrum of aTMOs
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