Title:	First principles calculations of the electronic structure of lithium intercalated oxygen deficient amorphous tungsten and titanium oxides
DNr:	SNIC 2015/1-412
Project Type:	SNIC Medium Compute
Principal Investigator:	Gunnar Niklasson <gunnar.niklasson@angstrom.uu.se>
Affiliation:	Uppsala universitet
Duration:	2015-12-01 – 2016-12-01
Classification:	10304 21001 20599
Homepage:	http://www.teknik.uu.se/fasta-tillstandets-fysik/
Keywords:

Abstract

Stoichiometric amorphous tungsten aWO3, and titanium aTiO2 oxides are key components for technological applications in sensors and electrochromic devices[1,2]. Non-stoichiometric aTiOx have potential applications in memristors [3]. Although the functionality of the amorphous oxides is well known the description on the involved physical processes remains elusive. This is in part due to the difficulty to apply standard methods to the study of amorphous materials. Methods to characterize amorphous solids are limited by their accuracy in describing the structure and electronic properties as a function of disorder. We have been carrying out an integrated study of aWO3 and aTiO2 oxides, which involves experimental measurements and computational simulations. We have used molecular-dynamics to produce amorphous structures and to verify the local-range structural order by direct comparison with X-ray diffraction and X-ray absorption data. However, due to disorder more demanding structural optimizations were performed to describe the experimental data properly. Density functional theory (DFT) was used to describe the electronic and optical properties in these amorphous oxides; specifically hybrid functionals were used. This requires large computational efforts in order to reach a good numerical accuracy. We have now fully characterized the local-structural order and electronic structure of stoichiometric aWO3 and aTiO2 oxides and those results are being written up for publication. We are extending our calculations to the description of electronic properties in oxygen deficient and lithium inserted aLiyWO3 and aLiyTiO2. However, it is of essential importance to carry out simultaneous calculation over several super-cells using different schemes in order to properly reflect the physics of the system. This requires increased computational resources, although we have optimized the simulation and calculation times by using as input pre-optimized structures and single gamma-point calculations. However, introductions of oxygen deficiency and doping make averaging over several structures and calculations necessary in order to accurately describe the electronic states. In this project we will to continue the study of the electronic and optical properties of lithium intercalated oxygen deficient aLiyWO3 and aLiyTiO2 oxides. Knowledge of the representative structure and electronic states makes it viable to extend our analysis to vibrational processes and polaron formation. We have access to experimental vibrational spectroscopy, whereas linear response DFT in this project would give the theoretical picture. This study is crucial to understand the phenomena and valuable to improve technological applications of amorphous transition metal oxides. The aim is to provide a unified description of the relationship between structure, electronic and polaron states, and their role in electronic transport and optical absorption. The access to the SNIC system has been very valuable for our studies. However we need to extend our computational capacity in order to obtain good results for comparison with experimental data. References 1. G.A. Niklasson and C.G. Granqvist, J. Mater. Chem. 17 (2007) 127-156 2. X. Chen, and S.S. Mao, Chem. Rev. 107, 2891 (2007). 3. J.J. Yang, D.B. Strukov, and D.R. Stewart, Nat. Nanotech. 8, 13 (2013)