X-ray absorption linear dichroism at the Ti K-edge of rutile (001) TiO2 single crystal.

T. C. Rossi,J. R. Rouxel,N. Ohannessian,D. Kinschel,M. Chergui,D. Pergolesi,C. Bacellar,D. Grolimund,O. Cannelli,G. F. Mancini

doi:10.1107/s160057752000051x

T. C. Rossi, J. R. Rouxel + Show 8 more

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https://doi.org/10.1107/s160057752000051x

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Abstract

X-ray absorption linear dichroism of rutile TiO2 at the Ti K-edge provides information about the electronic states involved in the pre-edge transitions. Here, linear dichroism with high energy resolution is analyzed in combination with ab initio finite difference method calculations and spherical tensor analysis. It provides an assignment of the three pre-edge peaks beyond the octahedral crystal field splitting approximation and estimates the spatial extension of the corresponding final states. It is then discussed for the first time the X-ray absorption (XAS) of pentacoordinated titanium atoms due to oxygen vacancies and it is found that, similarly to anatase TiO2, rutile is expected to exhibit a transition on the low-energy side of peak A3. Its apparent absence in the experiment is related to the degree of p-d orbital mixing which is small in rutile due to its centrosymmetric point group. A recent XAS linear dichroism study on anatase TiO2 single crystals has shown that peak A2 has an intrinsic origin and is due to a quadrupolar transition to the 3d energy levels. In rutile, due to its centrosymmetric point group, the corresponding peak A2 has a small dipole moment explaining the weak transition. The results are confronted with recent picosecond X-ray absorption spectroscopy on rutile TiO2 nanoparticles.

Highlights

Titanium dioxide (TiO2) is a wide-band-gap insulator which exhibits superior properties in a wide range of applications (Liu & Aydil, 2009; Miyoshi et al, 2018; Gao et al, 2019)
In order to assign the 3d and 4p orbitals contributing to the final state of the pre-edge transitions, we look at the density of states (DOS) in the pre-edge provided by finite difference method near-edge structure (FDMNES) [Fig. S7 (SI)] as well as the expected evolution from the spherical tensor analysis (Table 2)
Rossi et al Linear dichroism at the titanium atom (Ti) K-edge of rutile others with a smaller cluster size in the multiple scattering approach (Jeanne-Rose et al, 1997). This additional peak is consistent with DOS appearing at the Ti L2,3-edge upon formation of oxygen vacancies on the blue side of t2g and egDOS (Vasquez et al, 2016). These results show that a defectrelated resonance due to pentacoordinated Ti atoms may be expected on the low-energy side of peak A3, where the peak A2 is present in another polymorph of TiO2 (a-TiO2), which originates from the relaxed projected DOS of Ti atoms in the vicinity of an Ovac

Summary

Introduction

Titanium dioxide (TiO2) is a wide-band-gap insulator which exhibits superior properties in a wide range of applications (Liu & Aydil, 2009; Miyoshi et al, 2018; Gao et al, 2019). Its natural polymorph in bulk crystals is rutile TiO2 (r-TiO2) in which TiO6 octahedra with D2h point group symmetry are connected to ten neighbouring octahedra via a corner or an edge (Fig. 1). Anatase is another polymorph of TiO2 (a-TiO2) which becomes the most abundant phase in nanomaterials. It is made of TiO6 octahedra but in a different spatial arrangement which generates an indirect band gap, contrary to rutile. Recent progress in the description of the bound excitons spatial extension (Baldini et al, 2017a,b) has shown that experimental techniques which are more sensitive to the local character of photogenerated quasiparticles are required to obtain the fine details of their geometrical and electronic structure

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