Characterizing the Muonium Impurity in Anatase TiO2

In this contribution, we introduce the Muon Spin Rotation, Relaxation and Resonance technique (MuSR) and discuss our current study focused on understanding the characteristics of Muonium–like states (as an analog to isolated Hydrogen) in Anatase Titanium Dioxide (TiO2). MuSR utilizes 100% spin polarized positive muons (charge +e; spin 1/2; mass 1/9 of proton), which upon being implanted in a material, precess in the local environment and decay with a positron emitted preferentially along the spin direction at the time of decay. The time evolution of the muon spin polarization is tracked as an ensemble of these decay events. The muon’s sensitivity to small magnetic field fluctuations and electronic interactions make it a great tool for studying the local environment in bulk materials. In some cases, the muon captures an electron after implantation to form Muonium (Mu): an experimentally accessible analog to an isolated Hydrogen impurity (H). Muonium is a factor of nine lighter than isolated H, but with nearly the same Bohr radius and ground state energy, it behaves very similarly to H. In rutile TiO2 specifically, Mu and H are both found with the same Oxygen bonding configurations and an identical electronic structure [R.C. Vilao, et al. PRB 92 (2015) 081202 ̃ (R)]. The Mu configuration and any associated dynamics (e.g.: charge state cycles, local motion and diffusion) in the similar anatase phase of TiO2 are the focus of this investigation. Understanding H impurities in these materials is important since H is a common and unavoidable impurity that has a very significant effect on the electrical and optical properties of TiO2 [see e.g.: Lavrov et al., Phys Rev B 93 (2016) 045204; Erdal et al., J Phys Chem C 114 (2010) 9139]. TiO2 is of particular interest due to its broad range of applications – some examples are gas–sensing systems, H storage, electrochromic devices and for photocatalysis [See e.g.: Chen et al., Chem Rev 107 (2007) 2891; Diebold, Suf Sci Rep 48 (2003) 53; Zhang et al. Phys Chem Chem Phys 16 (2014) 20382]. Contributing authors: P.W. Mengyan (NMU), R.L. Lichti (Texas Tech University), J.S. Lord (Rutherford Appleton Lab, UK)