Ambipolar Property of Isolated Hydrogen in Oxide Materials Revealed by Muon

Abstract

The study on the electronic state of muon as pseudo-hydrogen (represented by the elemental symbol Mu) by muon spin rotation has long been appreciated as one of the few methods to experimentally access the electronic state of dilute hydrogen (H) in semiconductors and dielectrics. Meanwhile, theoretical predictions on the electronic state of H in these materials by first-principles calculations using density functional theory (DFT) do not always agree with the observed states of Mu, standing as an obstacle to integrating the findings of both Mu and H. In order to address this issue, we have re-examined the vast results of previous Mu studies in insulating/semiconducting oxides by paying special attention to the non-equilibrium character and the ambipolarity of Mu. As a result, we established a semi-quantitative model that enables systematic understanding of the electronic states of Mu in most oxides. First of all, Mu often occurs simultaneously in a neutral (Mu 0 ) and a diamagnetic state (Mu + or Mu − ) in wide-gap oxides, while DFT calculations predict that H is stable only for one of the charged states with the polarity determined by the equilibrium charge-transition level (E +/− ). Our model considers that Mu can form complex defects with both oxygen (Mu D ) and cations (Mu A ), resulting in a relaxed-excited state accompanying respective donor or acceptor levels (E +/0 or E −/0 ) corresponding to those predicted by the DFT calculations for H, and that their initial yields are determined by the interaction with Muinduced excitons. Furthermore, by introducing the assumption that the stability of these two states including their valence is determined by i) the relative position of E ±/0 in the energy band structure of the host and ii) a potential barrier associated with the transition between Mu D and Mu A , we find that the known experimental results can be explained systematically in accordance with E ±/0 . The model also reveals some common properties of Mu-related defects which were previously regarded as individual anomalies. One is the polaron-like nature of the electronic states associated with shallow donor Mu complexes, for which we argue a Copernican shift of the viewpoint to the Mu + D -bound excitons. Another is the fast diffusion of Mu 0 A , which is understood by the isolated feature of the acceptor-like electronic states. The possible impacts of these findings to a wide range of insulating compounds is discussed by drawing on examples including GaN, FeS 2 , and NaAlH 4 as key materials for the green technologies.

Local electronic structure of interstitial hydrogen in MgH2 inferred from muon study

“…A similar situation is suggested from an earlier µSR study for the effect of Ti-doping to NaAlH 4 [42,43] Regarding the Mu − KT state corresponding to H − i , the magnitude of ∆ KT is found to vary little over the entire temperature range, indicating that the Mu site remains unchanged over the time range of ∼10 1 µs. The fluctuation frequency ν KT shows nearly constant value of ∼0.1 MHz below 150 K, which is unlikely to be attributed to thermally activated diffusive motion of Mu − KT .…”