Hydrogen bistability in non-magnetic oxides

Recent extensive muonium spectroscopy measurements ($\mu $SR) [1] suggest a general correlation between stable muonium (Mu) forms in non magnetic oxides and their band gap, E$_{g}$: in the materials with Eg$<$4 eV, Mu is, as a rule, fully ionized; in the oxides with Eg$>$7 eV, most of the Mu exists in the atomic form, while both ionized and atomic forms of Mu coexist in the oxides with the band gap in the interval 4$<$Eg$<$7 eV. Based on the analogy between muonium and hydrogen, we propose a general model which may explain such a correlation and makes important predictions about hydrogen behavior in non-magnetic oxides. As an example, we use ab initio calculations to extract the parameters for this model to explain difference in hydrogen behavior in HfO$_{2}$, ZrO$_{2}$ films and in their silicates at low hydrogen concentrations [2]. We further discuss the role of hydrogen in mechanisms of tetragonal to a monoclinic phase transformations in ZrO$_{2}$ and HfO$_{2}$. 1. S.F.J. Cox, J.L. Gavartin, J.S. Lord et al. J. Phys.~: Condens. Matter, 2006 (in press). 2. R.P. Pezzi, L. Miotti, K.P. Bastos \textit{et al}. \textit{Appl. Phys. Lett.} \textbf{85} 3540 (2004).