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Hydrogen polaron donor states in oxides: a muSR perspective

Authors: R. C. Vilao, Helena Vieira Alberto, Ricardo Vieira, Joao M. Gil, Alois Weidinger, Roger L. Lichti, P. W. Mengyan, Brittany Baker, A. G. Marinopoulos, J. S. Lord

Ref.: 14th International Conference on Muon Spin Rotation, Relaxation and Resonance (μSR2017) (2017)

Abstract: Hydrogen donor states are modeled experimentally in ZrO and rutile TiO using muonium as a pseudoisotope. For the neutral donorlike configuration at an oxygenbound position the electron is found to be trapped near the hydrogen/muonium nucleus with the corresponding electron density localized at neighboring cations. This particular geometrical arrangement is characterized by a very small hyperfine interaction, dominated by the dipolar component. [1,2] This hyperfine interaction and its temperature dependence were spectroscopically characterized for a monocrystalline sample of TiO and compared to the corresponding EPR/ENDOR data [3], providing a rare direct comparison of the same configuration in muonium and in hydrogen. We additionally present a detailed description of the low temperature bonding configurations. The central part consists of Ti , at which the donor electron is localized on a Ti ion, surrounded by six O atoms. The muon is bound to one of the six oxygens of this TiO complex. However, these bonds are not equivalent. The ground state is formed by bonding to the inplane oxygens only, which lie in the same plane as Ti . In the excited state, jumps to outofplane oxygens of the same TiO octahedron occur. For zirconia, both in the cubic and in the monoclinic form, a similar configuration is identified in polycrystalline samples. However, the hyperfine interaction is too small, preventing the clear observation of its powderpattern spectrum. This is collapsed over the diamagnetic line and manifests itself as an additional broadening. An upper bound for the hyperfine interaction can nevertheless be estimated. The nature of the hyperfine interaction in ZrO can be understood from the abinitio predicted configuration for the neutral oxygen bound state, where the electron associated to the neutral oxygen bound configuration is in fact not centered at the muon/proton, but instead forms a polaronic state with the electron density localized asymmetrically at nearby Zr ions, with an extremely small hyperfine interaction dominated by the dipolar component. Such a geometrical arrangement can lead to a large compensation of the dipolar part from different regions of the electron distribution. [1] R. C. Vilão et al. Phys. Rev. B 92, 081202 (2015) [2] R. B. L. Vieira et al. Phys. Rev. B 94, 115207 (2016) [3] A. T. Brant et al. J. Appl. Phys. 113, 053712 (2013)