Hydrogen impurity in yttria: Ab initio and mu SR perspectives


The incorporation of interstitial hydrogen in yttria was studied by means of ab initio calculations based on density-functional theory (DFT) and muonium spin polarization spectroscopy (mu SR). The density-functional calculations, based on a semilocal functional within the GGA-PBE and a hybrid functional, uncovered multiple geometrical configurations for the neutral, H-0, and the negatively charged, H-, states of hydrogen, thus demonstrating the existence of metastable minimum-energy sites. It was observed that the low-energy configurations for H-0 and H- are similar: they prefer to relax in deep, interstitial sites, whereas the equilibrium configurations for the positively charged state, H+, were bond-type configurations with the hydrogen forming a covalent O-H bond with an O anion. For all neutral and negative configurations, localized defect levels were found inside the gap. Overall, the results for the formation energies obtained by the two different functionals are qualitatively similar; an amphoteric behavior was found for hydrogen after considering the lowest-energy structures for each charge state. The calculated acceptor transition level, obtained by the hybrid functional and seen near midgap, is consistent with mu SR data from literature. The results are consistent with the present mu SR data, where the observed diamagnetic signal is attributed to a donor-like muonium at the oxygen-bonded configurations and the paramagnetic signal to an acceptor-like deep muonium at the interstitial sites.



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Silva, EL; Marinopoulos, AG; Vilao, RC; Vieira, RBL; Alberto, HV; Duarte, JP; Gil, JM

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The work was supported by the Program COMPETE: FCOMP-01-0124-FEDER-010450 and by the Portuguese Fundacao para a Ciencia e a Tecnologia (FCT) under Ciencia 2007 and PTDC/FIS/102722/2008 research projects, and under the COMPETE: PEst-C/FIS/UI0036/2011 strategic project. The authors would also like to thank the computing support from the Department of Physics and the Milipeia cluster at the Laboratory for Advanced Computing of the University of Coimbra. Acknowledgments are also to be made to Fernando Nogueira and to the Condensed Matter Group of the Center for Computational Physics for fruitful discussions. The support of the muon teams at ISIS and PSI is also gratefully acknowledged.

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