Hydrogen impurity in yttria:<i>Ab initio</i>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>μ</mml:mi></mml:math>SR perspectives

Abstract

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 ($\ensuremath{\mu}$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${}^{\ensuremath{-}}$, 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${}^{\ensuremath{-}}$ 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 $\ensuremath{\mu}$SR data from literature. The results are consistent with the present $\ensuremath{\mu}$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.