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The electronic structure of hydrogen impurity in Lu2O3 was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in f-electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a DFT+U approach which accounts for the on-site correlation of the 4f electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within DFT+U the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in Lu2O3 if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei).

A study based on density functional theory calculations was performed for the three charge states of interstitial hydrogen in Yttria. The present calculations were carried out by employing the GGA-PBE and the HSE06 exchange-correlation functional. It is observed that the ground state H$^0$ and H$^-$ configurations are similar in behaviour; these prefer to relax in the O vacant site, with limited structural relaxation. For the neutral and the negative charged systems more two different geometrical configurations occur with higher energies, which evidence the existence of metastable sites of H in the Yttria lattice. The H$^+$ equilibrium state is found only when a strong bond between the impurity ion and an anion, O-H bond of $\sim$1 \AA, is formed. The formation energy of the interstitial impurity is studied, for which amphoteric behaviour is found for this defect and for the ground state structures. These results evidence that hydrogen counteracts the prevailing conductivity of the host lattice, thus passivating any existing electrical levels originated from other doping sources. To compare results with non-equilibrium and short time-scale $\mu$SR measurements, the formation energy of the higher energy configurations were also evaluated. For the Bond O geometrical site, H evidences a donor-dopant behaviour, hence enabling the coexistence of donor behaviour with the ground-state amphoteric behaviour. From the density of states results, and for the H$^0$ and H$^-$ charge states, a defect level was observed in the band gap. For the ground state Vacant O configuration and the metastable Interstitial structure the defect level was found to be slightly above the valence band maximum, hence indicating no dopant-donor character for the Yttria material. But, for the Bond O, local-minimum, configuration, the defect level is located above the middle of the band gap, closer the conduction band., Comment: 19 pages (one column), 12 figures This paper has been withdrawn by the author because there are major differences between the two versions that are online (title, authors, abstract, body). It is intended to do a new submission of the paper

A combination of transmission electron microscopy (TEM) and local density-functional theory (LDFT) is employed to analyze the microscopic structure of the rhombohedral (-1012) and the prismatic Sigma-3 (10-10) twin interfaces in alpha-alumina. LDFT provides interface energies, atomic and electronic structures for competing structure models. With high-resolution TEM the atomic structure at the interface is imaged quantitatively along two orthogonal zone axes. Electron energy loss spectroscopy in TEM yields the interfacial electronic structure with nano-scale spatial resolution. All experiments confirm the theoretically preferred model for each of the two grain boundaries quantitatively.

Physical Review Letters