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 (μSR). 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 μSR data from literature. The results are consistent with the present μSR 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.

Although spectroscopic investigation of surface chemisorbed CO 2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO 2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO 2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13 C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times ( T 1 ). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO 2 signals, which contributed to an empirical model of CO 2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T 1 values confirm the presence of CO 2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO 2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13 C-labeled CO 2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO 2 species, showing that 45% of chemisorbed CO 2 versus 55% of physisorbed CO 2 is formed from the overall confined CO 2 in amine-modified hybrid silicas. A total of six distinct CO 2 environments were identified from which three physisorbed CO 2 were discriminated, coined here as “gas, liquid, and solid-like” CO 2 species. The complex nature of physisorbed CO 2 in the presence and absence of chemisorbed CO 2 species is revealed, shedding light on what fractions of weakly interacting CO 2 are affected upon pore functionalization. This work extends the current knowledge on CO 2 sorption mechanisms providing new clues toward CO 2 sorbent optimization.

Muonium, a positive muon and an electron, is often used as an experimentally accessible substitute for hydrogen in materials research. In semiconductors and insulators, a large amount of information on the hydrogen behavior is deduced from this analogy; however, it is seldom demonstrated that this procedure is justified. We show here, via a comparison of the hyperfine interactions, that in TiO 2 muonium and hydrogen form the same configuration with the same basic electronic structure. A detailed description of the bonding characteristics of the muon to the Ti 3+ polaron is presented. The special role of muon motion within the so-called oxygen channel in the rutile structure, which occurs at a lower temperature than for hydrogen, is emphasized. Muonium (Mu) is a pseudoisotope of hydrogen in which the proton is replaced by a positive muon (μ + ), with a factor of 9 lighter mass. Muon spin spectroscopy (μSR) uses muons implanted with 100% spin polarization and offers a very sensitive method to study the properties of this isolated pseudohydrogen in solids [1,2]. It is usually assumed that information obtained from μSR can be transferred with appropriate modifications to H. However, overlapping experiments to support this assumption are scarce. A particularly relevant case is the doping character of H in semiconductors and oxides [3][4][5][6], where practically all calculations refer to the electronic structure of H whereas most experimental information comes from μSR [7][8][9][10][11][12][13]. Overlapping data exist only for ZnO where proton-ENDOR (electron-nuclear double resonance) data [14] can be compared directly with μSR results [15][16][17][18]. A number of properties (e.g., ionization energy) are indeed similar for the two species. However, the measured hyperfine interaction (hfi), scaled with the magnetic moments, differs by almost a factor of 10. This raised the question of whether the same configuration is measured, or if the H center may involve an additional defect [19,20].Here, we report a case where the same configuration can be established for H and Mu. We compare the hfi of the μSR experiment with the proton-ENDOR result, both for rutile TiO 2 . The H center in TiO 2 was extensively studied by Brant et al.[21] using electron paramagnetic resonance (EPR) and ENDOR, who found that the electron is located at the Ti ion reducing it from Ti 4+ to Ti 3+ . H is bound to one of the six O atoms surrounding Ti and the magnetic interaction between the proton and the electron is mainly dipolar. This specific hfi permits a sensitive comparison of the two experiments. We have observed a dramatic change of the μSR spectra with increasing temperature and a complete disappearance of the hyperfine splitting at 10 K. We show that this is due to rapid jumps of the muon between neighboring bonding positions to O atoms around Ti 3+ . The very strong angle dependence of the dipolar interaction and the averaging over values in different * ruivilao@fis.uc.pt positions lead to the reduction and final disappearance of the hfi...

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In muon-spin-rotation experiments, positive muons are implanted in the material and come to rest in the unrelaxed host lattice. The formation of the final configuration requires a lattice relaxation which does not occur instantly. The present paper is concerned with the transition from the initial stopping state to the final muon configuration. We identify the often observed fast relaxing signal in muon experiments (e.g., in several oxides studied recently) with the transition state in this conversion process. This state is paramagnetic with a small hyperfine interaction (in the order of MHz) which fluctuates and averages to almost zero. Because of its apparent diamagnetic frequency behavior, the fast signal was in the past assigned to Mu + or Mu − . We present evidence that this state is actually paramagnetic. The model presented in this paper is of importance for the interpretation of past and future μSR measurements.