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First-principles calculations were performed jointly with muon-spin (μSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In1−x,Gax)...

Sapphire $(\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3})$ has been investigated by the muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ method in several experiments in the past. The main $\ensuremath{\mu}\mathrm{SR}$ component is a diamagnetic-like signal with a fast relaxation. Because of this diamagnetic-like behavior, the signal was assigned to either positively charged muonium $({\mathrm{Mu}}^{+})$ or negatively charged muonium $({\mathrm{Mu}}^{\ensuremath{-}})$, but neither of the two assignments was satisfactory (the so-called ``sapphire puzzle''). We have proposed that the signal is due to a weakly paramagnetic muonium configuration (transition state) which is formed during the reaction of muonium with the host lattice. In the present paper, we report new experimental data on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and discuss these and earlier data in the ${\mathrm{Mu}}^{\ensuremath{-}}$ and in the transition state model. Calculations based on density functional theory were also performed with detailed findings on the energetics of the different muonium configurations and their migration energies. We conclude that the transition state model is more plausible than the ${\mathrm{Mu}}^{\ensuremath{-}}$ model, but the ${\mathrm{Mu}}^{\ensuremath{-}}$ interpretation cannot be excluded completely. In addition, the evidence is presented that the bare muon performs local motion but no long-range diffusion below room temperature in the microsecond time range.

Lattice vacancies are a major concern for the use of the Y3Al5O12 garnet (YAG) in optical applications. They are known to trap charge carriers preventing them from reaching luminescence centers. This reduces useful photon emission and deteriorates performance. Recent efforts to characterize such defects include experimental works by positron-annihilation spectroscopy (PAS) where extensive positron trapping was reported and attributed to defects made up of both cation and oxygen vacancies. The present study reports first-principles calculations for monovacancy and divacancy defects in YAG by means of conventional and two-component density-functional theory. The defect formation energies and corresponding charge-transition levels in the gap were initially determined. The ability of the lower-energy defects to act as positron-trapping centers was then examined. Corresponding positron lifetimes and binding energies to defects were calculated and compared to the experimental PAS data. The lifetimes of aluminum monovacancies agreed well with experiment if gradient corrections are included in the electron-positron correlation. Association of oxygen with aluminum vacancies was found to lead to stable negatively-charged divacancy complexes which also trap positrons. These defects are characterized by longer positron lifetimes, in agreement with the experimental observations.

Recent studies by positron-annihilation spectroscopy (PAS) in single-crystal and nanostructured yttria-stabilized zirconia (YSZ) revealed extensive positron trapping at vacancy-type point defects and other structural imperfections. The present work reports first principles calculations of formation energies and positron lifetimes of Zr vacancies in 10.3 mol% cubic YSZ. The lifetime calculations are based on two-component density-functional theory within the local-density approximation for the electron-positron correlation energy. Gradient-correction effects were also examined. Zr monovacancies were found to be potent trapping sites for positrons with large binding energies and lifetimes in excess of 220 ps. A strong dependency of the lifetimes on the Zr site and local coordination number was observed, a consequence of the structural disorder of the stabilized lattice. In contrast, oxygen vacancies did not act as attractive centers for positrons, either as single defects or as nearest neighbors to seven-fold coordinated Zr vacancies. Defect association of the Zr vacancies with hydrogen led to stable ([Formula: see text]-H) complexes with hydrogen bound to oxygen ions which are nearest neighbors to the vacancies. The negatively-charged ([Formula: see text]-H) defects were also found to trap positrons with corresponding lifetimes strongly dependent upon the hydrogen content within the complex.

The formation and migration energies of interstitial hydrogen in rutile TiO2 are obtained from first principles calculations. The computational approach was based on density functional theory with a semilocal generalised-gradient approximation functional, supplemented with an on-site Hubbard term to account for correlation among the Ti 3d electrons. Charge-transition levels are calculated and compared to previous theoretical studies. The donor character of hydrogen is examined in depth, focusing in particular on the tendency to form polaron-like configurations with the unpaired electron trapped at nearby titanium ions. Distinct minimum-energy paths of hydrogen migration and associated energy barriers were determined by the nudged elastic-band method. The present findings show clearly the strong anisotropy in the energy barriers for migration within the open c channels as opposed to migration crossing adjacent channels of the rutile lattice. For the rate-limiting step which leads to macroscopic diffusion along the c axis the corresponding rate and diffusion coefficient were also determined from transition-state theory. The results are discussed in connection to existing measurements of hydrogen diffusion and recent findings from electron paramagnetic resonance, electron-nuclear double resonance and muonium spectroscopies that probed the spatial localization of the electron spin.

The formation and migration energies of interstitial hydrogen in rutile TiO

Early transition metals (TMs) of the 3d and 4d rows are undesired contaminants in solar- and electronic-grade Si. From the theoretical standpoint, understanding the properties of these TMs in silicon still remains a challenging problem owing to the strong correlations among the TM d-electrons. The present study proposes a first-principles Hubbard-corrected DFT+U approach, with on-site parameters accounting separately for electron Coulomb (U) and exchange (J) effects. We use this approach together with conventional DFT to determine electrical levels and migration barriers of early 3d (Ti, V and Cr) and 4d (Zr, Nb and Mo) TMs in Si. Comparisons with experimental data allowed us to uniquely assign the deep levels in the gap appraising also the effect of on-site correlation. Our results also resolve existing controversies in the literature concerning the type and origin of the donor levels of Cr and Mo. For all the metals, with the exception of Cr, high barriers of interstitial diffusion are obtained, thus confirming that most of these TMs are slow diffusers in silicon.

Reliability of battery energy storage systems (BESS) used for online applications, such as electric vehicles and smart grid, depends heavily on the accuracy and rapidness of the state of charge (SOC) estimation. Moreover, to achieve a robust SOC estimation, the battery model parameter identification process is of significant importance. This paper examines a combination of the adaptive unscented Kalman filter (AUKF) and the fast upper diagonal recursive least square (FUDRLS) for the parameter identification and SOC estimation processes, respectively. The analysis focuses on on-line applications and the results are compared with previous work. Experimental validation based on various setups and load conditions is conducted, whereas the advantages of the proposed combination are highlighted.

Over the last decade the use of battery energy storage systems (BESS) on different applications, such as smart grid and electric vehicles, has been increasing rapidly. Therefore, the development of an electrical model of a battery, capable to estimate the states and the parameters of a battery during lifetime is of critical importance. To increase the lifetime, safety and energy usage, appropriate algorithms are used to estimate, with the lowest estimation error, the state of charge of the battery, the battery impedance, as well as its remaining capacity. This paper focuses on the development of model-based online condition monitoring algorithms for Li-ion battery cells, which can be extended to battery modules and systems. The condition monitoring algorithms were implemented after considering an optimal trade-off between their accuracy and overall complexity.

This paper describes a model-based evaluation analysis of grid connected Energy Storage Systems (ESS) that provide a set of grid services: energy arbitrage, distribution investment deferral and frequency regulation. The purpose of this evaluation is to determine the most profitable way to operate an ESS among different investment scenarios and dispatch schedules. Depending on the selected operational profile, revenue streams of the stacked services are aggregated and compared with capital and operating costs over the ESS lifetime, providing whether an investment on ESS is viable or not. Parameters that affect the feasibility of an ESS investment are highlighted. Results indicate that modular investments are prevalent as far as distribution investment deferral is considered, especially in combination with lucrative services as frequency regulation and energy arbitrage.

The increasing penetration of Renewable Energy Sources (RES) and generation uncertainties, brought to the fore new challenges and problems regarding efficient Distribution Networks (DNs) operation. Energy Storage Systems (ESS) can play a significant role in more reliable, secure and flexible DN operation since they can deal with difficult-to-predict changes. This study provides a detailed methodology among the corresponding mathematical formulation for the optimal sizing and allocation of ESS considering optimum operation schedule.

The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy ((Formula presented.) SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E((Formula presented.)), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, (Formula presented.), adopts exclusively hydroxide-bond OH configurations, (Formula presented.) and (Formula presented.) instead show a preference to occupy cage-like interstitial sites in the lattice. (Formula presented.) in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, (Formula presented.) in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all (Formula presented.) states showed that the ground-state interstitial (Formula presented.) configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the (Formula presented.) SR-spectroscopy data. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

Cation vacancies in metal oxides have high formation energies and, thus, are not as abundant as their oxygen-related counterparts. Nonetheless, they can be readily created during non-equilibrium processes. Positron-annihilation spectroscopy (PAS) is a well suited technique of probing such defects since negatively-charged lattice vacancies are deep potential wells which can act as positron traps. The present study reports first-principles calculations of positron trapping at Zr monovacancies in monoclinic zirconia. The binding of positrons and associated lifetimes for these defects were obtained within two-component density-functional theory under different approximations for the electron-positron correlation. The role of hydrogen was also explored. Low-energy vacancy-hydrogen defect complexes were determined for one and two hydrogen atoms bound to a single Zr monovacancy. Hydrogen decoration of the vacancy affected the localization of the positron leading to an appreciable decrease of the lifetime. The study was supplemented with PAS measurements on samples of monoclinic zirconia. From the PAS data two distinct, high-intensity components were resolved originating from different defect states. The corresponding lifetimes of 187 ps and 225 ps were very close to the theoretical predictions.

We present a systematic study of isolated hydrogen in diverse forms of ${\mathrm{ZrO}}_{2}$ (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides $({\mathrm{Y}}_{2}{\mathrm{O}}_{3},{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia $[{A}_{\mathrm{iso}}=3.02(8)$ GHz], but slightly anisotropic in the nanograin yttria-doped zirconia $[{A}_{\mathrm{iso}}=2.1(1)$ GHz, $D=0.13(2)$ GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.

The electronic structure of hydrogen impurity in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ 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 $\text{DFT}+U$ 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 $\text{DFT}+U$ the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ 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).

Owing to their strong interaction with carriers, early $3d\text{-row}$ (Ti, V, and Cr) and $4d\text{-row}$ (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+$U$. The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters $U$ and $J$, respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family.