Magnesium Vacancies Research Articles

New insights from DFT analysis: Mg vacancies and hydrogen doping in enhancing thermodynamic properties and hydrogen storage performance of Li2BeMgH6

To identify effective hydrogen storage materials, a novel study employing first-principles calculations based on density functional theory DFT on the lithium-based double perovskite hydride Li2BeMgH6. This material, with its innovative and significant attributes, emerges as a potential candidate for hydrogen storage, inspiring experimental researchers to pursue its synthesis for the experimental fabrication of hydrogen storage devices. Notably, it boasts a remarkable gravimetric capacity of 11.35 wt%, coupled with exceptional stability and desorption temperature, surpassing the standards set by the U.S. Department of Energy (DOE) for solid-state hydrogen storage (ΔH = −40 kJ/mol.H2 and Td = 289 K–393 K), with respective values of ΔH = −77.37 kJ/mol H2 and Td = 591 K for the cubic phase and ΔH = −84.64 kJ/mol H2, Td = 647 K for the rhombohedral structure. This comprehensive research aims to demonstrate the thermodynamic stability of this innovative material and improve its hydrogen storage properties in line with DOE requirements. The results obtained highlight the significant impact of two strategies for enhancing hydrogen storage properties in two stable phases (face-centered cubic (fcc) and rhombohedral (R3)): the creation of magnesium vacancy defects and hydrogen doping in the Mg vacancies in the studied system. The first strategy, involving the generation of 6 % defects in the Mg atoms, resulted in a notable improvement in storage properties, with a gain of 50.75 % for the cubic structure and 50.46 % for the rhombohedral structure. The second strategy, which consists of doping the vacant Mg sites with hydrogen, also demonstrated remarkable efficiency, leading to an increase of 50.05 % and 53.07 % in storage performance for the two phases, respectively.

Efficient Fabrication of High‐Density Ensembles of Color Centers via Ion Implantation on a Hot Diamond Substrate

AbstractNitrogen‐vacancy (NV) centers in diamonds are one of the most promising systems for quantum technologies, including quantum metrology and sensing. A promising strategy for the achievement of high sensitivity to external fields relies on the exploitation of large ensembles of NV centers, whose fabrication by ion implantation is upper limited by the amount of radiation damage introduced in the diamond lattice. In this work an approach is demonstrated to increase the density of NV centers upon the high‐fluence implantation of MeV N2+ ions on a hot target substrate (>550 °C). The results show that with respect to room‐temperature implantation, the high‐temperature process increases the vacancy density threshold required for the irreversible conversion of diamond to a graphitic phase, thus enabling to achieve higher density ensembles. Furthermore, the formation efficiency of color centers is investigated on diamond substrates implanted at varying temperatures with MeV N2+ and Mg+ ions revealing that the formation efficiency of both NV centers and magnesium‐vacancy (MgV) centers increases with the implantation temperature.

Visible-to-Near-Infrared Mechanoluminescence in Bi-Activated Spinel Compounds for Multiple Information Anticounterfeiting.

Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu3+/Eu2+, Sm3+/Sm2+, and Mn4+/Mn2+. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis-NIR) ML induced by the self-reduction of Bi3+ to Bi2+ in the spinel-type compound (MgGa2O4) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi3+/Bi2+ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (VMg″) is considered as the driving force for the self-reduction of Bi3+ to Bi2+. Furthermore, an oxygen vacancy (VO••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron-hole (VO••/VMg″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis-NIR ML materials toward wider application possibilities.

Investigating the Impact of Displacement Cascades on Tritium Diffusion in MgT2: A Molecular Dynamics Study.

Molecular dynamics methods were utilized to investigate displacement cascades and tritium diffusion in α-MgT2. It was observed from collision cascades results that the stable number of defects weakly depended on temperature, while the peak and stable number of defects linearly increased with increasing the primary knock-on atom energy. The results of the mean square displacement study revealed that defects had a significant impact on tritium diffusion. The clustering of magnesium self-interstitial atoms and diffusing tritium atoms results in an increased diffusion barrier, whereas the formation of clusters between tritium interstitial atoms is relatively difficult and has no significant impact on the diffusion barrier. The presence of magnesium and tritium vacancies has a minimal effect on the diffusion barrier due to the large number of diffusing tritium atoms that offset the adsorption of vacancies on diffusing atoms. Both magnesium and tritium interstitial atoms increase the collision probability of diffusing atoms, leading to an increased diffusion prefactor. Magnesium vacancies cause significant lattice distortion, increasing the diffusion barrier, while the impact of tritium vacancies on the diffusion barrier is small due to their minimal lattice distortion effect. The research uncovered significant disparities in the diffusion properties of hydrogen and tritium, indicating that the results of the study of hydrogen storage could not be applied to tritium.

Improvement of decomposition temperature and gravimetric density of [formula omitted] by transition metals and vacancies: A comparison study

An ab initio study is investigated to study the effects of transition metals and vacancies on the thermodynamic characteristics of magnesium hydride (MgH2) and compare the calculation results in terms of the decomposition temperature and the gravimetric density. The Green's function method is used to perform our calculations. In particular, the magnesium vacancies facilitate faster diffusion of hydrogen atoms within the MgH2 while the transition metals weaken the strong band between hydrogen and magnesium, which both improve the decomposition temperature. The creation of vacancies is more favorable than doping with transition metals in MgH2, with a concentration range of vacancies from 3.3 to 4.6% for practical use. In addition, the vacancies allow increasing the gravimetric density from 7.658 to 8.269 wt%, while using the transition metals reduces the density from 7.658 to 7.064 wt% for Mg1-xCrxH2 and 7.658 to 7.006 wt% for Mg1-xMnxH2.

UV/blue/green converted efficient red-NIR photoluminescence in Cr incorporated MgAl2O4 nanocrystals: Site selective emission tailored through cation inversion and intrinsic defects

The UV/visible activated near-infrared (NIR) phosphors have many applications in solid state lighting, night vision devices and bio-imaging. The early research reported the red-NIR emitting phosphors doped with Cr3+ centers upon visible light excitation. Here, in this work the intense red-NIR emission and color tuning is achieved for broad excitation range (UV/blue/green) through Cr dopant induced defect centers and cation inversion in MgCr Al2O4 ( = 0.5, 1, 3, 5 and 10 mol%) nanocrystals. The Cr3+ doped MgAl2O4 nanocrystals were synthesized by combustion method through stoichiometric substitution of Mg by Cr, while most of the Cr3+ ions occupied the octahedral sites of spinel host with the formation of antisite defects, Cr3+ clusters, magnesium and oxygen vacancies. These defect centers were probed through Rietveld refinement, photoluminescence (PL), x-ray photoelectron and nuclear magnetic resonance spectra analyses. At UV excitation, the intrinsic defects played an interesting role in exhibiting the blue-violet emission attributed to host lattice defects and red-NIR emission attributed to strong/weak ligand field octahedral Cr3+ sites, via charge transfer to Cr3+ ions. The PL spectra evinced the enhanced red-NIR emission intensity upon 266 nm excitation than upon blue and green light excitation. Further, the weak ligand field site emission is found to be dominating with increase in doping concentration. Thus, Cr doped MgAl2O4 nanocrystals showed their potency of exhibiting the intense red-NIR emission and color tuning (from red purple to bluish purple and then to red color) upon UV/blue/green excitation.

CO adsorption on MgO thin-films: formation and interaction of surface charged defects.

Two-dimensional (2D) materials formed by thin-films of metal oxides that grow on metal supports are commonly used in heterogeneous catalysis and multilayer electronic devices. Despite extensive research on these systems, the effects of charged defects at supported oxides on surface processes are still not clear. In this work, we perform spin-polarized density-functional theory (DFT) calculations to investigate formation and interaction of charged magnesium and oxygen vacancies, and Al dopants on MgO(001)/Ag(001) surface. The results show a sizable interface compressive effect that decreases the metal work function as electrons are added on the MgO surface with a magnesium vacancy. This surface displays a larger formation energy in a water environment (O-rich condition) even with additional Al-doping. Under these conditions, we found that a polar molecule such as CO is more strongly adsorbed on the low-coordination oxygen sites due to a larger contribution of the channeled electronic transport with the silver interface regardless of the surface charge. Therefore, these findings elucidate how surface intrinsic vacancies can influence or contribute to charge transfer, which allows one to explore more specific reactions at different surface topologies for more efficient catalysts for CO2 conversion.

Role of vacancies and transition metals on the thermodynamic properties of MgH2: Ab-initio study

Using the Korringa – Kohn – Rostoker method, the effect of doping with transition metals and creating magnesium vacancies into MgH2 is investigated ab to improve its thermodynamic properties. The results indicate that the heat of formation increases with increasing the concentration of magnesium vacancies and transition metals and vice versa for the decomposition temperature and stability. In particular, the density of states shows that the decrease in the stability as a function of the concentrations can be explained by the fact that there is a weak hybridization between the doped elements and hydrogen atoms, unlike the pure magnesium hydride, which is mainly composed of a strong hybridization between the hydrogen and magnesium atoms. In addition, the decrease in the number of magnesium elements allows the hydrogen atoms to move faster in the structure. This improves the thermodynamic properties of MgH2 by decreasing the temperature of decomposition and increasing the enthalpy of formation. Moreover, the optimal concentrations (x + y) for practical use are 6.1% for Mg1-x-yMnxH2, 6.4% for Mg1-x-yCrxH2, and 7.1% for Mg1-x-yVxH2.

Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2.

Case studies of 1T-TiSe2 and YBa2Cu3O7-δ have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB2 lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB2 in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the Tc of approx. 39 K. The high-resolution single-crystal XRD experiments in combination with multipolar refinements presented here can reproduce these results but show that the observed temperature-dependent density changes are almost entirely due to a decrease of atomic displacement parameters as a natural consequence of a reduced thermal vibration amplitude with decreasing temperature. Our investigations also shed new light on the presence or absence of magnesium vacancies in MgB2 samples─a defect type claimed to control the superconducting properties of the compound. We propose that previous reports on the tendency of MgB2 to form non-stoichiometric Mg1-xB2 phases (1 - x ∼ 0.95) during high-temperature (HT) synthesis might result from the interpretation of XRD data of insufficient resolution and/or usage of inflexible refinement models. Indeed, advanced refinements based on an Extended Hansen-Coppens multipolar model and high-resolution X-ray data, which consider explicitly the contraction of core and valence shells of the magnesium cations, do not provide any significant evidence for the formation of non-stoichiometric Mg1-xB2 phases during HT synthesis.

Effect of hydrogen on magnetic properties in MgO studied by first-principles calculations and experiments

We investigated the effects of both intrinsic defects and hydrogen atom impurities on the magnetic properties of MgO samples. MgO in its pure defect-free state is known to be a nonmagnetic semiconductor. We employed density-functional theory and the Heyd–Scuseria–Ernzerhof (HSE) density functional. The calculated formation energy and total magnetic moment indicated that uncharged {mathrm{V}}_{mathrm{Mg}}^{0} and singly charged {mathrm{V}}_{mathrm{Mg}}^{-1} magnesium vacancies are more stable than oxygen vacancies (VO) under O-rich growth conditions and introduce a magnetic moment to MgO. The calculated density of states (DOS) results demonstrated that magnetic moments of VMg result from spin polarization of an unpaired electron of the partially occupied valence band, which is dominated by O 2p orbitals. Based on our calculations, VMg is the origin of magnetism and ferromagnetism in MgO. In contrast, the magnetic moment of the magnetic VMg-MgO crystal is suppressed by hydrogen (H) atoms, and unpaired electrons are donated to the unpaired electronic states of VMg when the defect complex Hi-VMg is formed. This suggests that H causes a reduction in magnetization of the ferromagnetic MgO. We then performed experimental studies to verify the DFT predictions by subjecting the MgO sample to a thermal treatment that creates Mg vacancies in the structure and intentionally doping the MgO sample with hydrogen atoms. We found good agreement between the DFT results and the experimental data. Our findings suggest that the ferromagnetism and diamagnetism of MgO can be controlled by heat treatment and hydrogen doping, which may find applications in magnetic sensing and switching under different environmental conditions.

Direct measurements of space-charge-regions in individual nanometer-scale crystals of Al2O3-rich magnesium-aluminate-spinel using off-axis electron holography and electron energy-loss spectroscopy

Space charge regions (SCR) in ionic solids, a result of an uneven distribution of point defects, alter their thermodynamic, transport, mechanical, and optical properties.We examine the formation of SCR in nanometer-scale sized crystals of magnesium-aluminate-spinel with excess Al2O3, MgO·1.27Al2O3. For this composition, the expected dominant point defects are anti-sites and cation vacancies.We determine the types, distribution and formation energies of charged point defects that form the SCR. Off-axis electron holography and electron energy-loss spectroscopy enabled to measure the electrostatic potential and distribution of Al·Mg anti-site charged point defects, respectively in individual crystals.We extract the width of the SCR, LSCR, between 1.7 and 2.1 nm. We measured the formation energy of charged vacancies following two extreme states, only aluminum or magnesium vacancies resulting in EfV″′Al=1.09±0.16eV and EfV″Mg=1.04±0.19eV, respectively.For crystals considerably larger than LSCR, V'''Al and VʺMg cation vacancies are the majority species at the particle core resulting in a negative charge density. When approaching the surface of the crystal, the space charge region is richer in Al·Mg defects.Finally, we examine the SCR as the crystal size is reduced, approaching LSCR. Measurements at particle cores of these crystals results in a positive charge due to the overlap of the SCR.

Highly tunable magneto-optical response from magnesium-vacancy color centers in diamond

Defect quantum bits (qubits) constitute an important emerging technology. However, it is necessary to explore new types of defects to enable large-scale applications. In this article, we examine the potential of magnesium-vacancy (MgV) in diamond to operate as a qubit by computing the key electronic- and spin properties with robust theoretical methods. We find that the electronic structure of MgV permits the coexistence of two loosely separated spin-states, where both can emerge as a ground state and be interconverted depending on the temperature and external strain. These results demonstrate a route to control the magneto-optical response of a qubit by modulating the operational conditions.

Magnesium vacancies and hydrogen doping in [formula omitted] for improving gravimetric capacity and desorption temperature

Performing an ab initio analysis, we inspect the effect of magnesium vacancies and hydrogen doping on the magnesium hydride (MgH2). The Korringa – Kohn – Rostoker method integrated with the coherent potential approximation is used to perform our calculations. In particular, we find that the gravimetric capacity of MgH2 increases from 7.658 to 9.816 wt% when the concentrations of magnesium vacancies and hydrogen dopant atoms increase from 0 to 10%. Concretely, the results reveal that the magnesium vacancies and the hydrogen doping have a beneficial effect on the hydrogen storage properties of the hydride by decreasing its desorption temperature and stability. This decrease can be explained on the one hand by the diminution of the number of Mg atoms that establish strong bonds with H atoms, and on the other hand by using the density of states, which indicates that when the concentrations increase, the Mg and H states shift to the conduction band. We also obtain that the value of the desorption temperature can be controlled by varying the concentrations of magnesium vacancies and hydrogen dopant atoms from 4.2 to 5.8% in order to reach the optimum range 289–393 K for the practical use of fuel cell vehicles.

High Efficiency Emission of Eu2+ Located in Channel and Mg‐Site of Mg2Al4Si5O18 Cordierite and Its Potential as a Bi‐Functional Phosphor toward Optical Thermometer and White LED Application

AbstractEu2+ activated Mg2Al4Si5O18 cordierite phosphor is prepared by sol gel synthesis. The reduction process is carried out in a vacuum atmosphere. X‐ray diffraction, luminescence, decay times, quantum yield and emission thermal stability are studied. The emission from Eu2+ ions at different lattice sites, such as magnesium vacancies and inside the interstate channels, are identified and discussed. Considerations on crystal field strength and bond length allow to assign the blue emission band peak at 445 nm to Eu2+ ions located inside the interstitial channel, while the red band centered at 615 nm is related to Eu2+ ions on Mg site. It is found that the Mg2Al4Si5O18:Eu2+ phosphor excited by UV shows broad emission from 375 to 700 nm with high quantum efficiency (60%). The emission of Eu2+ in Mg2Al4Si5O18 has very good thermal stability, at 100 °C the intensity reaches 97% relative to the emission measured at room temperature. The investigations show that cordierite can be used as a temperature sensor with relative sensitivity of 0.45% at 400 °C. Moreover, the fabricated light‐emitting diode composed of 365 nm near‐UV chip and Mg2Al4Si5O18:Eu2+ phosphor exhibits bright white light with CIE coordinates (0.32, 0.34) for correlated color temperature of 6282 K and color rendering index of 82.

Study of cation vacancies with localized hole states in MgAl2O4 crystals

The density functional theory with the Hubbard U parameter method was used to evaluate the lattice structure, electronic structures, and optical spectra of MgAl2O4 crystals with cation vacancies. We analyzed the electronic structures of oxygen atoms around the cation vacancies. The holes prefers to distribute themselves on all the nearest oxygen atoms around the vacancy rather than to localize on a single oxygen atom. We propose the color center models VMg2−−[O23−]2+ and VAl3−−[O23−]3+ for the magnesium vacancy and the aluminum vacancy with localized hole states. Taking into account electron-phonon coupling, we obtained the optical spectra of the cation vacancies with localized hole states. The calculated absorption peaks of the cation vacancies with localized hole states are consistent with the experimental results. The emission peaks of the magnesium vacancy and the aluminum vacancy with localized hole states are at 2.63 eV and 2.62 eV, close to that of the F center at 2.69 eV. We predict that cation vacancies with localized hole states will act as a source of blue luminescence.

Possible origin of ferromagnetism in pristine magnesium oxide film

The electronic structure and magnetism of pristine magnesium oxide (MgO) (001) surface have been studied using first principle calculations based on density functional theory (DFT). Our results reveal that isolated magnesium (Mg) vacancy (VMg) can produce a magnetic moment of about 1.99 μB. The magnetic moment mainly comes from p-orbitals of oxygen atoms adjacent to VMg. The surface with one VMg will show half-metallic properties. Studies of magnetic coupling show that the surface with two VMg's will be in a spin singlet state (S = 0) at the distance of 2.98 or 8.42 Å between two VMg's, while the two VMg's are coupled ferromagnetically at the distance of 4.21, 5.96, or 6.66 Å. As the distance between two VMg's increases, the system will change from semiconductor state to half-metallic state, and finally to metal state. These findings are significant for d0 ferromagnetism in MgO film and useful for spintronic application.

Low-Temperature Methane Oxidation Triggered by Peroxide Radicals over Noble-Metal-Free MgO Catalyst.

Methane is a greenhouse gas that contributes to global warming. Hence, effectively removing the low concentration (<1000 ppm) of methane in the environment is an issue that deserves research in the field of catalysis. In this study, oxygen-magnesium bivacancies are simultaneously imbedded into MgO by designing an in situ reduction combustion atmosphere for oxygen release and substituting magnesium with carbon to induce the formation of magnesium vacancies. The DFT calculations reveal that the surface electron density of MgO is improved by the oxygen vacancy structure and the substitution of Mg by C in bulk; this accelerates migration of the charge from the material surface to the adsorbed oxygen species, which leads to abundant surface peroxide species that enable activation and oxidation of methane at a low temperature (below 200 °C). This work could provide a concept for developing non-noble or transition metal oxides for low-temperature activation and conversion of alkanes in the thermocatalytic field through reactive oxygen species.

Improving desorption temperature and kinetic properties in MgH2 by vacancy defects: DFT study

The aim of this work is the improvement of the desorption temperature and kinetic properties in MgH2 by magnesium vacancy defects VMg. We use the Korringa – Kohn - Rostoker (KKR) calculation combined with the coherent potential approximation (CPA) in this work. In particular, we find that the formation energy increases with the increasing VMg concentration and, vice versa, for the desorption temperature in MgH2. We also find that the magnesium vacancy defects have an effect on the gravimetric hydrogen capacity by making the magnesium hydride more lightweight. Moreover, the densities of states (DOS) indicate that the stability of MgH2 decreases with the increasing VMg concentration by shifting the Mg and H states to the conduction band (CB). In particular, we observe that it is difficult, after ∼4.8%, to storage the hydrogen into the system without cooling, because the desorption temperature becomes less than 0 °C. We also find that the optimal VMg concentration for the hydrogen vehicles is about 3.7% because its desorption temperature is close to the operating temperature of most modern vehicle engines.

Vacancy induced p-orbital ferromagnetism in MgO nanocrystallite

The collective magnetic response in MgO is due to the presence of magnesium vacancy (VMg) centers. Herein, this observation is verified under an experimental and density functional theory (DFT) framework. MgO grown at 500 °C, 650 °C and 800 °C attains a saturation magnetisation (Ms) of 0.13 emu/g, 0.15 emu/g and 0.22 emu/g, respectively. Ms drops from 0.22 emu/g to 0.04 emu/g when MgO (at 800 °C) is vacuum treated at 200 °C for 5 h. Ferromagnetism persists far above room temperature, attaining a Curie temperature (TC) of 822 K. Electron spin resonance (ESR) data of the samples at 77 K and 300 K confirm the presence of oxygen (VO) and magnesium (VMg) vacancy centers. DFT calculation reveals that only VMg contributes to a magnetic moment in MgO, with no apparent contribution from VO or VMg+O. An increase in the VMg concentration results in a concomitant increase in the magnetic moment. Spin polarised charge density distribution analysis shows spin polarised O 2p states in the vicinity of VMg. Based on these results, we speculate that a localized polaron is formed by the hole trapped on the oxygen sub-lattice, nearest to VMg. Finally, the p-p interactions of several nearest polarons contribute to the long-range ferromagnetism.

Energetic Substantiation of the Formation of Ytterbium Dimers in Single Forsterite Crystals

A structural computer simulation of ytterbium-containing forsterite crystals has been carried out. Atomistic simulation is performed using the GULP 4.1 software package (General Utility Lattice Program). Different mechanisms of ytterbium dissolution in forsterite crystals are considered, and the dissolution energies of isolated defects, as well as charged and neutral clusters of various configurations, are calculated. The results of the calculation manifest that the formation of ytterbium clusters with a magnesium vacancy gives a significant gain in the dissolution energy. The formation of neutral clusters (dimers) in the M1 site (YbMg1νMg1YbMg1) leads to an energy reduction by 1.7 eV compared with the statistical distribution of defects. As a result of simulation, it is shown the energetic substantiation of the formation of ytterbium dimers in forsterite crystals. A model of the most energetically favorable center in the M1 site is proposed—a dimer, consisting of a pair of trivalent ytterbium ions with a magnesium vacancy between them, forming a chain parallel to the crystallographic axis c.