Occupation Of Interstitial Sites Research Articles

Broadband spectral cyan emission phosphor for full-spectrum LED caused by interstitial site occupation

The phosphor with a highly condensed, rigid framework structure and a single crystallographic site often exhibit symmetrical narrow-band emission. It is challenging to achieve broadband emission by doping Eu2+ ions in similar structures. Here, we propose to control the occupation and quenching concentration of Eu2+ ions in a single-site matrix Sc2Si2O7 to increase efficiency and precise regulation of luminescence spectra substantially. The analysis of photoluminescence spectroscopy through steady-state, transient-state, and Gaussian fitting techniques has discovered two emission centers despite the presence of a single rare-earth substitution site. The theoretical calculations and bond valence sum subsequently prove that Eu2+ ions prefer substituting the Sc3+ and interval sites to emit intense cyan light. Under 340 nm excitation, broad cyan-emission (FWHM = 115 nm) is exhibited with a high quantum yield of 60.67 %. The present phosphor exhibits pronounced thermal stability, and the emission intensity can still keep 68.3 % at 170 °C compared to that at atmospheric temperature. The Sc2Si2O7: Eu2+ phosphor boasts exceptional potential as a highly efficient cyan component in full-spectrum WLEDs. By replacing the blue light component commonly found in WLEDs, the intelligent and healthy alternative Sc2Si2O7: Eu2+ phosphor can effectively decrease the harmful blue light. This work also highlights the critical need to analyze local phosphor distortions upon rare-earth substitution, especially in single crystallographic site structures.

Incorporation and substitution of ions and H2O in the structure of beryl

Abstract. Incorporation of ions into the crystal structure of beryl (Be3Al2[Si6O18]) can take place by direct ion-to-ion substitution of the framework components Al3+, Be2+ and Si4+ or by occupation of interstitial or structural channel sites. The most common impurities in beryl include transition metals, alkalis and H2O. It is accepted that the transition metals Mn, Cr and V directly substitute for Al at the octahedral site and induce colour. Similarly, the octahedral site can host Fe instead of Al. Nevertheless, it is shown that it remains disputed whether Fe can also be present at the tetrahedral, interstitial, or channel sites, and opposing hypotheses exist regarding these possibilities. However, in the case of Fe, not only the possible occupation of these sites remains under debate, but also their influence on the subsequent colour of beryl. Similarly, the residence of Li in the channels and at the Be tetrahedral or interstitial tetrahedral sites is still under debate. The presence of more than two types of H2O (type I and type II) in the structural channels of beryl is also unclear. This article aims to give an overview on the consensus and on the current debates found in the literature regarding these aspects. It mainly concentrates on the substitution by and the role of Fe ions and on channel occupancy by H2O.

Role of diffusing interstitials on dislocation glide in refractory body centered cubic metals

In this work, we employ a phase field dislocation dynamics technique to simulate dislocation motion in body centered cubic refractory metals with diffusing interstitials. Two distinct systems are treated, Nb with O interstitials and W with H interstitials, to consider both relatively small and large atomic size interstitials. Simulations without and with driving stress are designed to investigate the role of interstitial type and mobility on the glide of edge- and screw-character dislocations. The simulations reveal the various short- and long-range dislocation-interstitial interactions that can take place and their dependency on interstitial type, site occupation, stress state, and mobility of the interstitials relative to dislocations. We show that while interstitial O increases the breakaway stress for both screw and edge dislocations in Nb, interstitial H in low H concentrations makes screw dislocations easier and the edge dislocations harder to move. The simulations find that screw dislocation glide is enhanced by the presence of interstitials in both systems. Edge dislocation glide is enhanced in W–H and inhibited in Nb–O.

Theoretical Insights on the Charge State and Bifunctional OER/ORR Electrocatalyst Activity in 4d-Transition-Metal-Doped g-C3N4 Monolayers.

Exploring efficient and stable electrocatalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is vital to developing renewable energy technologies. However, due to the substantial and intricate design space associated with these bifunctional OER/ORR electrocatalysts, their development presents a formidable challenge, resulting in their cost-prohibitive nature in both experimental and computational studies. Herein, using the defect physics method, we systematically investigate the formation energies and bifunctional overpotential (ηBi) of 4d-transition-metal (4d-TM, 4d-TM = Zr, Nb, Mo, Ru, Rh, Pd, and Ag)-doped monolayer supercell g-C3N4 (4d-TM@C54N72) based on the density functional theory (DFT) calculations. Under N-rich and C-rich conditions, we find that the formation energies of RhN@C54N71 (Rh occupation N) and PdN@C54N71 (Pd occupation N) are smaller than that of other 4d-TMN@C54N71 (4d-TM occupation N site); for the 4d-TMint@C54N72 (4d-TM interstitial site occupation), the lowest-formation energy defects are Pdint@C54N72. These results indicate that they have better stabilities. Interestingly, for these formation energy lower systems, Pd0int@C54N72 (ηBi = 1.00 V) and Rh1+N@C54N71 (ηBi = 0.73 V) have ultralow overpotential and can be great candidates for bifunctional OER/ORR electrocatalysts. We find the reason is that adjusting the charge states of 4d-TM@C54N72 can tune the interaction strength between the oxygenated intermediates and the 4d-TM@C54N72, which plays a crucial role in the activity of reactions. Additionally, the data obtained through machine learning (ML) application suggest that the electronegativity (Nm) and bond length of 4d-TM and coordination atoms (dTM-OOH) are primary descriptors characterizing the OER and ORR activities, respectively. The charged defect tuning of the bifunctional OER/ORR activity for 4d-TM@C54N72 would enable electrocatalytic performance optimization and the development of potential electrocatalysts for renewable energy applications.

Effect of reaction path on high-pressure synthesis and stability of ruthenium hydrides

This study explores the behavior of ruthenium hydrides under high-pressure conditions through three thermodynamical paths using laser-heated diamond anvil cells. The synthesis of RuH0.9 occurs gradually exceeding the pressure of 23.5 GPa in the ambient temperature path, while RuH is successfully synthesized at pressures above 20 GPa and a temperature of 1500 K. High-temperature conditions are found to reduce the pressure required for synthesis. The results demonstrate that the hydrogen occupancy of octahedral interstitial sites in the ruthenium hydrides is found to reach saturation with complete hydrogen absorption in the high-temperature path. Moreover, the crystallinity of the ruthenium hydride samples improves at higher temperatures, with the grain size increasing from 10 nm in the ambient temperature path to submicron in the high-temperature path. However, the predicted RuH6 and RuH3 were not observed in the present work.

Statistical approaches to the problem of homogeneous melting of solids in the microcanonical ensemble

Melting is a common phenomenon in our daily life, and although it is understood in thermodynamic (macroscopic) terms, the transition itself has eluded a description from the point of view of microscopic dynamics. While there are studies of metastable states in classical spin Hamiltonians, cellular automata, glassy systems and other models, the statistical mechanical description of the microcanonical superheated solid state is lacking.Our work is oriented to the study of the melting process of superheated solids, which is believed to be caused by thermal vacancies in the crystal or by the occupation of interstitial sites. When the crystal reaches a critical temperature, it becomes unstable and a collective self-diffusion process is triggered. These studies are often observed in a microcanonical environment, revealing long-range correlations due to collective effects, and from theoretical models using random walks over periodic lattices. Our results suggest that the cooperative motion made possible by the presence of vacancy-interstitial pairs (Frenkel pairs) above the melting temperature can induce long-range effective interatomic forces even beyond the neighboring fourth layer. From microcanonical simulations it is also known that an ideal crystal needs a random waiting time until the solid phase collapses. Regarding this, our results also point towards a description of these waiting times using a statistical model in which there is a positive quantity X that accumulates from zero in incremental steps, until it exceeds a threshold value.

Element selection for crystalline inorganic solid discovery guided by unsupervised machine learning of experimentally explored chemistry

The selection of the elements to combine delimits the possible outcomes of synthetic chemistry because it determines the range of compositions and structures, and thus properties, that can arise. For example, in the solid state, the elemental components of a phase field will determine the likelihood of finding a new crystalline material. Researchers make these choices based on their understanding of chemical structure and bonding. Extensive data are available on those element combinations that produce synthetically isolable materials, but it is difficult to assimilate the scale of this information to guide selection from the diversity of potential new chemistries. Here, we show that unsupervised machine learning captures the complex patterns of similarity between element combinations that afford reported crystalline inorganic materials. This model guides prioritisation of quaternary phase fields containing two anions for synthetic exploration to identify lithium solid electrolytes in a collaborative workflow that leads to the discovery of Li3.3SnS3.3Cl0.7. The interstitial site occupancy combination in this defect stuffed wurtzite enables a low-barrier ion transport pathway in hexagonal close-packing.

Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

AbstractDespite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of 27Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra‐low fluence (≈2 × 1010 cm−2) implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre‐doped with stable Mg during growth, and lowest (2–6%) in n‐GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p‐ and n‐GaN prolonged implantations cause interstitial 27Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 1014 cm−2 result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.

Pressure-induced disordering of site occupation in iron–nickel nitrides

Controlled disordering of substitutional and interstitial site occupation at high pressure can lead to important changes in the structural and physical properties of iron–nickel nitrides. Despite important progress that has been achieved, structural characterization of ternary Fe–Ni–N compounds remains an open problem owing to the considerable technical challenges faced by current synthetic and structural approaches for fabrication of bulk ternary nitrides. Here, iron–nickel nitride samples are synthesized as spherical-like bulk materials through a novel high-pressure solid-state metathesis reaction. By employing a wide array of techniques, namely, neutron powder diffraction, Rietveld refinement methods combined with synchrotron radiation angle-dispersive x-ray diffraction, scanning electron microscopy/energy dispersive x-ray spectroscopy, and transmission electron microscopy, we demonstrate that high-temperature and high-pressure confinement conditions favor substitutional and interstitial site disordering in ternary iron–nickel nitrides. In addition, the effects of interstitial nitrogen atoms and disorderly substituted nickel atoms on the elastic properties of the materials are discussed.

Discovery of an Environmentally Friendly Water-Soluble Luminous Material with Interstitial Site Occupancy

As an essential precursor, K2MnF6 plays an important role in the preparation of Mn4+-activated fluoride materials, but its deterioration in pure water hampers development of a green route. Herein, ...

Effects of deuterium content on the thermal stability and deuterium site occupancy of TiZrHfMoNb deuterides

A series of high entropy alloy (HEA) hydrides/deuterides were synthesized and characterized to explore the relationship between thermal stability and the structure of TiZrHfMoNb deuterides. The X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry results reveal that the lattice constant and cell volume of TiZrHfMoNb hydrides increase with the increasing hydrogen content. The desorption temperature decreases with the increasing hydrogen content, indicating that the increased lattice constant arising from the introduction of deuterium atoms will destabilize the HEA deuterides. A density functional theory calculations demonstrate that the binding energies between hydrogen and metals in BCC TiZrHfMoNb hydrides decline with the increasing hydrogen content, meaning that the thermal stability decreases with the increasing hydrogen content. The HEA deuterides are found to be more stable than their hydrides, implying the remarkable isotopic effect of TiZrHfMoNb HEA. Neutron powder diffraction refinements confirm that deuterium atoms prefer to occupy both of the tetrahedral and octahedral interstitials in BCC structure, while a prior occupation of tetrahedral interstitial sites in FCC structure. The deuterium/hydrogen concentrations and the lattice constants are proposed to be the major factor regulating the thermal stability of HEA deuteride, making them suitable for different application field of hydrogen storage.

Hydrogen induced lattice expansion and site occupation analyzed by ion beam methods

We study the absorption of H in ultrathin crystalline Fe/V stacks using elastic recoil detection (ERD) and resonant nuclear reaction analysis (NRA) combined with Rutherford backscattering spectrometry (RBS). Probing with the resonant 1H (15N,αγ) 12C nuclear reaction allows us to profile the hydrogen concentration with high depth resolution, while ERD permits us to quantify the integral hydrogen inventory while minimizing possible ion-beam induced hydrogen desorption. We perform angular scans in order to directly identify the interstitial site occupancy. A detailed analysis of channeling patterns permits us the deduction of crystal strain in as-deposited films as well as H-loaded targets. We observe tetragonal strain formation resulting in c/a = 1.06 and find qualitative indications for tetrahedral site occupation.

Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films

Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics. Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime. A σDOS model is proposed, taking the measured substrate-induced stress contribution to the chemical potential into account. This enables the determination of the different sites’ volume fractions and of pure site energy distributions by fitting measured isotherms. Interstitial sites, grain/domain boundary sites and deep traps are distinguished. Dislocations and vacancies are shown to have a minor impact on the films’ trapping of hydrogen atoms, while deep traps are related to the films’ surface. Enhanced binding energies in nano crystalline films can be ascribed to the tensile strain effect of grain boundaries acting on the grains. Measured surface trapping energies fit to the respective bulk values, while the trapping of hydrogen in grain/domain boundaries of the films is significantly increased. This can be interpreted with different grain/domain boundary structures. Different from octahedral interstitial site occupation, tetrahedral site occupation is suggested for grain/domain boundaries of the films.

Improving the Ionic Conductivity of Li15P4S16Cl3 By Doping

All-solid-state Li-ion batteries are a next generation electrochemical energy storage technology and an emerging alternative to conventional Li-ion batteries owing to higher energy density, wider operation temperature range and better safety properties1. Solid state ionic conductor as the solid electrolyte (SEs) is the key component in all-solid-state Li-ion batteries. Among existing SEs, sulfides SEs attract extensive attentions due to high room temperature ionic conductivity and favorable mechanical properties. However, currently known sulfide SEs only present a few limited types of crystal structures, which limits the materials design and hinders our understanding of ionic conduction mechanisms. Exploring new compound and new crystal structures beyond known groups is much desired in order to expand the ionic conductor database and advances the knowledge about ionic conduction. Recently, our group successfully synthesized and identified a new lithium chloro-thiophosphate in Li2S-P2S5-LiCl system, namely Li15P4S16Cl3 2. This compound shows a low ionic conductivity of ~10-7 S/cm at room temperature. However, this crystal structure provides abundant choices for composition and unit cell size tuning, which may trigger high ionic conductivity. We successfully substituted P in Li15P4S16Cl3 with aliovalent cations to tune the Li content and the interstitial site occupancy. As a result, high room temperature conductivity of ~10-5 S/cm was achieved. The structure of the doped compounds was examined with using synchrotron X-ray and neutron diffractions. The Li+ diffusion pathway was also theoretically investigated with using ab initio molecular dynamics (AIMD) method and experimentally examined with using 7Li solid state nuclear magnetic resonance (NMR) techniques. Auvergniot, J.; Cassel, A.; Ledeuil, J.-B.; Viallet, V.; Seznec, V.; Dedryvère, R., Interface Stability of Argyrodite Li6PS5Cl toward LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4 in Bulk All-Solid-State Batteries. Chemistry of Materials 2017, 29 (9), 3883-3890.Liu, ZT.; Zinkevich, T.; He, XF.; Indris, S.; Xiong, S.; Liu, J.; Xu, W.; Bai, J.; Mo, Yifei; Chen, HL., Li15P4S16Cl3, a new lithium chloro-thiophosphate as a solid state ionic conductor. Submitted.

Hydrogen site location in ultrathin vanadium layers by N-15 nuclear reaction analysis

We present a method using resonant nuclear reaction analysis combined with optical transmission and heavy-ion Rutherford backscattering spectrometry to study the absorption of hydrogen in single crystalline thin vanadium films. Probing with the resonant 1H(15N,αγ)12C reaction allows for highly resolved hydrogen depth profiling, while measurements along the crystal axes render possible the direct identification of the interstitial site occupancy. First experiments were performed on thin vanadium hydrides in Fe(Cr)/V superlattices revealing differences in site occupancy.

Unexpected Magnetic Ordering on the Cr Substructure in UCr2Si2C and Structural Relationships in Quaternary U-Cr-Si-C Compounds.

Previous experimental and theoretical studies revealed that carbon insertion into the RCr2Si2 compounds drastically affects the magnetic behavior, since chromium does not carry any magnetic moment in RCr2Si2C (R = Y, La-Sm, Gd-Er) compounds in contrast to RCr2Si2 (R = Y, Sm, Gd-Lu, Th) compounds. In this study, we report on the unexpected magnetic ordering of chromium atoms in the isotype quaternary UCr2Si2C compound. While specific heat and magnetic measurements suggest a Pauli paramagnetic behavior, neutron powder diffraction reveals an antiferromagnetic ordering of the chromium substructure at high temperature ( TN > 300 K), while that of uranium remains nonmagnetically ordered down to 2 K. Its magnetic behavior, inverse in comparison to the RCr2Si2C carbides involving a magnetic lanthanide, is discussed in relation with the singularity of its crystal structure among the series. Moreover, the crystallographic structures and the structural stability of UCr2Si2C and of two other quaternary U-Cr-Si-C compounds (i.e., UCr3Si2C and U2Cr3Si2C3), based on the full occupancy of interstitial sites by carbon atoms, are discussed and compared to those of the related ternary intermetallics. Finally, the low-temperature form of UCr2Si2, corresponding to a displacive transformation around 210 K of the ThCr2Si2-type structure, is reinvestigated by considering a higher symmetry monoclinic unit cell ( C2/ m) instead of the previously reported triclinic cell ( P1̅). The antiferromagnetic ordering at low temperature ( TN = 30(2) K) of the uranium substructure is confirmed, and its magnetic structure is reanalyzed and discussed considering the monoclinic crystal structure.

Doping of metastable Cu3N at different Ni concentrations: Growth, crystallographic sites and resistivity

Copper nitride, Cu3N, is a metastable material whose properties can be changed considerably by doping with metals which opens for a variety of applications in several areas (sensors, electrical connects, batteries, memories, etc.). The present work is a systematic study in the system Cu-Ni-N of preferences regarding occupation of interstitial and substitutional crystallographic sites in the Cu3N structure as the metal dopant level increases and how the occupation influences growth behavior, texture, microstructure and resistivity. Ni doped Cu3N films of different chemical composition were grown by a gas-pulsed Chemical Vapor Deposition technique. The occupation of the different crystallographic sites of the Cu3N by the Ni atoms was obtained from analysis of X-ray diffraction data. At low Ni content, less than about 21% in metal content, Ni replaced the Cu atoms in the structure. In the intermediate Ni metal content range from about 21 to 40% the vacant centre position became available. After filling the centre position, substitution of Cu for Ni occurred up to a Ni content of about 80% (Cu0.8Ni3.2N) which is the solid solubility limit of Ni in Cu3N. The film resistivity decreased rapidly by adding nickel to the Cu3N structure from about 109μΩ·cm without any Ni doping to about 100μΩ·cm with 80% Ni in the metal content. After filling the centre position the change in resistivity when Cu atoms were substituted for Ni was very small. Finally, the growth mechanism, texture and microstructure changed significantly with the uptake of Ni atoms in the structure.

Phase and Facet Control of Molybdenum Carbide Nanosheet Observed by In Situ TEM.

Transition metal carbides are of great potential for electrochemical applications. The phase and facet of molybdenum carbides greatly affect the electrochemical performance. Carburization of MoO3 inside a transmission electron microscope to monitor the growth process of molybdenum carbides is performed. Carbon sources with different activities are used and the controllable growth of molybdenum carbides is investigated. The results show that the relatively inert amorphous carbon film produces Mo2 C, where the interstitial sites formed by hexagonal closed packing molybdenum atoms are partially occupied by carbon atoms. In contrast, the carbon decomposed from the sucrose has a high portion of sp3 hybridized and crosslinked carbon atoms with high reactivity, leading to the formation of MoC with full occupation of interstitial sites by carbon atoms. In addition, the MoC growth experiences a (111) to (100) facets change with the increase of temperature. The (111) facet formed at low temperature has Mo-terminated or C-terminated surface with higher surface energy and higher reactivity, while the (100) facet with 1:1 C/Mo ratio on the surface exhibits enhanced stability. The phase and facet control by carbon source and temperature allow us to tune the crystal structures and surface atoms as well as their electrochemical properties.

Effects of Ni doping induced band modification and Ni3Se2 nanoinclusion on thermoelectric properties of PbSe

The electrical and thermal transport properties of Ni-doped PbSe (PbSe:Nix; 0.01 ≤ x ≤ 0.06) have been studied. Further, the complex crystal chemistry of Pb-substitutional/tetrahedral interstitial site occupancy of Ni has been studied to understand the influence of divalent dopants on the thermoelectric transport properties of PbSe. Ni incorporation in PbSe introduces Ni impurity bands located near the valence band and also induces bandgap narrowing (BGN) by shifting the Fermi level deep into the valance band. This affects the charge carrier transport to decrease the resistivity and to maintain high Seebeck coefficient of ∼230 μV/K. Again, the disorder created by dopant atoms leads to anharmonic lattice vibrations responsible to the lower lattice thermal conductivity in PbSe:Nix as compared to that of the pristine PbSe. The concurrently increased electrical conductivity and decreased thermal conductivity enhance the thermoelectric performance of PbSe:Ni0.04 by increasing its power factor and figure of merit to 142% and 180% of the corresponding values for the pristine PbSe.

Effect of Al on the dehydrogenation of LiBH4 from first-principles calculations

Al atom is designed to add into LiBH4 by occupation of Li atom, B atom and interstitial sites. The effects of Al on the dehydrogenation of LiBH4 are investigated by first-principles calculations. It is found that substitution with Al destabilizes LiBH4, leading to a reduction in the dehydrogenation energy (Ed) with the order of Li8B8H32 > Li8B7AlH32 > Li7AlB8H32 > Li8B8AlH32. This descending order is also reflected on the scaled bond order of BH (BOsBH) and the band gap (Eg). Our theoretical studies show that in LiBH4–Al systems, the weaker BH covalent bonding interaction, the metal-like character and the formation of AlB bond all favor H release. Al atom prefers to substitute for B atom, but this only weakly affects the dehydrogenation energy. Although Li8B8AlH32 compound with low dehydrogenation energy exhibits favorable dehydriding performance, the energy cost for Al occupation of interstitial site needs to be reduced for practical application.