Vacant Sites Research Articles

Recombination dynamics and manybody effect of excitons in large-area monolayer MoS2 capped with (111) NiO epitaxial layer

CVD grown monolayer MoS2 films on c-sapphire substrates are vacuum annealed and capped with (111) NiO epitaxial films using pulsed laser deposition technique. Time, energy and polarization resolved optical techniques are used to understand the effect of capping on the excitonic properties of the monolayer MoS2. It has been observed that trion contribution in the photoluminescence (PL) spectra increases after the capping, suggesting an enhancement of electron concentration in the conduction band. This has been attributed to the capping driven reduction of physisorbed air molecules from the sulphur vacancy ( VS ) sites. Note that the air molecules act as passivating agents for the VS -donors. Low temperature polarization resolved PL spectroscopy and ultrafast pump and probe transient absorption spectroscopy (TAS) show an increase of the biexcitonic population in the system after the encapsulation. The TAS study further reveals longer lifetime for both A and B excitons in capped samples implying a reduction of non-radiative recombination rate of the excitons after the capping. It has also been observed that in the capped samples, K/K′ valleys are populated with trions under sufficiently high pump powers. This has been attributed to the lower non-radiative recombination rates of the high energy photoexcited carriers and the faster transfer of either electrons or holes from the high energy pockets to the K/K′ valleys. The study further reveals different many-body effects in excitons and trions.

Pd promoted oxygen species activation of ZnO nanosheets for enhanced hydrogen sensing performance and its DFT investigation

The sensing performance of ZnO-based metal oxide semiconductors to H2 can be significantly enhanced after decoration of noble metals, and it is of great significant to understand the mechanism and structure-activity relationship between materials and sensing performance, which facilitate the designation of high-performance sensing materials. Herein, we have synthesized Pd-decorated two-dimensional (2D) ZnO nanosheets. A contrastive study based on H2 gas sensing performance of pristine ZnO and Pd-loaded ZnO (PZO) are carried out. According to the results, PZO exhibit superior sensing performance towards H2 (Ra/Rg=17.6, 160 °C), which is much higher than the pristine ZnO nanosheets. Moreover, the PZO also exhibit robust sensing stability and selectivity, detailed structural investigations illustrate that the electronic and surface properties of Pd-ZnO interface play the vital role in H2 sensing process. Density functional theory (DFT) calculation illustrates that H atoms trend to be dissociated at the surface Pd0 sites and the more surface oxygen vacancies are extremely desirable for H2 sensing. The superior H2 sensing of PZO is mainly attributed to the synergistic mechanism of Pd catalytic site and oxygen vacancy (OV). Our works pave the way for designation of high performance H2 sensing materials for the safety of development and utilization of H2.

Toward an understanding of the role of structural organization and point defects in the formation of functional pure and Tm3+-doped (Ca3-xSrx)(VO4)2 for diode-pumped and SRS lasers

Whitlockite-type polycrystalline and single-crystal (Ca3−xSrx)(VO4)2 solid solutions were successfully obtained by solid-state synthesis and Czochralski method, respectively. Synchrotron powder X-ray diffraction allowed to reveal actual compositions of polycrystalline (Ca3−xSrx)(VO4)2 solid solutions with х = 0.3, 0.54, 0.9, 1.2, 1.35 refined by Rietveld method, which are slightly different from the initial ones. Synchrotron X-ray single-crystal diffraction analysis of Czochralski-grown undoped (Ca1.8Sr1.2)(VO4)2 (CSVO) revealed its actual composition (Ca2.982(11)Sr0.031)(VО4)2. Difference between initial and refined compositions is analyzed and discussed. In the CSVO structure, Sr2+ ions partially occupy monocapped Ca2 and two-capped Ca3 trigonal prismatic sites and Ca2+ ions are in the Ca5 and Ca5A sites, forming two independent strongly distorted tetrahedral Ca5O4 and octahedral Ca5AO6 oxygen environments. Synchrotron X-ray single-crystal diffraction analysis together with X-ray absorption spectroscopic study of (Сa2.4Sr0.6)(VO4)2 doped with 0.5 and 1.0 wt% Tm2O3 (СSVO:0.5Tm and CSVO:1.0Tm) allowed to establish the absence of cations in the Ca5A site, the localization of Sr2+ ions in the Ca2 and Ca3 sites and additionally in octahedral Ca4 site together with Tm3+ ions, forming an individual eight-coordinated environment, in the refined compositions (Ca2.776(7)Sr0.210(4)Tm3+0.008(1))(VO4)2 and (Ca2.855(7)Sr0.140(7)Tm3+0.009(1)□0.022(7))(VO4)2, respectively, with vacancies (marked by a square,□) in the two-capped Ca1 trigonal prismatic site in the CSVO:1.0Tm structure. Additional interstitial oxygen, participating in the formation of the Tm3+ environment, found in the residual electron density, together with vacancies in the Ca1 site provides electroneutrality to CSVO:0.5Tm and CSVO:1.0Tm crystals. Several structural models of the coordination environment of Tm3+ ions were analyzed and the most correct one was proposed.

Revealing Dynamic Evolution of the Anode‐Electrolyte Interphase in All‐Solid‐State Batteries with Excellent Cyclability

AbstractAll‐solid‐state‐batteries (ASSBs) based on a halide solid electrolyte (SE) and a lithium‐metal based anode typically have poor cyclability without a buffer layer (such as Li3PS4 or Li6PS5Cl) to prevent the degradation reactions. Here excellent cycling stability of ASSB consisting of an uncoated single‐crystal LiNi0.8Co0.1Mn0.1O2 cathode and a Li3YCl6 (LYC) SE separator in direct contact with a Li‐In anode are demonstrated. Through a combination of electrochemical measurements, synchrotron micro‐X‐ray absorption and diffraction analyses, and density functional theory calculations, reveal for the first time that along with the standard Li+ transport during charge/discharge, indium in the Li‐In anode also participates in the redox reactions. The in‐situ generated In3+ preferentially occupies the vacant Li sites in the trigonal LYC lattice, leading to the formation, growth, and eventual stabilization of an anode‐electrolyte interphase (AEI) layer consisting of an In‐doped Li3‐xInxYCl6 (x = ≈0.2) phase. It is discussed how the presence of such an AEI layer prevents LYC decomposition and suppresses dendrite formation and propagation, enabling stable cycling of ASSB with ≈90% capacity retention over 1000 cycles. This work sheds light on the dynamic evolution of the halide SE and alloy anode interphase, and opens new avenues in the future design of long‐lasting high‐energy all‐solid‐state‐batteries.

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation.

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce-O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru-O-Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10-50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

The migration and occupation of Li+ and Na+ in illite and montmorillonite during the heating process

Abstract The impact of temperature on the migration of cations within layers of clay minerals is of profound significance for the design and practical application of materials derived from clay minerals. This study focuses on Li+ and Na+ as representative cations, together with illite (Ilt) and montmorillonite (Mnt) as representative clay minerals. The study investigates the behaviour of cation migration and occupation within clay minerals across varying temperatures. A series of samples were prepared meticulously by immersing illite and montmorillonite in Li+ and Na+ solutions, subsequently subjecting them to different temperatures (unheated, 100, 150, 200, 250 and 300°C) for 24 h. Through the use of techniques such as X-ray diffraction (XRD), cation exchange capacity (CEC), Fourier-transform infrared spectroscopy (FTIR), magic angle rotating solid nuclear magnetic resonance spectroscopy (MAS NMR), and X-ray photoelectron spectroscopy (XPS), the study discerns structural transformations in illite and montmorillonite, and tracks the migration and occupation of Li+ and Na+. The findings reveal that following heating, Na+ and Li+ do not infiltrate the lattice of illite. For montmorillonite, Na+ also does not migrate into the montmorillonite lattice, however, in contrast, Li+ does exhibit migration into this lattice. Notably, the migration and occupation of interlayer Li+ within montmorillonite exhibit discernible temperature dependence. Specifically, upon reaching 150°C, interlayer Li+ migrate to ditrigonal cavities within the tetrahedral layers. As the temperature elevates to 200°C, Li+ further permeate vacant octahedral sites through the ditrigonal cavities, culminating in the formation of a localised trioctahedral structure.

Gas-Solid Phase Reaction Derived Silver Bismuth Iodide Rudorffite: Structural Insight and Exploring Photocatalytic Potential of CO2 Reduction.

Photocatalytic reduction of CO2 is a promising strategy to mitigate the effects of global warming by converting CO2 into valuable energy-dense products. Silver bismuth iodide (SBI) is an attractive material owing to its tunable bandgap and favorable band-edge positions for efficient CO2 photoreduction. In this study, SBI materials, including AgBi2I7, AgBiI4, Ag2BiI5, and Ag3BiI6 are first synthesized, through gas-solid reaction by controlling the stoichiometric ratio of reactants. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) results revealed that the distance between Ag-I is proportional to the degree of Ag ions delocalization, which occupies the vacant sites. That greatly retards the charge recombination at vacant sites. In addition, the surface potential via photo-assisted Kelvin probe force measurements of various SBI catalysts shows that Ag3BiI6 exhibits the highest surface potential change due to the rich delocalized Ag ions. This results in effective charge carrier transport and prevention of charge recombination at vacant sites. Taking the above advantages, the averaged CO and CH4 production rates for Ag3BiI6 achieved 0.23 and 0.10µmolg-1h-1, respectively. The findings suggest that Ag3BiI6 has a high potential as a novel photocatalyst for CO2 reduction and sheds light on the possibility of solving environmental contamination and sustainable energy crises.

Interaction between water and point defects inside volume-constrained α-quartz: An ab initio molecular dynamics study at 300 K

Quartz-based minerals in earth’s crust are well-known to contain water-related defects within their volume-constrained lattice, and they are responsible for strength-loss. Experimental observations of natural α-quartz indicate that such defects appear as hydroxyl groups attached to Si atoms, called Griggs defect (Si-OH), and molecular water (H2O) located at the interstitial sites. However, factors contributing to the formation of Griggs and interstitial H2O defects remain unclear. For example, the role of point defects like vacancy sites (O2− and Si4+), and substitutional (Al3+) and interstitial (Li+, K+, Ca2+, Mg2+, etc.) ions has remained largely unexplored. Here, we performed ab initio molecular dynamics at 300 K to examine the energetics and structure of water-related defects in volume-constrained α-quartz. Several configurations were systematically interrogated by incorporating interstitial H2O, O2− and Si4+ vacancies, substitutional Al3+, and interstitial Li+, Ca2+ and Mg2+ ions within α-quartz. Interstitial H2O defect was found to be energetically favorable in the presence of Substitutional Al3+, and interstitial Ca2+, Mg2+, and Li1+. In the presence of O2− and Si4+ vacancies, H2O showed a strong tendency to dissociate into OH—to form Griggs defect—and a proton; even in the presence of substitutional and interstitial ions. These ions distorted the α-quartz lattice and, in the extreme case, disrupted long-range order to form local amorphous domains; consistent with experimental reports. Our study provides an initial framework for understanding the impact of water within the crystal lattice of an anhydrous silicate mineral such as quartz. We provide not only thermodynamic and process-related information on observed defects, but also provides guidelines for future studies of water’s impact on the behavior of silicate minerals.

High-efficiency photocatalyst based on Au nanoparticles loaded on defective ZnO nanorods

Optimizing the semiconductor-metal interaction is the key to developing metal single-atom cocatalysts with 100 % metal atom utilization and unique electronic properties. Metal atom sites are exclusively coordinated with semiconductor, featuring a chemical-bond-driven tunable interaction. Here vacancy defect-rich ZnO nanorods (NRs) are fabricated by a facile hydrothermal approach. Au nanoparticles (NPs) are preferentially anchored on surface zinc vacancy sites of ZnO NRs in an ultra-low concentration (10−4 mol/L) of HAuCL4 solution. The Au/ZnO nanohybrids exhibit an excellent photocatalytic activity toward removal of Rhodamine B and levofloxacin. Combining ferromagnetism and valence-band XPS spectra analysis, it is demonstrated that the strong interaction of surface vacancy defects in ZnO and Au generates a distinct upshift of the valence band edge, resulting from the antibonding interaction between O 2p and Au 5d orbitals which significantly weakens the Au–O bonds. This facilitates the release of the lattice oxygen and thus generates more oxygen vacancies. The remarkable photoactivity of defective Au/ZnO is due to the synergistic interaction between the intrinsic oxygen vacancy and Au. Optimizing the hybridization of transition metal d and O 2p states at the atomic scale can induce more active-sites, improve atomic utilization efficiency and thus provide a new approach for enhancing photoactivity.

CO2 adsorption and storage over FAU-zeolites for environmental remediation: Understanding the role of oxygen vacancy and promotor loading

The CO2 level is rising in the atmosphere and has reached well above 400 ppm. Thus, there is growing concern about increasing CO2 levels in the atmosphere as it is a greenhouse gas, constituting 20–25 % of total greenhouse gas emissions. CO2 is quite a resilient molecule and is a by-product of any combustion-driven process. The CO2 conversion is a non-spontaneous process due to its strong stability. The IEA and IPCC constantly mandate laws to monitor and regulate CO2 emissions in the atmosphere. One efficient way is to scrub off CO2 coming out of the exhaust. Several technologies have been developed around this concept, like adsorption and absorption. In this work, we have used zeolite for CO2 adsorption. Four different zeolites with a Si/Al ratio of one, which possess varied pore diameters of 3 Å, 4 Å, 5 Å and 10 Å are tested for CO2 adsorption. The zeolite with the pore diameter of 5 Å exhibited the best performance, which adsorbed 0.3 mmol/g (13.203 mg/g) of CO2. The adsorption capacity of zeolites is further enhanced by creating oxygen vacant sites and loading sodium (Na) promotor. The creation of oxygen vacancy ∼11 % for MZE600 sample raises the CO2 adsorption by 1.6 times compared to the ZE. Various concentrations of Na promotor are loaded on zeolite namely 5,10,15,20,25,35 %. About 11.97 mmol/g (527 mg/g) of CO2 adsorption was observed for 25 % loading of Na promotor on zeolite (M25NaZE). With the sequential increase in promotor loading (up to 25 %), CO2 adsorption increases. Further increase in loading reduces the CO2 adsorption, attributed to blockage of pores on sorbent and plausible agglomeration of the promotor. The M25NaZE sample is stable for a minimum of 10 regeneration cycles and exhibits linear response to variation in CO2 concentration from 2.5 % to 16 % and feed flow rate of 50–500 mL/min.

Promote the efficient co-combustion of toluene & CO in Pt/CeO2 catalyst: Orientally adjusting the oxidized SMSI to optimize acid site and oxygen vacancy

By effectively controlling the strong metal-support interaction (SMSI), the formation of acid sites and oxygen vacancies (Ov) on the Pt/CeO2 catalyst was optimized. The efficient co-combustion of toluene and CO on Pt/CeO2-450 was achieved. In the dual-pollutant system, when the conversion rate was 90 %, the corresponding conversion temperatures of toluene and CO were 165 °C and 173 °C, respectively. With the increase of SMSI, the catalytic activity of Pt/CeO2 showed a trend of first increase and then decrease. This was proportional to the content of weak acid sites, medium acid sites, and Ov generated on their surfaces. Compared with the combustion of a single pollutant, the result displays that toluene and CO had significant competitive adsorption on Pt/CeO2. The inhibition of toluene on CO also showed the same trend as the catalytic activity with the increase of SMSI. Under the double inhibition, Pt/CeO2 exhibited preferential selectivity for toluene combustion. However, the in situ DRIFTS results showed that this competitive adsorption phenomenon did not interfere with the combustion reaction path. The co-combustion of toluene and CO follows independent reaction paths. This work provides valuable insights and ideas for realizing the development of efficient and environmentally friendly catalysts through SMSI regulation.

Investigation of structural, electronic, mechanical, and thermoelectric properties of the defect chalcopyrite semiconductor ZnIn[formula omitted]Se[formula omitted] from density functional theory

In this study, the detailed structural, electronic, mechanical, and thermoelectric properties of ZnIn2Se4 defect chalcopyrite semiconductor are carried out using density functional theory & semi-classical Boltzmann transport theory. From the band structure analysis, the compound is confirmed to be a direct band gap semiconductor with a band gap of 1.20 eV. The presence of flat valence bands near the Fermi level indicates a higher effective mass of holes. From the elastic and mechanical properties, it has been found that the compound is mechanically stable and revealed to be ductile. The phonon dispersion study of the compounds shows that the compound is dynamically stable with no imaginary phonon modes. The electronic transport properties were studied within a temperature range of 300K to 900K as a function of chemical potential (μ), temperature(T), and carrier concentrations (n). The μ dependent electronic transport properties show enhanced values in the p-type doping region because of the cationic site vacancy. The dependency of electronic transport properties on n and T agrees well with Mott’s relation. The highest ZT is found to be 0.96 at 900K for a low carrier concentration of 1019cm−3. The phonon thermal conductivity of the compound is found to be 2.52 W/mK near room temperature and 0.84 W/mK at 900K using the Slack method. All these results support ZnIn2Se4, a defect chalcopyrite semiconductor, as a favorable candidate for thermoelectric applications.

Probing the sites on metal oxide cluster nodes of metal organic framework MIL-96, MIL-100 and MIL-110

Metal oxide cluster MOF nodes are a good platform to understand the reactivities and catalysis. MIL-96, MIL-100 and MIL-110 are drastically different in their node structures. Following our previous reported results for MIL-100, we report the modified synthesis and characterization of the node sites of MIL-110 and MIL-96 and their catalytic sites by using methanol dehydration as probe reaction. We show that these MOFs can be synthesized from the same solution by adding various amounts of formic acid as modulator. Herein, the chemistry of hydroxyl, formate and methoxy groups were used to probe the three MOFs’ node sites. IR data show the bonding structures of these ligands are highly diverse, with reactivities and flexibilities depending on node sites. We identified terminal OH group and various µ2-OH groups in single, paired or hydrogen bonded modes on the MOF nodes. Formate ligands were introduced to the node sites during the synthesis or by a post treatment in formic acid/DMF solution. Surprisingly, the bonding modes of formate ligands on Al3(OH)3 node structure of MIL-96 can be reversibly switched between monodentate and bidentate through a process induced by water molecules. Further, methoxy ligands—likely to be the intermediate for catalytic reaction—were also observed on the node sites under methanol dehydration conditions at 250 ℃. The activities of MIL-100 and MIL-96 for methanol dehydration determined by TOF per node were similar, both approximately 3-fold higher than that of MIL-110. The data suggests that catalytic sites incorporate adjacent vacancy and defect node sites, which constitute a minority among all observed MOF node sites. We foresee the opportunities in expanding the chemistry and understanding into other MOFs with metal oxide cluster nodes.

Unveiling the nature of atomic defects in graphene on a metal surface.

Low-energy argon ion bombardment of graphene on Ir(111) induces atomic-scale defects at the surface. Using a scanning tunneling microscope, the two smallest defects appear as a depression without discernible interior structure suggesting the presence of vacancy sites in the graphene lattice. With an atomic force microscope, however, only one kind can be identified as a vacancy defect with four missing carbon atoms, while the other kind reveals an intact graphene sheet. Spatially resolved spectroscopy of the differential conductance and the measurement of total-force variations as a function of the lateral and vertical probe-defect distance corroborate the different character of the defects. The tendency of the vacancy defect to form a chemical bond with the microscope probe is reflected by the strongest attraction at the vacancy center as well as by hysteresis effects in force traces recorded for tip approach to and retraction from the Pauli repulsion range of vertical distances.

Bi‐Functional Chelating Ligand Enables Ultra‐Stable Blue Emissive CsPbBr3 Nanoplatelets Film

AbstractQuantum‐confined perovskite CsPbBr3 Nanoplatelets (NPLs) have recently emerged as promising blue‐emitting materials for perovskite light‐emitting diodes (PeLEDs). Yet, their susceptibility to optical instability in solid films under ambient conditions poses a significant hindrance. This study introduces a novel chelating strategy that utilizes metal manganese ions and multidentate ligands, markedly improving the structural stability, and emission efficiency of NPLs. The approach involves adding Diethylenetriaminepentaacetic acid (DTPA) to the perovskite precursor solution, which allows for strong coordination to surface [PbBr6]4− octahedrons via its multiple chelation sites. Ensuing metal manganese ion integration during the purification phase addresses Pb2+ and Br− site vacancies, culminating in near‐perfect octahedral structures with significantly fewer vacancies. These metal manganese ions are then further immobilized on the NPLs surface by the chelating effect of unbound DTPA functional groups. The resultant CsPbBr3 NPLs films demonstrate an impressive PLQY of 66%, showcasing remarkable air stability with consistent blue emission for up to 5 days. The CsPbBr3 NPLs‐based PeLEDs show electroluminescence at 460 nm with a current efficiency of 1.07 cd A−1 and a maximum luminance of 220 cd m−2. The proposed chelating strategy positions perovskite NPLs as an extremely promising prospect in future applications of high‐definition displays and high‐quality lighting.

A DFT-based design of B/N/P-co-doped oxo-triarylmethyl as a robust anode material for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are emerging as a promising alternative to lithium-ion batteries (LIBs) due to their superior safety features and cost-effectiveness. In this work, heteroatoms-co-doped oxo-triarylmethyl (B/N/P@oxTAM) as a favorable anode material for MIBs was investigated using density functional theory calculations. The B/N/P@oxTAM is a highly porous structure and has a better affinity for Mg-ions to attach to vacancy site. Partial density of states, open-circuit voltage, theoretical specific capacity, and diffusion energy barrier were calculated and discussed. A significant decrease in the HOMO-LUMO gap with no structural deformation occurred, suggesting the high cycling performance of B/N/P@oxTAM for MIBs. Moreover, the designed anode material demonstrated full loading with six Mg-ions at different active sites, indicating a high theoretical specific capacity of 513.75 mAh g−1 and a low open-circuit voltage of 0.07 V. The presence of a heterocyclic ring (borabenzene) with a diffusion energy barrier of 0.039 eV increased the diffusion of Mg-ions. Therefore, B/N/P@oxTAM can be used as a viable anode material for MIBs with extended life cycle and quick charge-discharge rates due to its low open-circuit voltage and diffusion energy barrier, as well as its high theoretical specific capacity value.

Impact of temperature on short-range charge ordering in LiFePO4/FePO4

Energy is stored in a LiFePO4 battery electrode through the intercalation of Li. As Li incorporate into the crystal lattice of Fe(III)PO4, electrons reduce Fe(III) into Fe(II). The interactions of Li and its vacant site (Va) with these localized electrons (holes), so-called polarons, cause phase separation during battery operation. These fundamental interactions are however difficult to quantify using standard electronic structure calculations. In this paper, we utilize DFT+U with occupation matrix control to compute interaction energies at varying Li-Fe(II) and Va-Fe(III) pair separations. The increased energy with separation warrants the use of an electrostatic description. The DFT+U data are fitted to a Coulombic potential with two-body corrections and used in a Monte Carlo scheme. The coordination of the species determines their short-range ordering, showing that the Li and Va create chains bridged by their associated polarons which dissociate into dimers at higher temperatures. This dissociation happens at higher temperatures for Va than for Li, indicating a more pronounced clustering behavior during the formation of FePO4. Notably, a significant amount of uncoordinated Li exists at elevated temperatures, challenging the simplified picture of complete Li-Fe(II) pairing. Published by the American Physical Society 2024

First-principles analysis of the local atomic environment in calcium lanthanum sulfide ceramics

Density functional theory is utilized to investigate the energetics and mechanisms of sulfur loss across different stoichiometries of La2S3:CaS ceramics. These atomistic results reveal that the local atomic environment around each sulfur atom, consisting of six cation sites, can be used to predict the vacancy formation energy of the atom. Three nominal compositions are considered, 50:50, 65:35, and 90:10 La2S3:CaS, for two different atomic configurations, La2S3-like and La3S4-like. Relations were developed utilizing multivariable linear models. It is shown that the number of cation vacancies and lanthanum atoms in the local environment are among the most important factors in determining the energetics of a sulfur vacancy. Bader charge analysis reveals this is related to the degree of localization of charge change around the vacancy site and how close to the preferred oxidation states the lanthanum is. Results also suggest that while defect-free La2S3-like structures are energetically favorable, once a critical vacancy content is reached, the La3S4-like structure becomes the lower energy structure, which agrees qualitatively with recent experimental observations. This has important implications on how defect content and stoichiometry in this system are related. These findings also provide useful guidance on how these materials might best be processed.

Atypical components in the composition of nepheline from rocks of the Khibiny massif according to IR spectroscopy data

Nepheline is one of the main rock-forming minerals of the rocks of the Khibiny alkaline massif. Nepheline composition changes depending on the crystallization temperature, which makes it an indicator of rock formation conditions. In addition, nepheline is one of the main concentrators of hydrocarbon gases. The study of nepheline from different rocks of the Khibiny massif by local IR spectroscopy with Fourier transformation (micro-FTIR) has revealed the presence of water and acetylene in its structure. The water content estimated according to the method of statistical quantitative determination of water in nominally anhydrous minerals varies in the range from 0 to 0.78 wt.%. The highest water content has been observed in high-temperature nepheline of early formations (foyaite of the central and outer parts of the massif), and the lowest – in relatively low-temperature nepheline of later rocks of the Main Ring Fault (urtite, apatite-nepheline rock and poikilitic (kalsilite)-nepheline syenite – rischorrite). The study reveals that water enters the structure of nepheline in the form of H2O molecules that occupy vacancies in the potassium sites. The water content in nepheline is controlled by the number of vacancies in its structure. The presence of acetylene has been determined on a qualitative level. Additional studies are required to assess its quantitative content.

Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet

Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5−δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.