Lattice Density Functional Theory Research Articles

Water Condensation in the Nanoscale Pores of Pt/C Catalyst Particles and Its Impact on Catalyst Utilization: A Simulation Based on a Reconstructed Structure from Nanoimaging.

In polymer electrolyte membrane fuel cells, carbon-supported platinum (Pt/C) catalyst particles require sufficient water condensation within the nanoscale pores to effectively utilize the interior Pt catalysts. Since experimental visualizations with nanoscale precision of this phenomenon are not yet possible, we utilized a Pt/C catalyst particle reconstructed from segmented nanoimaging of a catalyst powder, which served as the computational domain for lattice density functional theory (LDFT) simulation of water condensation. Paired with experimental water uptake data, LDFT successfully simulated high-resolution water condensation, capturing both thin-film and capillary water condensation phenomena. Using a simple proton movement method within the water network, we reproduced the Pt utilization data from a CO stripping experiment. Our findings highlight that at low relative humidity (RH), Pt utilization is influenced by thin water film formations, mainly dictated by the wettability properties of surfaces within primary pores and the Pt/C catalyst particle's exterior. Conversely, at high RH, Pt utilization is attributed to capillary water condensation in medium-to-large sized pores. This approach contributes a qualitative and quantitative discussion on hypotheses regarding the mechanism of Pt utilization, supporting recent studies (e.g., Girod, R.; Nat. Catal. 2023, 6, (5), 383-391).

Utilization of Pt Nanoparticle Catalysts during Capillary Condensation Phenomenon in PEMFC: A Lattice DFT Simulation Based on 3D Reconstruction of TEM Images

The use of porous carbon support particles in Cathode Catalyst Layer (CCL) of Polymer Electrolyte Membrane Fuel Cell (PEMFC) has gained significant interest due to their ability to enable proper dispersion of Platinum (Pt) nanoparticles [1]. This offers a key advantage of avoiding the Pt nanoparticles from potential poisoning by sulfonate anion functional groups in the ionomer that is deposited on the surface of the support particles [2]. Furthermore, the presence of condensed liquid water is essential for effective utilization of the Pt nanoparticles that are dispersed both inside and outside of the porous carbon support particles. This is because water plays a crucial role as a proton pathway for hydrogen ions during the electrochemical reaction, thus a requisite amount of condensed liquid water is also needed to promote efficient catalytic activity and maximize the performance of Pt nanoparticle catalysts. It has been demonstrated that the microstructure characteristics of the porous carbon support particles have a significant impact on the performance of Pt utilization, which is attributed to the varying behavior of water condensation [2]. Microstructure properties such as pore size, surface area, and wettability properties, can influence the distribution of condensed liquid water, thereby affecting the electrochemical performance of Pt nanoparticles in the catalyst layer of PEMFC. Understanding the effect of capillary water condensation in the porous carbon support particles is therefore crucial for maximizing the utilization and catalytic activity of Pt nanoparticles in fuel cell systems.To investigate the behavior of water condensation in the CCL particles of PEMFC, we conducted simulations based on reconstructed structural data obtained from Transmission Electron Microscopy (TEM) images, as shown in Figure 1. The CCL particle used in this study is taken from the Toyota MIRAI Gen 2 model, an emerging commercial PEMFC-based vehicle in Japan. Here, we used the lattice Density Functional Theory (DFT) to simulate the process of capillary water condensation within the pore network of the particle. The lattice DFT method, which is based on the mean-field lattice gas model, can be interpreted as a coarse-grained equivalent of the conventional DFT [3]. The significant advantage of using lattice DFT is to provide a simple model and computationally efficient approach for studying the behavior of capillary water condensation. To validate the parameters used in our lattice DFT simulations, we compared the isotherm curve obtained from our simulation results with experimental data. Through this comparison, we were able to fine-tune the parameters of the lattice DFT simulation to fit the simulated isotherm curve with that of the experiment. When the parameters are optimized, the resulting water distribution obtained from the lattice DFT simulation, as illustrated in Figure 1, serves as a crucial input to investigate the utilization/activation of Pt nanoparticles within and outside the carbon support particle under varying relative humidity (RH) conditions. Since the resulting water distribution depends on RH, we expect that the utilization/activation of Pt nanoparticles also depend on RH. In this study, we defined utilization/activation of Pt nanoparticle when there is a continuous path of condensed water connecting the Pt nanoparticle to the surface of the carbon support particle. This criterion is similar to the case when we assume that the ionomer is distributed uniformly on the surface of the carbon support particle. Our investigation shows that the utilization/activation behavior of Pt nanoparticles follows closely the isotherm curve of the capillary water condensation.The complex internal structure of CCL particles in PEMFC makes it challenging to visualize and understand their workings through experiments alone. Therefore, our approach of using lattice DFT simulations offers valuable insights to the role of capillary water condensation behavior in the utilization/activation of Pt nanoparticles. This approach may also pave the way for future research in optimization strategies for PEMFC, ultimately contributing to the advancement of clean energy technologies.This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan (Grant number JPNP20003).[1] Islam, M.N., et al. (2022), Nat. Commun. 13, 6157.[2] Ramaswamy, N., et al. (2020) J. Electrochem. Soc. 167 064515.[3] Monson, P. (2012) Micropor. Mesopor. Mater. 160, 47–66.Figure 1. Reconstructed structural data obtained from TEM images of a PEMFC CCL particle used in Toyota MIRAI Gen 2 model, with visualization of condensed water obtained from a sample lattice DFT simulation. Figure 1

Stability of nanoparticle laden aerosol liquid droplets.

We develop a model for the thermodynamics and evaporation dynamics of aerosol droplets of a liquid, such as water, surrounded by gas. When the temperature and the chemical potential (or equivalently the humidity) are such that the vapor phase is in the thermodynamic equilibrium state, then, of course, droplets of the pure liquid evaporate over a relatively short time. However, if the droplets also contain nanoparticles or any other non-volatile solute, then the droplets can become thermodynamically stable. We show that the equilibrium droplet size depends strongly on the amount and solubility of the nanoparticles within, i.e., on the nature of the particle interactions with the liquid and, of course, also on the vapor temperature and chemical potential. We develop a simple thermodynamic model for such droplets and compare predictions with results from a lattice density functional theory that takes as input the same particle interaction properties, finding very good agreement. We also use dynamical density functional theory to study the evaporation/condensation dynamics of liquid from/to droplets as they equilibrate with the vapor, thereby demonstrating droplet stability.

What can lattice DFT teach us about real-space DFT?

In this paper we establish a connection between density functional theory (DFT) for lattice models and common real-space DFT. We consider the lattice DFT description of a two-level model subject to generic interactions in Mermin's DFT formulation in the grand canonical ensemble at finite temperature. The case of only density-density and Hund's rule interaction studied in earlier work is shown to be equivalent to an exact-exchange description of DFT in the real-space picture. In addition, we also include the so-called pair-hopping interaction which can be treated analytically and, crucially, leads to non-integer occupations of the Kohn-Sham (KS) levels even in the limit of zero temperature. Treating the hydrogen molecule in a minimal basis is shown to be equivalent to our two-level lattice DFT model. By means of the fractional occupations of the KS orbitals (which, in this case, are identical to the many-body ones) we reproduce the results of full configuration interaction, even in the dissociation limit and without breaking the spin symmetry. Beyond the minimal basis, we embed our HOMO-LUMO model into a standard DFT calculation and, again, obtain results in overall good agreement with exact ones without the need of breaking the spin symmetry.

Density-matrix functional theory of the attractive Hubbard model: Statistical analogy of pairing correlations

The ground-state properties of the Hubbard model with attractive local pairing interactions are investigated in the framework of lattice density-functional theory. A remarkable correlation is revealed between the interaction-energy functional $W[\mathbit{\ensuremath{\eta}}]$ corresponding to the Bloch-state occupation-number distribution ${\ensuremath{\eta}}_{\ensuremath{\kappa}\ensuremath{\sigma}}$ and the entropy $S[\mathbit{\ensuremath{\eta}}]$ of a system of noninteracting fermions having the same ${\ensuremath{\eta}}_{\mathbit{k}\ensuremath{\sigma}}$. The relation between $W[\mathbit{\ensuremath{\eta}}]$ and $S[\mathbit{\ensuremath{\eta}}]$ is shown to be approximately linear for a wide range of ground-state representable occupation-number distributions ${\ensuremath{\eta}}_{\mathbit{k}\ensuremath{\sigma}}$. Taking advantage of this statistical analogy, a simple explicit ansatz for $W[\mathbit{\ensuremath{\eta}}]$ of the attractive Hubbard model is proposed, which can be applied to arbitrary periodic systems. The accuracy of this approximation is demonstrated by calculating the main ground-state properties of the model on several 1D and 2D bipartite and nonbipartite lattices and by comparing the results with exact diagonalizations.

Hybrid Pore-Scale Adsorption Model for CO2 and CH4 Storage in Shale

Making reliable estimates of gas adsorption in shale remains a challenge because the variability in their mineralogy and thermal maturity results in a broad distribution of pore-scale properties, including size, morphology and surface chemistry. Here, we demonstrate the development and application of a hybrid pore-scale model that uses surrogate surfaces to describe supercritical gas adsorption in shale. The model is based on the lattice density functional theory (DFT) and considers both slits and cylindrical pores to mimic the texture of shale. Inorganic and organic surfaces associated with these pores are accounted for by using two distinct adsorbate–adsorbent interaction energies. The model is parametrized upon calibration against experimental adsorption data acquired on adsorbents featuring either pure clay or pure carbon surfaces. Therefore, in its application to shale, the hybrid lattice DFT model only requires knowledge of the shale-specific organic and clay content. We verify the reliability of the model predictions by comparison against high-pressure CO2 and CH4 adsorption isotherms measured at 40 °C in the pressure range 0.01–30 MPa on four samples from three distinct plays, namely, the Bowland (UK), Longmaxi (China), and Marcellus shale (USA). Because it uses only the relevant pore-scale properties, the proposed model can be applied to the analysis of other shales, minimizing the heavy experimental burden associated with high-pressure experiments. Moreover, the proposed development has general applicability meaning that the hybrid lattice DFT can be used for the characterization of any adsorbent featuring morphologically and chemically heterogeneous surfaces.

Deconstructing water sorption isotherms in cement pastes by lattice density functional theory simulations

AbstractThe moisture content in cement pastes influences their mechanical properties and durability. However, the complex, multiscale nature of cement pastes makes it challenging to isolate the contributions of each scale to their macroscopic water sorption isotherms. In particular, the contribution of the calcium–silicate–hydrate gel (the binding phase of cement pastes) remains only partially understood. Here, we introduce a density functional theory lattice model describing water sorption in calcium–silicate–hydrate, which properly reproduces experimental water sorption isotherms in cement pastes. Based on this model, we deconstruct the contribution of each pore scale (interlayer spacing, gel pores, and capillary pores) to the total sorption isotherm. We show that, when the relative humidity is below 80%, the calcium–silicate–hydrate gel accounts for more than 90% of the moisture content adsorbed in cement pastes. In turn, we find that the contribution of the interlayer space within the calcium–silicate–hydrate grains is governed by the competition between the rate of interlayer space opening and the increasing propensity for water to fill larger pores upon increasing relative humidity. Overall, our results highlight the key role played by the calcium–silicate–hydrate in governing the sorption isotherms of cement pastes.

Enhanced Sorption of Supercritical CO2 and CH4 in the Hydrated Interlayer Pores of Smectite.

Understanding the long-term confinement of supercritical fluids in the clay pores of subsurface rocks is important for many geo-energy technologies, including geological CO2 storage. However, the adsorption properties of hydrated clay minerals remain largely uncertain because competitive adsorption experiments of supercritical fluids in the presence of water are difficult. Here, we report on the sorption properties of four source clay minerals-Ca-rich montmorillonite (STx-1b), Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-for water at 20 °C up to relative humidity of 0.9. The measurements unveil the unsuitability of physisorption analysis by N2 (at 77 K) and Ar (at 87 K) gases to quantify the textural properties of clays because of their inability to probe the interlayers. We further measure the sorption of CO2 and CH4 on swelling STx-1b and nonswelling IMt-2, both in the absence (dehydrated at 200 °C) and the presence of sub-1W preadsorbed water (following dehydration) up to 170 bar at 50 °C. We observe enhanced sorption of CO2 and CH4 in STx-1b (50 and 65% increase at 30 bar relative to dry STx-1b, respectively), while their adsorption on IMt-2 remains unchanged, indicating the absence of competition with water. By describing the supercritical adsorption isotherms on hydrated STx-1b with the lattice density functional theory model, we estimate that the pore volume has expanded by approximately 6% through the formation of sub-nanometer pore space. By presenting a systematic approach of quantifying the smectite clay mineral's hydrated state, this study provides an explanation for the conflicting literature observations of gas uptake capacities in the presence of water.

Evaluating liquid-water generation in catalyst layers of polymer electrolyte fuel cells based on adsorption isotherm measurements and lattice density functional theory calculations

Evaluating liquid-water generation in catalyst layers of polymer electrolyte fuel cells based on adsorption isotherm measurements and lattice density functional theory calculations

Supercritical adsorption in micro- and meso-porous carbons and its utilisation for textural characterisation

Understanding supercritical gas adsorption in porous carbons requires consistency between experimental measurements at representative conditions and theoretical adsorption models that correctly account for the solid’s textural properties. We have measured unary CO2 and CH4 adsorption isotherms on a commercial mesoporous carbon up to 25 MPa at 40 °C, 60 °C and 80 °C. The experimental data are successfully described using a model based on the lattice Density Functional Theory (DFT) that has been newly developed for cylindrical pores and used alongside Ar (87K) physisorption to extract the representative pore sizes of the adsorbent. The agreement between model and experiments also includes important thermodynamic parameters, such as Henry constants and the isosteric heat of adsorption. The general applicability of our integrated workflow is validated by extending the analysis to a comprehensive literature data set on a microporous activated carbon. This comparison reveals the distinct pore-filling behaviour in micro- and mesopores at supercritical conditions, and highlights the limitations associated with using slit-pore models for the characterisation of porous carbons with significant amounts of mesoporosity. The lattice DFT represents a departure from simple adsorption models, such as the Langmuir equation, which cannot capture pore size dependent adsorption behaviour, and a practical alternative to molecular simulations, which are computationally expensive to implement.

Pore scale study of gas sorption hysteresis in shale nanopores using lattice Boltzmann method

Tremendous amount of gas can be adsorbed in shale nanopores as a result of the high surface-to-volume ratio, and capillary condensation might occur in mesopores (2–50 nm) for subcritical gases, leading to sorption hysteresis. However, the sorption (adsorption and desorption) behavior in different pore systems in shale is not very well understood. Herein we use a D3Q19 lattice Boltzmann model (LBM) that incorporates the mesoscopic fluid-fluid and solid-fluid interactions to study sorption hysteresis in nanopores. The model is developed based on our previous work using a D2Q9 LBM, and is calibrated to lattice density functional theory (LDFT). Improved thermodynamic consistency is observed using exact difference method and tunable interparticle forcing scheme. Sorption simulation in an open and semi-closed slit pore shows different extent of hysteresis, which is in good agreement with Grand Canonical Monte Carlo simulation (GCMC) results. We then reconstruct two types of nanopore systems that are commonly observed in shale, namely the interparticle and intraparticle pores, and study and compare nitrogen sorption hysteresis. The spatial distribution of the condensed nitrogen is visualized and two distinct types of sorption hysteresis loops based on the classification by International Union of Pure and Applied Chemistry (IUPAC) are observed, which indicates that the experimentally measured nitrogen sorption curves can be used to distinguish the dominant pore type in shale. The 3D LBM developed in this study can be used to explore a wide variety of thermodynamic processes that happen at nanoscale, and it can also be incorporated with hydrodynamics to study coupled problems of fluid adsorption/transport in shale.

A Simple Monte Carlo Simulation For the Two Dimensional Attractive Hubbard Model

We use the Auxiliary Field Quantum Monte Carlo technique to perform a simple study of the two dimensional attractive Hubbard model and calculate the total ground state energy to compare with another technique in the literature for U = –4t. Our finite size extrapolated result corroborate the approximated lattice density-functional theory method for this value of U at half-filling. To stabilize the Auxiliary Field Quantum Monte Carlo algorithm one must resort to one of the two decomposition methods: the modified Grand-Schmidt decomposition and the singular value decomposition. Two characteristics like accuracy and total execution time were considered to compare the decomposition algorithms, and the results are presented bellow.

Relation between oxygen gas diffusivity and porous characteristics under capillary condensation of water in cathode catalyst layers of polymer electrolyte membrane fuel cells

We present a coupled simulation method for evaluating oxygen diffusivity in cathode catalyst layers of polymer electrolyte membrane fuel cells under capillary condensation of water partially filling the pore network. The voxel data of a catalyst layer formed with platinum-supported carbon and Nafion ionomer are prepared from sequential cross-sectional images taken by a cryo-focused ion beam scanning electron microscope. Capillary condensation of water, which effectively alters the pore network, is simulated using the lattice density functional theory. Then, the effective porosity εeff, chord length distribution, and characteristic length of the partially wet catalyst layer are evaluated. The effective diffusion coefficients of oxygen in the Knudsen regime are evaluated using the mean-square displacement method. The ratio εeff/τ, with τ being the tortuosity factor, roughly exhibits intermediate behavior between the Bruggeman correlations of sphere- and cylinder-filling for εeff ≳ 0.2, while εeff/τ drastically deviates from the Bruggeman correlation for εeff ≲ 0.2. The effective oxygen diffusion coefficients at 350 K are predicted as a function of relative humidity and total pressure based on the kinetic theory of gases. The proposed method gives deeper insights into oxygen diffusivity under capillary condensation, contributing to more efficient material design.

Recent Progress in Lattice Density Functional Theory

Recent developments in the density-functional theory of electron correlations in many-body lattice models are reviewed. The theoretical framework of lattice density-functional theory (LDFT) is briefly recalled, giving emphasis to its universality and to the central role played by the single-particle density-matrix γ . The Hubbard model and the Anderson single-impurity model are considered as relevant explicit problems for the applications. Real-space and reciprocal-space approximations to the fundamental interaction-energy functional W [ γ ] are introduced, in the framework of which the most important ground-state properties are derived. The predictions of LDFT are contrasted with available exact analytical results and state-of-the-art numerical calculations. Thus, the goals and limitations of the method are discussed.

Supercritical CO2 and CH4 Uptake by Illite-Smectite Clay Minerals.

Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their submicron features have direct relevance to many geoenergy applications. The quantification of gas uptake over a broad range of pressures is key toward assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals-Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2)-using CO2 and CH4 up to 30 MPa at 25-115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N2 (77 K) and Ar (87 K), while larger uptakes are measured with CO2 (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The lattice density functional theory model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.

Study of subcritical and supercritical gas adsorption behavior in different nanopore systems in shale using lattice Boltzmann method

Adsorption has been studied for decades in either petroleum or chemical industry. However, the detailed adsorption behavior is yet not well understood in nanoporous media with complex pore structures and surface properties, especially for subcritical gases when capillary condensation might occur. Shale (mudrock) has complex pore systems as a result of heterogeneous mineralogy and diagenesis, and interparticle (interP) and intraparticle (intraP) pores are two main components. Understanding gas adsorption and phase transition behavior in different pore systems is essential for petrophysical characterization, reserve estimation and production forecast. In this study, we use lattice Boltzmann method (LBM) to study nitrogen (subcritical) and methane (supercritical) adsorption behavior in reconstructed shale nanopore systems. The model accounts for the van der Walls (vdW) forces between fluid molecules by incorporating a modified pseudopotential model based on Peng-Robinson equation of state (PR-EOS). Rock-fluid interaction is also considered by introducing phenomenological surface forces, which take two different forms for subcritical and supercritical gases respectively. The correlation between parameters defined in LBM and physical properties (i.e. density, pressure, isosteric heat of adsorption) is established systematically. Using the developed model, we observe quantitative agreement between LBM and lattice density functional theory (LDFT)/experimental data in terms of adsorption isotherms and density distributions. We then compare adsorption behavior in two ‘model’ interP and intraP pore systems and a synthesized polyethylene porous medium with more complex pore shapes. We observe three adsorption stages for subcritical gas adsorption: a) early capillary condensation near grain contacts or sharp corners, forming isolated pendular rings or fluid clusters; b) growing, merging, and spreading of the isolated condensed phase, forming circular gas/liquid meniscus; c) the condensed phase becomes fully-connected across the geometry, leaving isolated gas bubbles. We observe a signature behavior for the adsorption isotherm of intraP pore system, where a stepped feature occurs around relative pressure of 0.8, and the adsorption uptake for the polyethylene porous medium is bounded by the upper and lower limits given by the interP and intraP pore systems. On the other hand, we find that supercritical methane adsorption in mesoporous media is less sensitive to pore shape, but rather controlled by the specific surface area. The existence of micropores also affects the adsorption uptake.

Lattice density-functional theory for quantum chemistry

We propose a lattice density-functional theory for {\it ab initio} quantum chemistry or physics as a route to an efficient approach that approximates the full configuration interaction energy and orbital occupations for molecules with strongly-correlated electrons. We build on lattice density-functional theory for the Hubbard model by deriving Kohn-Sham equations for a reduced then full quantum chemistry Hamiltonian, and demonstrate the method on the potential energy curves for the challenging problem of modelling elongating bonds in a linear chain of six hydrogen atoms. Here the accuracy of the Bethe-ansatz local-density approximation is tested for this quantum chemistry system and we find that, despite this approximate functional being designed for the Hubbard model, the shapes of the potential curves generally agree with the full configuration interaction results. Although there is a discrepancy for very stretched bonds, this is lower than when using standard density-functional theory with the local-density approximation.

Prediction of CO2 adsorption-induced deformation in shale nanopores

Understanding of CO2 adsorption in shale provides significant information about the potential for geological storage of CO2 in shale formations to mitigate the impact of carbon emissions on the climate. This work focuses on the adsorption behavior of CO2 in shale with various pore sizes at a wide temperature range including 31.28 °C, 40 °C, 65 °C, and 90 °C and pressures up to 15 MPa through a comprehensive model. Using a thermodynamic approach, we combine lattice density functional theory with finite element formulation to estimate CO2 adsorption behavior along with its induced strain in shale nanopores. We study the effect of geometric confinement, including pore size and pore geometry, on the excess adsorption and swelling strain of shale under different temperatures. When the model was applied to sorption of CO2 in Posidonia Shale samples, all reported experimental features of the phenomenon were reproduced. Although the developed model applied to the specific case of CO2 adsorption-induced swelling in shale, it is also applicable to any problem regarding adsorption of any fluid confined in a porous medium and its triggered swelling strain.We found that the highly confined geometry of the duct pore, which applies a large attractive surface field to adsorbed layers, leads to a faster, and a higher amount of adsorption compared with that of a slit pore. This effect is more significant in larger pore widths. The results also demonstrate a sharp rise in the adsorption isotherm and swelling strain near the bulk critical point of CO2. This phenomenon attributes to the highly compressive behavior of CO2 near its critical point, which leads to a sharp increase in bulk density and, thus, in adsorbed phase density and its resultant swelling strain. The results also denote that total volumetric strain is a function of temperature, pore width, and pore shape. The effect of pore shape and temperature is more prominent for pores with larger sizes.

Orange-Emitting Li4Sr4[Si4O4N6]O:Eu2+-a Layered Lithium Oxonitridosilicate Oxide.

We report on the structure and properties of the lithium oxonitridosilicate oxide Li4Sr4[Si4O4N6]O:Eu2+ obtained from solid-state metathesis. The crystal structure was solved and refined from single-crystal X-ray data in the space group P42/ nmc (No. 137) [ Z = 2, a = 7.4833(6), c = 9.8365(9) Å, and R1(obs) = 0.0477]. The structure of Li4Sr4[Si4O4N6]O:Eu2+ is built up from a layered 2D network of SiN3O tetrahedra and exhibits stacking disorder. The results are supported by transmission electron microscopy and energy-dispersive X-ray spectroscopy as well as lattice energy, charge distribution, and density functional theory (DFT) calculations. Optical measurements suggest an indirect band gap of about 3.6 eV, while DFT calculations on a model free of stacking faults yield a theoretical electronic band gap of 4.4 eV. Samples doped with Eu2+ exhibit luminescence in the orange spectral range (λem ≈ 625 nm; full width at half-maximum ≈ 4164 cm-1; internal quantum efficiency at room temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of lithium oxonitridosilicate oxides.

Structural and electrochemical properties of Na2FeSiO4 polymorphs for sodium-ion batteries

The advance of battery technology in electric vehicle and large-scale energy storage applications drives the development of new battery materials, such as sodium iron silicate Na2FeSiO4. In this work, structural characteristics, deintercalation voltages, ionic conductivities, and mechanical properties of Na2FeSiO4 polymorphs are investigated by using potential-based lattice dynamics and density functional theory methods. To simulate the polymorphism of Na2FeSiO4, a total of 14 structural models are constructed from the structure types ABC2X4 of Na- and Li-based silicates. Free energies are calculated within the quasi-harmonic approximation to evaluate the relative stabilities of 13 dynamically stable structures under temperature and pressure conditions. The energetically favored structures reveal that Na2FeSiO4 polymorphs tend to form the structures with three-dimensional (3D) framework of FeO4 and SiO4 tetrahedra. In comparison with Li-equivalent, Na2FeSiO4 is predicted to have the characteristics of smaller volume change, lower deintercalation voltage, 3D or quasi-3D ionic mobility, and better mechanical property, which may deliver a prospect of higher electrochemical performance. The mechanism of substitution Na for Li on the effects of structural and electrochemical properties is also discussed. Helpful information is expected to provide for the preparation of high-performance cathode materials of Na-ion batteries and the fundamental understanding of Na-intercalation chemistry.