Rocksalt Structure Research Articles

Phase change memory materials: Why are alloys of Ge, Sb, and Te the materials of choice?

Rewritable optical storage is dominated by alloys of a small number of elements, particularly Ge, Sb, and Te, and Ge/Sb/Te alloys in the composition range (GeTe)1−x(Sb2Te3)x (0 ≤ x ≤ 1) have been the materials of choice: all have metastable rock salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status arises from the close similarity of the valence orbitals of Ge, Sb, and Te, which arises from the irregular changes in atomic orbitals and properties as the atomic number increases (“secondary periodicity”). The instability of cubic (metallic) Bi to a (semimetallic) rhombohedral structure (H. Jones, 1934) can be adapted to Ge/Sb/Te alloys and explains the crucial metastable (rock salt) structures of these compounds. Vacancies almost always occur next to Te atoms, which form one sublattice of the rock salt structure.

Localisation of vibrational modes in high-entropy oxides

The recently-discovered high-entropy oxides (HEO’s) offer a paradoxical combination of crystalline arrangement and high disorder. They differ qualitatively from established paradigms for disordered solids such as glasses and alloys. In these latter systems, it is well known that disorder induces localised vibrational excitations. In this article, we explore the possibility of disorder-induced localisation in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, the prototypical HEO with rock-salt structure. To describe phononic excitations, we model the interatomic potentials for the cation–oxygen interactions by fitting to the physical properties of the parent binary oxides. We validate our model against the experimentally determined crystal structure and optical conductivity. The resulting phonon spectrum shows wave-like propagating modes at low energies and localised modes at high energies. Localisation is reflected in signatures such as participation ratio and correlation amplitude. Finally, we argue that mass disorder can be increased to enhance localisation. We consider a hypothetical material, high-entropy telluride-oxide, where tellurium atoms are admixed into the anion sublattice. This shows a larger localised fraction, with additional localised modes appearing in the middle of the spectrum. Our results demonstrate that HEO’s are a promising platform to study Anderson localisation of phonons.

Enhancing the Electrochemical Performances of Disordered Li1.23Ni0.3Nb0.3Fe0.16O0.85F0.15 Cathode Material for Lithium-Ion Batteries by LiNbOx Coating.

For the next generation of lithium-ion batteries (LIBs), it is primary to seek high capacity and long-lifetime electrode materials. Li-excess disordered rock-salt structure (DRS) cathodes have gained much attention due to their high specific capacity. However, Li-excess can lead to a decrease in the structural stability of an electrode material. A new Li-rich DRS oxyfluorides, Li1.23Ni0.3Nb0.3Fe0.16O0.85F0.15 (F0.15) with a series amounts of LiNbOx (LN) coating (0, 5, 10, and 15 wt % denoted as F0.15-LN0, F0.15-LN5, F0.15-LN10, and F0.15-LN15, respectively), are successfully synthesized and evaluated as cathode materials in LIBs. Among them, F0.15-LN10 exhibits the highest initial discharge specific capacity of 296.1 mAh g-1 (at a current density of 20 mA g-1) with the capacity retention rate of 14% higher than that of the uncoated F0.15 after 80 cycles. Even at 300 mA g-1, F0.15-LN10 still delivers the highest discharge specific capacity of 130 mAh g-1. After 20 cycles, the charge-transfer impedance of F0.15-LN10 remained the smallest. The characterizations indicate that LN coating reduces the surface polarization of the cathode materials, slows the interfacial side reactions between the electrolyte and the electrode, and speeds up the Li+ diffusion. These results demonstrate that LN coating is an effective strategy to improve the electrochemical performance.

First-principles study on the structure, mechanical and thermodynamic properties of (Ti, Hf, Nb, Ta)C high-entropy carbide ceramics

High-entropy carbide ceramics (HECCs) have demonstrated extraordinary properties and broad application potential. However, the action mechanisms of metallic elements on the properties have not been understood in depth. In this study, the structural stability and the mechanical and thermodynamic properties of (Ti, Hf, Nb, Ta)C HECCs across a broad composition spectrum were systematically analyzed based on the density functional theory and Debye–Grüneisen model. The crystal structure was constructed and validated through experimental testing. The mechanical properties such as elastic moduli, ductility, hardness, and Poisson's and Pugh's ratios were calculated. Furthermore, the temperature dependence of bulk moduli, thermal expansion coefficients, and heat capacities of the HECCs were investigated. To interpret the elastic and toughening mechanisms of non-equiatomic HECCs, the electronic structures were investigated. The results indicate that the HECCs with a single solid solution phase of the FCC rock-salt structure are thermodynamically stable. The HECCs containing higher Nb and Ta contents have greater strength and stiffness, and those containing higher Ti and Hf contents exhibit greater hardness and brittleness than the equiatomic HECC. In addition, non-equiatomic HECCs containing higher Nb and Ta contents have larger bulk moduli and lower linear thermal expansion coefficients, which can be attributed to the stronger covalent interaction among metallic elements and carbon. This study is expected to have an impact on designing ideal HECCs for high-temperature applications under extreme environments.

ICEMS Characterization of NixFe3-xO4 (0.7 ≤ x ≤ 1.7) thin films grown by ion beam sputtering on (0001) Al2O3 substrates

We report here on the ICEMS characterization of nickel ferrite (NixFe3−xO4) thin films having different nickel contents grown on alumina substrates by Ion Beam Sputtering. The spectra corresponding to the films with nominal x = 0.7, 1.0 and 1.2 are characteristic of compounds crystallizing in a spinel-related structure showing two different magnetic sextets associated with Fe3+ located in the tetrahedral and octahedral sites of such structure. The spectra show an additional broad third sextet with a large isomer shift which suggests the occurrence of electron hopping between Fe2+ and Fe3+ ions sitting in the octahedral sites. With increasing nickel content, the linewidth of the sextets increases and their corresponding hyperfine magnetic fields decrease. This is an indication of an increase in structural disorder in the deposited films as their nickel concentrations increase. The cation distribution of the iron ions over the tetrahedral and octahedral sites appears also to depend on the nickel content. The film with x = 1.2 shows a significant increase in the fraction of octahedral iron ions as compared with the expected nominal value suggesting that, for this composition, some Ni2+ could also occupy tetrahedral sites. The Mössbauer spectrum corresponding to the film with x = 1.7 shows a magnetic pattern with very broad lines similar to those shown by amorphous or disordered materials. The average isomer shift is quite high (around 0.40 mms− 1) and characteristic of Fe3+ in octahedral oxygen coordination. This indicates that for the largest nickel content studied (x = 1.7), the film does not contain Fe3+ in tetrahedral environments suggesting that the spinel structure is no longer present. This correlates well with the X-Ray Diffraction data which indicate a structural change from spinel to a disordered rock-salt structure for this particular film with high nickel content.

Eco-friendly high entropy oxide rock-salt type structure for oxygen evolution reaction obtained by green synthesis

Global energy consumption increases year after year, causing the depletion of non-renewable sources. According to the International Energy Agency (IEA), global demand for electrical energy is expected to increase by 3.3 % in 2024. Therefore, developing new renewable sources is urgent, including new devices for energy storage and conversion, particularly those based on electrochemical reactions. Water splitting is a clean and sustainable technology capable of facing this issue by producing oxygen and hydrogen from water and electricity. However, an issue related to this technology is the slow kinetics of oxygen evolution reaction, making it necessary to develop new electrocatalysts with high electrochemical performance. To meet this requirement, this work deals, for the first time, with a high entropy oxide with a rock-salt structure synthesized by a green sol–gel synthesis using red seaweed (Rhodophyta) as a polymerizing agent. Sol-gel synthesis allows the large-scale production of nanomaterials with high uniformity and dispersion of the involved chemical elements. The literature, which discussed the synthesis of these oxides, reveals that agents harmful to the environment are employed, including sodium hydroxide, acetic acid, hexadecyltrimethylammonium bromide, urea, and ammonium hydroxide. The composition of the high entropy oxide is (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O. As electrocatalyst for oxygen evolution reaction, it exhibits a low overpotential (336 mV vs. RHE at 10 mA cm−2), a Tafel slope of 68 mV dec-1, and excellent durability. The electrochemical performance of the high entropy oxide prepared in this work is superior to other electrocatalysts of the same class that were produced using transition metal-based precursors.

Phase transition mechanism and long‐term stability of LiF‐assisted sintered Li2MgxZrO3+x microwave dielectric ceramics

AbstractHerein, Li2MgxZrO3+x (x = 0–1) +2 wt.% LiF ceramics with a rock salt structure were prepared by the solid‐state reaction method. The phase transition mechanism was systematically investigated by X‐ray diffraction, and first‐principles calculations. With a gradual increase in MgO content, the samples underwent a monoclinic‐Li2ZrO3 to tetragonal‐Li2ZrO3 to tetragonal‐Li2MgZrO4 to cubic‐phase transition. The variations of the microwave dielectric properties due to the phase transition were systematically analyzed by the multi‐phase mixtures law. In addition, CO2 adsorption of the samples was quantitatively analyzed by X‐ray photoelectron spectroscopy, and the long‐term stability of the Q × f of the samples was investigated. The optimal microwave dielectric properties were obtained in Li2Mg0.2ZrO3.2 ceramic at 950°C: εr = 19.5, Q × f = 44 500 GHz, and τf = 2.6 ppm/°C. In addition, the ceramic samples were compatible with Ag, which indicates that Li2MgxZrO3+x (x = 0−1) +2 wt.% LiF is a promising material for low‐temperature co‐fired ceramics.

Influence of Synthesis Parameters on the Short-Range Structure and Electrochemical Performances of Li2MnO2F.

In the last years, disordered rocksalt structure (DRS) materials were proposed as a positive electrode for lithium-ion batteries. In particular, the fluorinated DRS materials were proposed to be more stable upon cycling than pure oxide counterparts. These materials are mainly obtained by mechanosynthesis in order to incorporate a significant number of F ions and maintain a disordered structure. Since the local structural arrangement is crucial for battery application, we aim to monitor its evolution upon the synthesis of Li2MnO2F from two sets of precursors: Mn2O3, Li2O, and LiF or LiMnO2 and LiF. The synthesis progress was thus followed, by 7Li and 19F MAS NMR coupled to XRD to probe the structure at different scales. This allowed us to identify an optimal milling time to reach the final compounds. We show that they exhibit similar morphology (by SEM), medium- and short-range orders (by XRD, 7Li and 19F NMR, EXAFS), and average Mn oxidation degree (by XANES). The electrochemical performances of the two compounds are almost similar, with high specific capacities of 319 mAh·g-1 ("from LiMnO2") and 304 mAh·g-1 ("from Mn2O3") for the first charge to 4.8 V vs Li+/Li, proving their interest as post-NMC candidates as positive electrode materials.

Nitrogen-Rich Molybdenum Nitride Synthesized in a Crucible under Air.

The triple bond in N2 is significantly stronger than the double bond in O2, meaning that synthesizing nitrogen-rich nitrides typically requires activated nitrogen precursors, such as ammonia, plasma-cracked atomic nitrogen, or high-pressure N2. Here, we report a synthesis of nitrogen-rich nitrides under ambient pressure and atmosphere. Using Na2MoO4 and dicyandiamide precursors, we synthesized nitrogen-rich γ-Mo2N3 in an alumina crucible under an ambient atmosphere, heated in a box furnace between 500 and 600 °C. Byproducts of this metathesis reaction include volatile gases and solid Na(OCN), which can be washed away with water. X-ray diffraction and neutron diffraction showed Mo2N3 with a rock salt structure having cation vacancies, with no oxygen incorporation, in contrast to the more common nitrogen-poor rock salt Mo2N with anion vacancies. Moreover, an increase in temperature to 700 °C resulted in molybdenum oxynitride, Mo0.84N0.72O0.27. This work illustrates the potential for dicyandiamide as an ambient-temperature metathesis precursor for an increased effective nitrogen chemical potential under ambient conditions. The classical experimental setting often used for solid-state oxide synthesis, therefore, has the potential to expand the nitride chemistry.

Learning from machine learning: the case of band-gap directness in semiconductors

Having a direct or indirect band gap can influence the potential applications of a semiconductor, for indirect band gap materials are usually not suitable for optoelectronic devices. Even though this is a fundamental property of semiconducting materials, discussed in textbooks, no unified theory exists to explain why a material has a direct or indirect band gap. Here we used an interpretable machine learning model, the multiVariate dAta eXplanation (VAX) method, to gather information from a dataset of materials extracted from the Materials Project. The dataset contains more than 10000 entries, and atomic properties such as the number of electrons, electronic affinity and orbital energies were used as features to build random forest models that successfully explain the directness of the band gaps. Our results indicate that symmetry is an important feature that dictates the target property, which is the reason why our analysis is made based on sub-groups with similar structures. These sub-groups include materials with zincblende, rocksalt, wurtzite, and perovskite structures. Besides the symmetry of the materials, the existence or not of d bands and the relative energy of atomic orbitals were found to be important in defining whether a material’s band gap is direct or indirect. In conclusion, interpretable machine learning methods such as VAX can be useful in obtaining physical interpretation from materials databases.

The ultralow loss Li2Mg3Ti1−xAl4x/3O6 microwave dielectric ceramics and its design of dielectric resonator antenna

The Li2Mg3Ti1−xAl4x/3O6 (0 ≤ x ≤ 0.1) ceramics are synthesized using the solid phase method, showing excellent microwave dielectric properties with εr = 14.05, Q × f = 144 500 GHz, and τf = −43 ppm/°C at x = 0.04. Rietveld refinements, based on the XRD data, reveal that the Li2Mg3Ti1−xAl4x/3O6 ceramics crystallize in the rock salt structure with Fm-3m space group. The relationship between the crystal structure and intrinsic microwave dielectric properties is studied in detail. The variation in the dielectric constant is related to ionic polarization and relative density. The quality factor (Q × f) demonstrates a strong correlation with lattice energy, relative density, and concentration of Ti3+. The variation of τf is influenced by the bond thermal expansion coefficient and bond valence. The dielectric resonant antenna, manufactured from Li2Mg3Ti0.96Al0.0533O6-0.09TiO2 ceramic with near-zero τf, shows a bandwidth of 230 MHz at 7.40 GHz and a high radiant efficiency, which indicates that high-quality factor Li2Mg3Ti0.96Al0.0533O6 ceramic has significant potential in relevant fields such as 5G and smart driving.

Nb-doping of cation-disordered rocksalt oxides with B2O3 surface modification for superior performance cathode

As the growing price and inadequate reserves of cobalt, the demand for cobalt-free materials with superior energy density to apply in Li-ion batteries has increased distinctly. Li-rich cation disordered rocksalt (DRX) material has aroused research hotspot because of high specific capacity and wide compositional space. However, irreversible oxygen loss as well as sluggish kinetic raise the problem in large polarization and inevitable capacity decay in cycling process, which have impeded the commercialization of these electrode materials. In this work, we have synthesized cobalt-free boron trioxide (B2O3)-modified Li1.2Ni0.3Ti0.3Nb0.2O2 compounds with cation disordered rocksalt structure through mechanochemical method. It is a fact that B2O3-modified cathodes possess outstanding cyclic stability as well as rate properties. Particularly, the Li1.2Ni0.3Ti0.3Nb0.2O2 electrode with 2.0 wt% B2O3 displayed a competitive capacity of 192.4 mAh/g at 10 mA g−1 and retained 134.7 mAh/g after 100 charge/discharge times. The redox mechanism is explored through ex-situ XPS measurement, verifying the reversible cationic redox of Ni2+/Ni4+ accompanying small Ti4+/Ti3+ reduction and anionic O2−/O− evolution, which results in unusual high capacity. Such an optimized surface modification combines with the Nb doping, can promote Li ion transport, reduce polarization, protect cathode from reacting with electrolyte. These findings provide important implications to improve performance of DRX cathodes.

Building a robust surface structure toward stable ultrahigh-nickel cathodes

Ni-rich cathodes especially with Ni content over 90 % are promising next-generation cathode materials for advanced high energy lithium − ion batteries. However, in comparison with Ni ≤ 80 % Ni-rich cathodes, these ultrahigh Ni-rich materials face more severe challenges in terms of harmful H2 − H3 phase transformations and structural degradation during cycling. Herein, we present a robust surface structure composted of defect-rich disordered structures and LiF/Li2SiO3 coating for stabilizing the ultrahigh Ni-rich cathode LiNi0.96Co0.03Mn0.01O2 (NCM96) by modification with (NH4)2SiF6. The surface disordered structures including Li/TM mixing, spinel, and rock salt structure play a crucial role in inhibiting the irreversible H2-H3 phase transition and reducing lattice volume changes by lattice coherence effect. Moreover, the disordered surface in collaboration with the co-coating restrains the rapid growth of surface rock salt phase into the bulk, enhancing structural stability. The modified sample NSF-0.5 delivers a capacity of 238 mAh/g at 0.1C and maintains a capacity retention of 88.5 % at 0.5C after 100 cycles in half cell as well as 82.3 % at 1C after 300 cycles in full cell, significantly superior to the NCM96. The findings provide a simple and cost-effective surface modification strategy to enhance the structural stability and electrochemical performance of ultrahigh nickel cathodes for high-energy density LIBs.

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides.

In order to help understand the structural stability of KCoO2-type ternary nitrides AMN2, referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO2-type AMN2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN2 nitrides exhibit α-NaFeO2-type structure with t ∼ 0.898-0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN2 and the greater tolerance factor of 1.048 for BaTiN2. The experimentally already synthesized AMN2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.

Mitigated Oxygen Loss in Lithium‐Rich Manganese‐Based Cathode Enabled by Strong Zr–O Affinity

AbstractOxygen loss is a serious problem of lithium‐rich layered oxide (LLO) cathodes, as the high capacity of LLO relies on reversible oxygen redox. Oxygen release can occur at the surface leading to the formation of spinel or rock salt structures. Also, the lattice oxygen will usually become unstable after long cycling, which remains a major roadblock in the application of LLO. Here, it is shown that Zr doping is an effective strategy to retain lattice oxygen in LLO due to the high affinity between Zr and O. A simple sol‐gel method is used to dope Zr4+ into the LLOs to adjust the local electronic structure and inhibit the diffusion of oxygen anions to the surface during cycling. Compared with untreated LLOs, LLO–Zr cathodes exhibit a higher cycling stability, with 94% capacity retention after 100 cycles at 0.4 C, up to 223 mAh g−1 at 1 C, and 88% capacity retention after 300 cycles. Theoretical calculations show that due to the strong Zr–O covalent bonding, the formation energy of oxygen vacancies has effectively increased and the loss of lattice oxygen under high voltage can be suppressed. This study provides a simple method for developing high‐capacity and cyclability Li‐rich cathode materials for lithium‐ion batteries.

Band gap engineering through calcium addition in (Mg, Co, Ni, Cu, Zn)O high entropy oxide for efficient photocatalysis

Multicomponent metal oxides have shown the potential to be excellent catalysts mainly because of synergistic interactions between the active sites. We report (Co, Mg, Ni, Cu, Zn)1-xCaxO (x = 0.05, 0.1) high entropy oxide (HEO) photocatalyst with single phase rocksalt structure (space group Fm3¯m) synthesized by solution combustion synthesis (SCS) technique. HEOs contain elements with varying electronegativities and crystal field splitting and thus exhibit a relatively lower band gap when compared to the individual constituent oxides. Adding Ca2+ results in increased covalency due to the greater exchange interaction between O 2p and transition metal (TM) 3d orbital, engendering further reduction in band gap and consequently improving photocatalytic activity in the visible region. In addition, the introduction of point defects, mainly metal/oxygen vacancy, to accommodate the larger size Ca2+ cation also contributed to the catalytic activity. The photocatalytic activity enhancement is demonstrated by visible light-assisted photodegradation of methylene blue (MB). The MB dye degradation rate constant for (Co, Ni, Cu, Zn, Mg)0.9Ca0.1O catalyst was found to be ∼0.0445 min−1, which was about six times faster than that of the pristine (Co, Mg, Ni, Cu, Zn)O.

Co-deposition of bismuth-nitrogen films on MgO (001) by molecular beam epitaxy

We attempted to grow a thin film of BiN by co-deposition of bismuth and nitrogen on rock-salt structure MgO (001) substrates. Furthermore, we studied the effect of variation of the growth temperature and the nitrogen to bismuth flux ratios on sample growth. For the samples grown and conditions used, we do not find strong evidence for the formation of a bulk Bi-N alloy. Even for very high nitrogen to bismuth flux ratio, we observed only bismuth and no nitrogen using bulk Rutherford back-scattering spectroscopy measurements, and only 1%–2% nitrogen was seen through surface Auger electron spectroscopy measurements. The in-plane lattice measurements show that the resulting Bi (110) samples are strained, which is presumably caused by lattice mismatch between the sample and the substrate when grown without any buffer layer. The use of a high-temperature buffer layer helps to release strain in the sample but only along one axis. Measurements of the atomic layer spacing using x-ray diffraction and also scanning tunneling microscopy confirm the Bi (110) thin film sample structure.

A comparative study on the hardness, stress and surface free energy of HfN films grown by HIPIMS and direct-current magnetron sputtering

HfN films with rocksalt structure have been grown by high-power impulse magnetron sputtering (HIPIMS) with various duty cycles, in a comparison with direct-current magnetron sputtering (dcMS) process. Such two sputtering processes yield the quite close compositions. Increasing the duty cycle from 7.5 % to 9 % and 11 % in HIPIMS process, a compressive stress rises up from 1.5 to 1.9 and 2.1 GPa, which is much smaller than the value of 3.8 GPa from dcMS. The hardness of such low-stressed HfN films has been identified to be 25.5–25.9 GPa, which agrees well with the theoretical one of 22.5 GPa by the density functional theory without the consideration of stress. In addition, the surface free energy of 26.3–29.0 mJ/m2 has been determined in HfN films grown by HIPIMS, much lower than that of 31.5 mJ/m2 of the films grown by dcMS.

Realization of cathodoluminescence in the 180 nm spectral range by suppressing thermal stress in mist chemical vapor deposition of rocksalt-structured MgZnO films

Rocksalt-structured (RS) Mg x Zn1−x O films with x = 0.65–1.0 were grown on MgO (100) substrate using the mist CVD method. A comparative study of the RS-Mg0.92Zn0.08O films grown under slow and rapid-cooling rates apparently showed simultaneous reductions in the surface pit density, FWHM values for the X-ray diffraction peak, and defect-related cathodoluminescence (CL) for the film grown under the slow-cooling rate. CL spectra for the RS-Mg x Zn1−x O films grown under the slow-cooling rate eventually showed near-band-edge emission peaks in the 180–190 nm spectral range for MgO molar fraction x ≥ 0.92 at RT.

Na6MCl8 rock-salt compounds with M = Mg, Ca, Ba, Zn, Sr as components for solid-state sodium ion batteries.

We investigate a new series of rock-salt type structures, Na6MCl8 with M = Mg, Ca, Ba, Zn and Sr using advanced atomistic simulations. Calculated results show a direct relationship between the size of the M2+ cation and lattice parameters as well as the defect formation energy variation. The NaCl Schottky defect type is highly favourable, and the Na6BaCl8 structure possesses the lowest values of defect formation energies. These structures are predicted to be mechanically stable and ductile, implying their compatibility with possible use as electrodes/electrolytes. The Na6MCl8 structures exhibit semiconductor characteristics with an energy gap ranging between 4.1-4.6 eV, which differs from the previous value of Na6MgCl8. A 3D migration pathway is identified in each rock-salt structure. Despite the small variation of the Na diffusivity and conductivity at 250 K within the structures considered, the Na6BaCl8 is characterized by the highest conductivity at 250 K, while the Na6MgCl8 structure has the highest conductivity and diffusivity values. The outstanding properties predicted for a Na ion battery suggest future development of synthetic strategies for their actual preparation.