Alkali Metal Sulfides Research Articles

Cladded phononic nodal frame state in biatomic alkali-metal sulfides

Cladded phononic nodal frame state in biatomic alkali-metal sulfides

A first-principles study on crystal structures and metallization of sodium-rich sulfides under high pressure

We performed systematical theoretical simulations on phase diagrams, crystal structures, electron properties, and phonon features of Na–S system under high pressures. NaS, Na2S, and Na4S, were found to be stable under pressures. The superconducting transition critical temperature was estimated to nearly 0 K at 100 GPa in Na3S due to the weak electron–phonon coupling. Furthermore, by the comparison on the structures, the electron features, and alkali metal ions of stoichiometric proportion, we found that not only the pressure but also the number of sodium atoms in the formula unit of alkali metal atoms can promote the insulator–metal transformation in the alkali metal sulfides, such as Li–S, Na–S, and K–S systems.

Na4Ga8S14: a Ga-enriched wide band gap ternary alkali-metal sulfide with unique [Ga12S42] 12-membered rings.

A Ga-enriched ternary alkali-metal sulfide Na4Ga8S14 has been synthesized by a high temperature solid-state reaction. It crystallizes in the centrosymmetric Pbca (no. 61) space group with cell parameters a = 13.5260(4) Å, b = 11.4979(3) Å, c = 29.9592(9) Å, and Z = 8, and exhibits a three-dimensional (3D) network structure constructed from unique [Ga12S42] 12-membered rings, one-dimensional ∞[Ga4S11] chains, individual [GaS4] units and Na+ ions. The experimental band gap of Na4Ga8S14 was measured as ∼3.57 eV. Theoretical calculations indicate that the title compound is a direct band gap compound and the band gap is mainly determined by [GaS4] units. Meanwhile, statistical analysis shows that the atomic ratio N (N = AIAII/Ga, where AI = alkali-metal, AII = alkaline earth-metal) can be used to regulate the connection of [GaS4] units from zero-dimensional (0D) isolated groups, one-dimensional (1D) chains, and two-dimensional (2D) layers to 3D frameworks in Ga-containing alkali- and/or alkaline earth-metal chalcogenides. The results enrich the diversity of alkali-metal sulfides and give an insight into the structural regulation of alkali- and/or alkaline earth-metal chalcogenides.

Li2S doping into CZTSe drives the large improvement of VOC of solar cell

Alkali metal doping or sulfurization are commonly applied in Cu2ZnSnSe4 (CZTSe) solar cell to improve the open-circuit voltage (VOC). However, alkali metal sulfide affording both alkali metal and sulfur is seldom to be studied, which restrains the development of kesterite solar cells. In this study, we evaporate Li2S during selenization process and hope to provide both alkali metal and sulfur to CZTSe film. The result indicates that Li shows a gradient distribution near the surface of CZTSe film and the content of S is slight. The film quality is improved and the recombination at grain boundaries is decreased after Li2S treatment. Besides, the bandgap of the absorber gets wider. Under the synergy of sulfur and lithium (mainly from lithium), the work function of the treated absorber gets higher and the conduction band offset (CBO) is in the ideal range. Combined with these contributions, the VOC of the champion device treated by Li2S dramatically increase by 120 mV. This study discloses that alkali metal brings the main effect on the performance of the kesterite solar cell even an alkali metal sulfide is evaporated, which deepens the understanding of sulfurization of CZTSe and also promote the progress of kesterite solar cells.

A review of the oxidation–pressure concept (OPC) and extended Zintl–Klemm concept (EZKC), and the emergence of the high-pressure Ni2In-type phase of lithium sulfide (Li2S) rationalized by reference to a newly defined stability enhancement ratio (S)

Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61, 220–224] on the pressure-induced Ni2In phase of Li2S, at 30 GPa, three concepts related to high-pressure phase transitions are reviewed here. This paper firstly reviews evidence that chemical oxidation (by inclusion of oxygen atoms) can produce a similar effect to the application of physical high pressure and temperature, in an effect labelled as the oxidation–pressure concept. Secondly, the pressure-induced Ni2In phase of Li2S is the final phase in the double transition antifluorite → anticotunnite → Ni2In, as is observed in other alkali metal sulfides. This new phase for Li2S could be expected after knowledge of the high-pressure Cmcm phase of Li2SO4, which is a distortion of the hexagonal I-Na2SO4 phase, both having M 2S subarrays of the Ni2In-type. Thirdly, in order to clarify these links, a simple methodology is proposed for gauging the level of increased stability (by defining a stability enhancement ratio, S) when the extended Zintl–Klemm concept (EZKC) has been applied. The method uses relative values of the lattice potential energies estimated for Li2S and for the pseudo-lattice Ψ-BeS derived by applying the EZKC to Li2S, after which, Li2S can be reformulated as Li+[LiS]− ≡ Li+[Ψ-BeS].

Characteristics of Particulate Matter Emitted from Agricultural Biomass Combustion

In this work, the emission of particulate matter (PM) from combustion of agricultural biomass was investigated in comparison to woody biomass. The mechanism of PM emission was studied by means of mass-based particle size distributions (PSDs), inorganic elemental component analysis and morphology at variant combustion temperatures, and different biomass feedstocks. The mass-based PSDs of PM10 of cotton stalk, rice husk, and camphor wood exhibit a bimodal distribution, while that of corn stalk exhibits a unimodal distribution. The emission of PM10 of agricultural biomass is much higher than that of woody biomass, and it is mainly composed of PM1, in which Na and K are enriched as alkali metal chloride and sulfide. On the other hand, Mg and Ca are enriched as the main inorganic compounds in PM1–10 for woody biomass. A higher combustion temperature is favorable for the formation of fine PM particles against a reduction of PM10. PM1 and PM1–10 formation mechanisms are different for different biomass feedstocks...

Electrical Energy Storage By a Mg/Cus Accumulator: An Alternative to Lithium Batteries

Rechargeable magnesium batteries are considered to be very promising candidates for next generation non-lithium batteries.[1-5] They use divalent Mg2+ cations instead of monovalent Li+ leading to high theoretical capacities. In addition, magnesium (used as anode) has the lowest standard electrode potential compared to other multivalent metals like aluminum or zinc which will be reflected in high cell voltages. The higher safety, environmental compatibility of the electrode materials, and the low costs of magnesium due to its high abundance compared to lithium are further advantages. Numerous research activities focus on the investigation of new cathode materials.[6-9] In addition, a high electrochemical stability of the electrolyte, especially towards the magnesium anode, is absolutely essential. To develop new electrolytes with improved physical and electrochemical properties therefore is another challenging task in this field.[10-11] In our presentation a different concept of a magnesium battery will be shown. The general idea is to use a CuS cathode which dissolves upon discharge when Cu2+ is reduced to Cu. At the anode Mg is oxidized to Mg2+ forming MgS as a solid discharge product. Here, in contrast to already known magnesium batteries, the divalent S2- anions will be used for the charge transfer instead of the Mg2+ cations. In consequence, a suitable sulfide containing electrolyte is required. It should be noted that, provided that the proposed electrode reactions are fully reversible, the suggested Mg/CuS accumulator can theoretically also be set up in the “discharged” state, i.e. MgS vs. Cu instead of Mg vs. CuS.Besides the processing of the Mg, Cu, MgS, and CuS electrodes the preparation, simulation and electrochemical testing of potential electrolyte candidates is the main focus of this project. Alkali metal sulfides solved in mixtures of organic solvents are chosen as reference electrolytes to be compared with new electrolytes based on ionic liquids. The electrochemical properties are characterized by cyclic voltammetry and impedance spectroscopy. The electrochemical stability of the system and the reversibility of the proposed electrode reactions have to be analyzed by half-cell cycling and full-cell charge/discharge tests. At this, Raman spectroscopy will be a valuable analytical tool to investigate the individual electrode reactions, i.e. the formation and dissolution of MgS and CuS. Experiments are accompanied by MD simulations and physical properties like diffusion coefficients, ion conductivity, etc. can be understood at a molecular level.

Neue Sulfido-Antimonate der schweren Alkalimetalle: Synthese, Kristallstruktur und chemische Bindung von (K=Rb=Cs)3SbS3 und Cs3SbS4 · H2O / New Sulfido Antimonates of the Heavy Alkali Metals: Synthesis, Crystal Structure and Chemical Bonding of (K=Rb=Cs)3SbS3 and Cs3SbS4 · H2O

The new sulfido antimonates(III) (Rb=Cs)3SbS3 were prepared from the alkali metal sulfides Rb2S=Cs2S2 and elemental antimony and sulfur or Sb2S3 at reaction temperatures of about 700 °C. The known isotypic potassium compound was similarily synthesized from the elements. The structures of the light-yellow crystals were refined using single-crystal X-ray data. Both compounds are isotypic to the respective Na salt forming the Na3AsS3 structure type (cubic, space group P213, K/Rb/Cs: a = 947:21(7)=982:28(5)=1025:92(5) pm, Z = 4, R1 = 0:0159=0:0560=0:0582). The Ψ - tetrahedral SbS3- 3 anions with Sb-S bond lengths of 242 pm are arranged in a cubic face centered packing, in which the three crystallographically different A+ cations occupy the tetrahedral and octahedral voids, overall exhibiting a distorted octahedral sulfur coordination. The chemical bonding and the characteristics of the stereochemically active lone electron pair have been investigated by means of FP-LAPW band structure calculations. Needle-shaped crystals of the monohydrate of the antimony(V) salt Cs3SbS4 · H2O were obtained from a suspension of Sb2O3, CsOH and elemental sulfur. Cs3SbS4 · H2O crystallizes in a new structure type (monoclinic, space group P21/c, a = 987:17(10), b = 994:83(7), c = 1600:46(14) pm, β = 126:895(8)°, Z = 4, R1 = 0:0234). As expected, the Sb-S distances (233.1 - 234.7 pm) in the nearly ideally tetrahedral anion SbS3- 4 are considerably shorter than in the antimonates(III) but match the bond lengths in the anhydrous sulfido antimonate(V) Cs3SbS4. Due to their similar fcc-like anion packing and the stereochemically active lone electron pair of Sb in the antimonates(III), the whole series of compounds A3SbIII,VS3/4 shows a uniform structure relation, which is elucidated using crystallographic group-subgroup relations

Unique thermodynamic relationships for Δf H o and Δf G o for crystalline inorganic salts. I. Predicting the possible existence and synthesis of Na2SO2 and Na2SeO2

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M(2)S subnets of M(2)SO(x) (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M(2)S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397-398; Vegas (2000), Crystallogr. Rev. 7, 189-286; Vegas et al. (2002), Solid State Sci. 4, 1077-1081]. These structural changes suggest that if M(2)SO(2) would exist, its cation array might well have an anti-CaF(2) structure. On the other hand, in an analysis of the existing thermodynamic data for M(2)S, M(2)SO(3) and M(2)SO(4) we have identified, and report, a series of unique linear relationships between the known Δ(f)H(o) and Δ(f)G(o) values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M(2)SO(x) (x = 3 and 4), and the similarly between M(2)S(2) disulfide (x = 0) and disulfur oxyanion salts M(2)S(2)O(x) (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δ(f)H(o)) for as yet unprepared compounds, M(2)SO (x = 1) and M(2)SO(2) (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609-3620], the values of the absolute entropies were estimated and from them, the standard Δ(f)S(o) values, and then the Δ(f)G(o) values of the salts. A tentative proposal is made for the synthesis of Na(2)SO(2) which involves bubbling SO(2) through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate ΔG(o) to be approximately -500 kJ mol(-1). However, examination of the stability of Na(2)SO(2) raises doubts and Na(2)SeO(2) emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na(2)S and Na(2)SeO(4). Like Na(2)SO(2), this compound is predicted to have an anti-CaF(2) Na(2)Se subnet.

FIRST-PRINCIPLES STUDY OF STRUCTURAL AND ELECTRONIC PROPERTIES OF ALKALI METAL CHALCOGENIDES: M2CH[M: LI, NA, K, RB; CH: O, S, SE, TE

Ground-state structural properties of alkali metal selenides and tellurides (M2X) [M: Li, Na, K, Rb; X: Se, Te] have been studied in the framework of density functional theory for the first time using local density approximation, Perdew–Burke–Ernzerhof generalized gradient approximation (GGA) and Wu–Cohen (WC) GGA parameterization schemes. Also structural properties of alkali metal oxides and sulfides (M2X) [M: Li, Na, K, Rb; X: O, S] have been reinvestigated using WC GGA. Electronic band structures of alkali metal chalcogenide compounds have been calculated using the aforementioned schemes as well as the modified GGA proposed by Engel and Vosko. The calculated structural parameters for all 16 materials have been compared with earlier theoretical and experimental results, and certain trends are discussed. Furthermore, electronic density of states for these compounds has also been presented herein and behavior in electronic properties analyzed. Li, K and Rb chalcogenides have been found to have indirect bandgap, whereas Na chalcogenides appear to have direct bandgap.

FIRST-PRINCIPLES STUDY OF ELECTRONIC STRUCTURE AND GROUND-STATE PROPERTIES OF ALKALI-METAL SELENIDES AND TELLURIDES (M2A)[M: Li, Na, K; A: Se, Te

First-principles calculations have been performed to investigate the electronic structure and ground-state properties of alkali-metal Selenides (M2Se) and Tellurides (M2Te) [ M: Li, Na, K ] using the Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) method. The exchange correlation energy is described within the local density approximation (LDA) using the von Barth and Hedin parameterization scheme. At ambient conditions, these compounds are found to crystallize in the face center cubic antifluorite (anti- CaF 2-type) structure. Ground-state properties such as total energy, equilibrium lattice parameter, and bulk modulus are calculated for these compounds. The calculated equilibrium lattice parameter is in agreement with experimental result. From the electronic structure calculations, we find that Li 2 Se , Li 2 Te , K 2 Se , and K 2 Te are indirect bandgap semiconductors, whereas Na 2 Se and Na 2 Te are direct bandgap semiconductors. The present results are compared with the earlier results of series of alkali-metal sulfides (M2S) and alkali-metal oxides ( M 2 O ), allowing us to make predictions about the total energy, bulk modulus, valence-band width, and bandgap behavior of the rest of the alkali-chalcogenide crystals.

FP-APW+lo calculations of the electronic and optical properties of alkali metal sulfides underpressure

The electronic and optical properties ofM2S (M = Li,Na, K and Rb) compounds in the cubic antifluorite structure have been calculated, using afull relativistic version of the full-potential augmented plane-wave plus local orbitalsmethod based on density functional theory, within both the local density approximation(LDA) and the generalized gradient approximation (GGA). Moreover, the Engel–VoskoGGA formalism (EV-GGA) is applied so as to optimize the corresponding potential forband structure calculations. The calculated equilibrium lattices and bulk moduli arein good agreement with the available data. Band structure, density of states,electron charge density and pressure coefficients of energy gaps are given. Resultsobtained for band structure using EV-GGA are larger than those with LDA andGGA. It is found that the spin–orbit coupling lifts the triple degeneracy at theΓ point and the double degeneracy at the X point. The analysis of the electron charge densityshows that the M–S bonds have a significant ionic character. The complex dielectric functionsε2(ω) for alkali metal sulfides were calculated for radiation up to 30 eV and the assignment of thecritical points to the band structure energy differences at various points of the Brillouinzone was made. The pressure and volume dependence of the static dielectric constant andthe refractive index are calculated.

First principles study of the elastic properties in X 2S (X=Li, Na, K and Rb) compounds under pressure effect

We have performed first principles full-potential augmented plane wave plus local orbitals (FP-APW+lo) calculations within the local density approximation (LDA) and the generalized gradient approximation (GGA), with the aim to determine and predict the elastic constants and their pressure dependence for the alkali-metal sulfides Li 2S, Na 2S, K 2S and Rb 2S. The ground state properties are found to agree with the experimental and other theoretical results. The calculated elastic constants at zero pressure for Li 2S and Na 2S are compared with the available experimental data and other theoretical results. With regard to K 2S and Rb 2S, we are not aware of any experimental or theoretical data for the elastic constants and their pressure dependence, so that the data here represent the first determination of these quantities. The shear modulus, Young’s modulus and the Poisson’s ratio for these compounds are derived. The Debye temperature is also estimated from the average sound velocity.

Stability of Alkali Metal Halide Polymorphs as a Function of Pressure

We investigated the regions of thermodynamic stability of possible modifications of the alkali metal halides as a function of pressure and type of alkali metal and halogen. Both Hartree-Fock and density functional calculations (for six different functionals) were performed. The results are in good agreement with experiment, and the trends in the computed quantities such as transition pressures and lattice parameters as a function of the ab initio method are similar to those found in earlier studies of the alkali metal sulfides. We predict that in most of these systems the so-called 5-5 modification should be metastable at standard pressure and be thermodynamically stable at slightly negative pressures. The sizes of the pressure ranges over which the various modifications are stable showed characteristic trends as a function of the type of the constituent elements, thus generalizing the traditional pressure-homologue rule for transition pressures and stable phases in ionic solids.

Pressure-induced antifluorite-to-anticotunnite phase transition in lithium oxide

Using synchrotron angle-dispersive x-ray diffraction and Raman spectroscopy on samples of ${\mathrm{Li}}_{2}\mathrm{O}$ pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite ($\ensuremath{\alpha}$, Fm-3m) to orthorhombic anticotunnite ($\ensuremath{\beta}$, Pnma) phase at $50(\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at ambient temperature. This transition is accompanied by a relatively large volume collapse of $5.4\phantom{\rule{0.3em}{0ex}}(\ifmmode\pm\else\textpm\fi{}0.8)%$ and large hysteresis upon pressure reversal (${P}_{\mathit{\text{down}}}$ at $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$). Contrary to a recent study, our data suggest that the high-pressure $\ensuremath{\beta}$-phase $({B}_{o}=188\ifmmode\pm\else\textpm\fi{}12\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ is substantially stiffer than the low-pressure $\ensuremath{\alpha}$-phase $({B}_{o}=90\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$. A relatively strong and pressure-dependent preferred orientation in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{O}$ is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides.

Energy Band Genesis from Sublattice States in Sulfides of Alkali Metals with an Antifluorite Lattice

Based on the density functional, self-consistent band spectra from crystals and charged sublattices of alkali metal sulfides are calculated by the method of pseudopotential in the basis of localized orbitals. Different charge states of sublattices compatible with the electric neutrality of crystals are addressed, including extrememost states: neutral sublattices, and unfilled metal and double charged anionic sublattices. It is shown that a transfer of fractional charges by the anionic sublattices results in a reasonable recovery of parameters of the valence bands of crystals. The lower conduction bands for this charge distribution are similar in crystals and fractionally charged anionic sublattices. The differences found out are accounted for by sublattice hybridization.

Structure prediction of high-pressure phases for alkali metal sulfides.

For a given chemical system we present a systematic approach to predict structures, which may exist at high pressure, by investigating the global enthalpy landscape. We combine global optimizations, based on empirical potential energy functions, and local optimizations (volume, cell shape, and atomic positions) on both Hartree-Fock and density functional theory level. We predict the existence of high-pressure phases for the alkali metal sulfides of the composition M2S (M = Li, Na, K, Rb, Cs), together with the transition pressures among these phases.

The Role of Sublattices in Chemical Bonding in Essentially Ionic Crystals

A technique for calculating the self-consistent electron density of sublattice and crystal as a whole has been developed based on the local electron density functional theory (DFT). Difference density, obtained by subtraction of the densities of the individual sublattices from the total electron density, is introduced as a quantity defining the relationship between the sublattices. Self-consistent calculations have been performed for series of compounds with a NaCl lattice: alkali and noble metal halides and alkali metal oxides and sulfides with an antifluorite lattice. As a result of these calculations, we formulated the key principle of chemical bonding based on sublattices. In the anion sublattice, alike atoms form a covalent type of bond with each other by transferring some portion of the charge to the interstice. The same mechanism of bonding is realized in sublattices of metal atoms containing the levels of the d-states in the valence region. In sublattices of simple metals, the valence density behaves as “metallic,” changing slightly within the unit cell, with electrons again moving from the metal sites into the interstice. Addition of the valence densities of sublattices leads to two essentially different situations: in one case, the metal sites coincide with the density maxima at the interstice of the anion sublattice, and in the other, they do not. In the first case, the superposition of the sublattice densities at the lattice sites leads to anion-cation hybridization, forming an essentially ionic chemical bond with a possible admixture of the covalent bond. In the second case, the anion sublattice is a covalently bonded framework incorporating the metal sublattice. This takes place in alkali metal oxides and sulfides.

Electronic Structure of Alkali Metal Oxides and Sulfides

The energy band structure, density of electronic states, imaginary part of the complex permittivity, and valence charge distribution over different crystal planes are calculated for eight M2A (M : Li, Na, K, or Rb; A : O or S) compounds within the formalism of the local electron density functional with the use of the nonlocal pseudopotentials. The series dependences of the band structure parameters are established. It is demonstrated that the band structures of these crystals differ significantly for the anion and cation. A conclusion on the different characters of hole transport in (Li, Na)2A and (K, Rb)2A is drawn based on the specific features of their upper valence band structure. The general regularities and distinctive features of the spectral dependence e2(E) for the examined M2A series are discussed. The valence density maxima are localized on anions and interstices, so that chains are formed from the second anionic neighbors covalently bonded by the charge between the anions. Wells with the minimum density formed between the anion chains in oxides may form channels for cation transport and are responsible for the ionic conductivity.

The thermal decomposition of hydrogen sulfide over alkali metal sulfides and polysulfides

The decomposition of hydrogen sulfide to hydrogen and sulfur on alkali metal sulfides M 2S (M = Li, Na and K) and polysulfides M 2S x ( x = 2–4; M = Na and K) has been studied in a flow system at 400–800°C. Hydrogen yields were measured as a function of temperature in order to compare the effectiveness of the alkali metal (poly)sulfides in promoting the decomposition. It was found that both Na 2S and K 2S are rapidly sulfided by H 2S to give the corresponding disulfides M 2S 2, whereas Li 2S acts as a catalyst for the thermal decomposition of H 2S. Although Na 2S 2 was further sulfided by H 2S, sodium polysulfides were not effective in enhancing hydrogen yields and all polysulfides produced an amorphous product of approximate composition Na 2S 2.4–2.6. With potassium polysulfides the final product was K 2S 3 in all cases and the sulfidation of K 2S 2 by hydrogen sulfide resulted in an enhancement of hydrogen yields at 500–700°C.