Electropositive Cations Research Articles

Why Mg2IrH6 Is Predicted to Be a High‐Temperature Superconductor, But Ca2IrH6 Is Not

The X2MH6 family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high‐temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the IrH64‐ anions are key for the superconducting mechanism, and they induce coupling in the eg* set, which are antibonding between the H 1s and the Ir dx2−y2 or dz2 orbitals. Because calcium possesses low‐lying d‐orbitals, eg* → Ca d back‐donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms and may hold implications for superconductivity in other systems where the antibonding anionic states are filled.

Why Mg2IrH6 Is Predicted to Be a High-Temperature Superconductor, But Ca2IrH6 Is Not.

The X2MH6 family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the IrH64-anions are key for the superconducting mechanism, and they induce coupling in the eg* set, which are antibonding between the H 1s and the Ir dx2-y2 or dz2 orbitals. Because calcium possesses low-lying d-orbitals, eg* → Ca dback-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms andmay hold implications for superconductivity in other systems where the antibonding anionic states are filled.

Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln2 Ae3 MIV 3 S12 Family.

Structural dissymmetry and strong second-harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnSn (n=5, 6) groups and highly electropositive cations (lanthanides (Ln3+ ) and alkaline earth (Ae2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2 Sr3 Sn3 S12 (Pmc21 ) and Ln2 Ca3 Sn3 S12 (P-62m); whereas 17 rigid GeS4 or SiS4 tetrahedra-constructed Ln2 Ae3 Ge3 S12 and Ln2 Ae3 Si3 S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc21 ) in the Ln2 Ae3 MIV 3 S12 family indicates that chemical substitution of the tetrahedral GeS4 /SiS4 units with SnSn breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn(4+) S5 unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4×AgGaS2 and Δn=0.008) of La2 Ca3 Sn3 S12 to the strong phase-matching (PM) SHG effect (3.0×AgGaS2 and Δn=0.086) of La2 Sr3 Sn3 S12 . Therefore, Sn(4+) S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.

Electropositive Metal Doping into Lanthanum Hydride for H− Conducting Solid Electrolyte Use at Room Temperature

AbstractHydride ion conductors have made remarkable progress in recent years; in particular, the fluorite‐type LaH3‐δ series exhibits high conductivity around room temperature. However, its intrinsic character of hydrogen non‐stoichiometry still makes its application as a solid electrolyte challenging, for which high electronic insulation is essential. Here, Sr‐substituted LaH3‐δ with slight O2− incorporation, represented as La1‐xSrxH3‐x‐2yOy (0.1 ≤ x ≤ 0.6, y ≤ 0.171), is synthesized, which exhibits H− conductivity of 10−4 – 10−5 S cm−1 at room temperature. The galvanostatic discharge reaction using an all‐solid‐state cell composed of Ti|La1‐xSrxH3‐x‐2yOy|LaH3‐δ shows that the Ti electrode is completely hydrogenated to TiH2 for x ≥ 0.2, whereas a short circuit occurs for x = 0.1. These experimental observations, together with calculation studies on the density of states and the defect formation energy, provide clear evidence that electropositive cation, such as Sr, doping critically suppresses the electron conduction in LaH3‐δ. Achieving a superior H− conducting solid electrolyte is a novel milestone in the development of electrochemical devices that utilize its strong reducing ability (E°(H−/H2) = −2.25 V vs SHE), such as batteries with high energy density and electrolysis/fuel cells with high efficiency.

Wide band gaps in nonlinear optical materials AMgAgGa6S11 (A = K, Rb) achieved by the incorporation of dual s-block elements in Ag-based sulfide

Infrared (IR) nonlinear optical (NLO) material with a concurrently wide band gap and significant NLO coefficient in a single crystal is an incompatible but addressable challenge. Hence, for the first time, the AgGaS2 structure evolved into an Ag-based sulfide system integrating dual s-block elements, guiding the discovery of two NLO crystals named AMgAgGa6S11 (A = K, 1; Rb, 2). Structurally, title phases possess a novel NLO functional motif [AgGa6S17] assembled by two C2-type supertetrahedra through a shared [AgS4] tetrahedron, in which the electropositive cations A+ and Mg2+ are embedded. Thus, compounds 1 and 2 display emulative band gaps (3.15 and 3.25 eV, exceeding that of 2.65 eV in AgGaS2) toward optimizing the laser-induced damage thresholds (3.1 and 3.2 × AgGaS2 @1064 nm). Remarkably, the noncentrosymmetric phases 1 and 2 exhibit moderate second-harmonic generation (SHG) responses (0.6 and 0.7 × AgGaS2 @1910 nm), the greater-than-commercial half of the AgGaS2. To the best of our current knowledge, an experimental birefringence of 1 and 2 (0.029 and 0.027, @546 nm) is first observed in quinary ordered chalcogenides, indicating that samples 1 and 2 meet phase-matching conditions at the incident laser of 1910 nm. All these observations demonstrate that crystals 1 and 2 can become prospective NLO materials. This research will pave the way for discovering excellent IR NLO crystals with wide band gaps and impressive NLO coefficients by incorporating multiple strong electropositive elements into chalcogenides.

Enhancing the thermoelectric power factor of Mg3Sb2with Sn doping on electronegative sites of Sb: Effects of reducing the electronegativity difference

The weighted mobility is a better descriptor of the optimal performance of electronic transport for thermoelectric materials. The smaller the electronegativity difference between two atoms, the more covalent the bonding character between these two atoms. A series of p-type Sn-doped Mg3Sb2 was prepared by mixing and grinding Mg, Sb, and wet chemically synthesized Sn powders, followed by cold pressing and heating in an evacuated-and-encapsulated ampoule at 500 °C. Both the electronic and thermal transport of p-Mg3Sb2-xSnx (x = 0, 0.02, 0.04, and 0.06) are investigated at temperatures ranging from 325 to 700 K. Sn doping on the electronegative Sb site lowered the electrical resistivity to 161 mΩ-cm at 325 K for x = 0.04, which is 91% lower than the pristine Mg3Sb2. The power factor is enhanced upon Sn doping as a result of increased weighted mobility peaking at x = 0.04. The highest power factor of the Mg3Sb1.96Sn0.04 alloy is 2.55 μW/cm-K2 having 16 times enhancement as compared to the pristine Mg3Sb2 at 700 K. As a result, Mg3Sb1.96Sn0.04 has a maximum zT of ∼0.30 at 700 K. The strategy of reducing the difference of electronegativity between electropositive cations and electronegative anions by partial replacement of Sn on electronegative Sb site is effective to increase the weighted mobility and hence enhance the power factor of p-Mg3Sb2.

Sc2C, a 2D Semiconducting Electride.

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc2C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc2C and Al2C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc2C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

Modular Organoboron Catalysts Enable Transformations with Unprecedented Reactivity.

ConspectusElectron-deficient boron-based catalysts with metal-free but metallomimetic characteristics provide a versatile platform for chemical transformations. However, their catalytic performance is usually lower than that of the corresponding metal-based catalysts. Furthermore, many elaborate organoboron compounds are produced via time-consuming multistep syntheses with low yields, presenting a formidable challenge for large-scale applications of these catalysts. Given this context, the development of organoboron catalysts with the combined advantages of high efficiency and easy preparation is of critical importance.Therefore, we envisioned that the construction of a dynamic Lewis multicore system (DLMCS) by integrating the Lewis acidic boron center(s) and a Lewis basic ammonium salt in one molecule would be particularly efficient for on-demand applications because of the intramolecular synergistic effect. This Account summarizes our recent efforts in developing modular organoboron catalysts with unprecedented activities for several chemical transformations. A series of mono-, di-, tri-, and tetranuclear organoboron catalysts was readily designed and prepared in nearly quantitative yields over two steps using commercially available feedstocks. Notably, these catalysts can be modularly tailored by fine control over the electrophilic property of the Lewis acidic boron center(s), electronic and steric effects of the electropositive ammonium cation, linker length between the boron center and the ammonium cation, the number of boron centers, and the nucleophilic anion. This modular design allows systematic manipulation of the reactivity and efficacy of the catalysts, thus optimizing suitable catalysts for versatile chemical transformations. These include the coupling of CO2 and epoxides, copolymerization of CO2 and epoxides, ring-opening polymerization (ROP) of epoxides, and ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides.The utilization of mononuclear organoboron catalysts provided a turnover frequency of 11050 h-1 for the CO2/propylene oxide coupling reaction, an unprecedented efficiency of 5.0 kg of polymer/g of catalyst for the copolymerization of CO2 and cyclohexene oxide, and a record-breaking catalytic efficiency of 7.4 kg of polymer/g of catalyst for the ROCOP of epoxides with cyclic anhydrides. A turnover number of 56500 was observed at a catalyst loading of 10 ppm for the ROP of epoxides using the dinuclear catalysts. The tetranuclear organoboron catalysts realized the previously intractable task of the copolymerization of CO2 and epichlorohydrin, producing poly(chloropropylene carbonate) with the highest molecular weight of 36.5 kg/mol reported to date.Furthermore, the study revealed that the interaction between the dynamic Lewis multicore, that is, the intramolecular synergistic effect between the boron center(s) and the quaternary ammonium salt, plays a key role in mediating the catalytic activity and selectivity. This was based on investigations of the crystal structures of the catalysts, key intermediates, reaction kinetics, and density functional theory calculations. The modular tactics for the construction of organoboron catalysts presented in this Account should inspire more advanced catalyst designs.

Alkyne Hydrogenation Catalysis across a Family of Ga/In Layered Zintl Phases.

Transition-metal-free Zintl-Klemm phases have received little attention as heterogeneous catalysis. Here, we show that a large family of structurally and electronically similar layered Zintl-Klemm phases built from honeycomb layers of group 13 triel (Tr) or group 14 tetrel (Tt) networks separated by electropositive cations (A) and having a stoichiometry of ATr2 or ATrTt (A = Ca, Ba, Y, La, Eu; Tr = Ga, In; Tt = Si, Ge) exhibit varying degrees of activity for the hydrogenation of phenylacetylene to styrene and ethylbenzene at 51 bar H2 and 40-100 °C across a variety of solvents. The most active catalysts contain Ga with, formally, a half-filled pz orbital, and minimal bonding between neighboring Tr2 or TrTt layers. A 13-layer trigonal polytype of CaGaGe (13T-CaGaGe) was the most active, cyclable, and robust catalyst and under modest conditions (1 atm H2, 40 °C) had a surface specific activity (590 h-1) comparable to a commercial Lindlar's catalyst. Additionally, 13T-CaGaGe maintained 100% conversion of phenylacetylene to styrene at 51 bar H2, even after 5 months of air exposure. This work reveals the structural design elements that lead to particularly high catalytic activity in Zintl-Klemm phases, further establishing them as a promising materials platform for hydrogen-based heterogeneous catalysis.

Chemical Bonding Origin of the Thermoelectric Power Factor in Half-Heusler Semiconductors

Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, making the Zintl distinction clearly invalid. This covalence is shown to strongly affect the band structure, thermoelectric properties and response properties in the materials, with improved thermoelectric properties observed for those materials that least follow the Zintl concept. This expands our knowledge of the chemical bonding motifs governing physical properties, and gives a critical view on the simplistic chemical concepts too often applied for design of complex materials.

Two-Dimensional and Three-Dimensional Tetrel-Arsenide Frameworks Templated by Li and Cs Cations

Two new ternary tetrel pnictide phases, Li1.08Ge4.92As7 and Cs0.11Zn0.05Si0.95As, have been synthesized. Both compounds crystallize in new structure types, which demonstrate the flexibility of Ge(Si)–As layers when accommodating electropositive cations of various sizes. Li1.08Ge4.92As7 displays a Ge-As layer motif that has not been observed in ternary Ge-As systems, while Cs0.11Zn0.05Si0.95As exhibits a 3D framework composed of fragments of the 2D parent-phase SiAs. For Li1.08Ge4.92As7, a combination of computational modeling and solid-state 7Li MAS NMR was used to establish the correct crystal structure with one mixed occupied Ge/Li site. The compositions of the phases were confirmed through energy-dispersive X-ray spectroscopy. Transport properties and UV–Vis spectroscopy reveal both reported compounds to be semiconductors. Cs0.11Zn0.05Si0.95As shows high thermal stability with congruent melting at 1300 K and an exceptionally low thermal conductivity of 0.5 W m–1 K–1 at 300 K. Temperature-dependent Li-ion hopping dynamics in Li1.08Ge4.92As7 was characterized using 7Li solid-state NMR.

Instantaneous and Phosphine-Catalyzed Arene Binding and Reduction by U(III) Complexes.

Neutral arenes such as benzene have never been considered suitable ligands for electropositive actinide cations, yet we find that even simple UIII UX3 aryloxide complexes such as U(ODipp)3 bind and reduce arenes spontaneously at room temperature, forming inverse arene sandwich (IAS) complexes XnU(μ-C6D6)UXm (X = ODipp, n=2, m=3; X = OBMes2 n=m=2 or 3) (ODipp = OC6H3iPr2-2,6; Mes = 2,4,6-Me3-C6H2). In some of these cases, further arene reduction has occured as a result of X ligand redistribution. These unexpected spontaneous reactions explain the anomalous spectra and reported lack of further reactivity of strongly reducing UIII centers of U(ODipp)3. Phosphines that are not considered suitable ligands for actinides can catalyze the formation of the IAS complexes. This enables otherwise inaccessible asymmetric and less congested IAS complexes to be isolated and the bonding in this series compared.

Fabrication of two-dimensional 3d transition metal oxides through template assisted cations hydrolysis method

With ample exposed active sites and fast reaction kinetics, thin layered 3d transition metal oxides have always been regarded as a promising candidate catalysts in the field for energy storage and conversion. Due to the complex coordination fields and highly hybridized metal-oxygen bonds, the generalized synthesis method of thin-layered 3d TMOs with large size is still lacking. Here, graphene oxide (GO) template-assisted method is developed to fabricate large-size two-dimensional 3d transition metal oxides (TMO), e.g. Mn2O3, Mn3O4, Fe2O3, Co3O4, CoO, NiO. The electrostatic attraction between electronegative GO templates and electropositive metal cations is used to confine the growth of hydroxide precursor along GO surface. The precise control of pH and temperature is essential for tailoring the balance between heterogeneous nucleation and heteroepitaxy growth which leads to different morphology, size and granularity of the final metal oxides. A mild annealing treatment of layered hydroxide precursor is necessary to achieve smooth and uniform surface morphology for thin layered TMO nanosheets. The unique advantages of these thin layered structures have been proved by the enhanced oxygen evolution reaction catalytic performance, because the distorted crystalline structure contributes to the oxygen exchange and accelerates the charge transfer process. Especially for thin layered Co3O4, it exhibits a superior OER activity with a low overpotential of 275 mV at 10 mA cm−2, even better than that of benchmark IrO2 electrocatalyst. This unique synthesis strategy is believed to facilitate the design and fabrication of advanced two-dimensional materials.

Decoupling the effect of vacancies and electropositive cations on the anionic redox processes in Na based P2-type layered oxides

The activation of anionic redox couple is recognized as one of the best way to increase the energy density of positive electrode materials in both Li and Na-ion batteries. However, for such hope, to materialize a better understanding of the parameters governing the activation, reversibility and efficiency of the anionic redox in NaMO2 layered compounds is still sorely needed. Herein, we report a new P2–Na0.63[□0.036Mg0.143Mn0.820]O2 compound that combines vacancies and Mg doping as well-known sources for anionic redox activation and benchmark its electrochemical performances against P2–Na0.72[Mg0.31Mn0.69]O2. We found that vacancies and Mg doping trigger independently anionic redox processes that differ in terms of redox voltage and reversibility. The one associated to vacancies occurs at the lowest potential and is irreversible. Moreover, we evidenced by monitoring the structural evolution of the pristine phase during cycling the benefice of anionic processes in ensuring the stabilization of P2-type structure at high voltage over a wide range in Na content. These findings highlight the importance of the anionic redox process origin (e.g. vacancies vs. highly electropositive cations) in governing the material electrochemical properties, while providing a new way to efficiently stabilize, without capacity loss, the P2-type structure through the charge process in non A-rich compounds.

Structural Instability Driven by Li/Na Competition in Na(Li1/3Ir2/3)O2 Cathode Material for Li-Ion and Na-Ion Batteries.

With increasing hopes placed on the Na-ion battery technology to complement the current Li-ion battery systems, it is important to improve the energy density of Na-based cathode materials. Na-rich rocksalt oxides, Na1+xM1-xO2 (M = transition metal), combining cationic and anionic redox activity, could provide the necessary increase in capacity to achieve this goal, but their synthesis remains challenging compared to the Li analogues. As an alternative, mixed compounds Na(AxM1-x)O2, with A being an electropositive cation such as Li, Mg, or Zn sitting in the transition-metal layer, have been reported. As a continuation, we herein prepared the mixed Na(Li1/3Ir2/3)O2 phase and compared its structure and electrochemical properties with the well-known Li2IrO3 and Na2IrO3 parent materials. By mixing Na and Li in the material, the stacking sequence of the transition-metal honeycomb layers in Na(Li1/3Ir2/3)O2 is modified compared to the two parent materials, resulting in the presence of extra superstructure peaks in X-ray diffraction data. Using electrochemical characterization and an in-situ X-ray diffraction technique, the mixed Na(Li1/3Ir2/3)O2 was found to be unstable both in Li and Na batteries and to separate into Na-rich NaxIrO3 and Li-rich LixIrO3 phases due to the competition between electrochemical (de)insertion, cation exchange with the electrolyte, and segregation of Na and Li in the material. These findings highlight important challenges and offer useful insight into guide the design of new mixed Na(AxM1-x)O2 cathode materials with high capacity for Na-ion batteries.

Large electropositive cations as surfactants for the growth of polar epitaxial films

Using density functional theory (DFT) we demonstrate that the adsorption of large cations such as potassium or cesium facilitates the epitaxial growth of polar ${\mathrm{LaAlO}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{LAO})$ on ${\mathrm{SrTiO}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{STO})$. The low ionization potential of K favors efficient electron transfer to the STO conduction band and results in a two-dimensional electron gas which exactly compensates for the diverging potential with increasing layer thickness. For large cations like K or Cs, DFT total energy considerations show that they remain adsorbed on the LAO surface and do not enter substitutionally into LAO. These results suggest a scheme for growing clean LAO/STO interface systems, and polar systems in general, by performing the growth process in the presence of large, low ionization potential alkali-metal ions.

Hydride ions in oxide hosts hidden by hydroxide ions

The true oxidation state of formally ‘H−’ ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of 1H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H+. Here we systematically investigate the link between geometrical structure and chemical shift of H− ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH− ion (formally H+ state) exceeds that in an H− ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H− and the distance from the H− ion to the coordinating electropositive cation. This relationship is pivotal for resolving H− species that are masked by various states of H+ ions.

Rapid Microwave Preparation and ab Initio Studies of the Stability of the Complex Noble Metal Oxides La2BaPdO5 and La2BaPtO5

We present a rapid microwave-assisted solid-state approach to prepare complex platinum-group metal oxides with the formula La2BaMO5 (M = Pd, Pt). While conventional furnace-based preparations of these compounds take several days and often require oxidizing conditions, the microwave-assisted pathway enables the target compounds to be obtained with high phase purity in about 20 min of reaction time in air without the multiple regrindings that are required of conventional solid-state synthesis. These complex oxides are stable in various atmospheres up to 1000 °C unlike the simple noble metal oxides, which are reduced even at room temperature. Density functional theory-based calculations have been employed to establish the stability of these complex oxides and to understand the electronic structure origins of the stability, most notably the influence of electropositive cations. It is shown that the presence of electropositive ions in the oxide crystal structure "softens" the oxygen anion and results in more covalent (Pd/Pt)-O interactions.

Vibrational spectra and structure of borohydrides

A comparison of the vibrational spectra of many inorganic borohydrides allows us to distinguish compounds with isolated BH4- ions and compounds containing complex ions such as Sc(BH4)4-. The characteristic spectral features of both types of compounds are identified, showing that the B–H bonding is quite different in both cases. A detailed analysis of the vibrations of the isolated BH4- ions provides new information about their local structure. Angular deformations of individual borohydride ion are analyzed quantitatively. It appears that the compounds containing isolated BH4- ions belong to those with the most electropositive cations and the highest decomposition temperature, while the complex borohydrides show significantly lower decomposition temperatures and possible diborane formation.

Preparation of calcium-doped boron nitride by pulsed laser deposition

Calcium-doped BN thin films CaxBNy (x=0.05–0.1, y=0.7–0.9) were grown on α-Al2O3(001) substrates by pulsed laser deposition (PLD) using h-BN and Ca3N2 disks as the targets under nitrogen radical irradiation. Infrared ATR spectra demonstrated the formation of short range ordered structure of BN hexagonal sheets, while X-ray diffraction gave no peak indicating the absence of long-range order structure in the films. It was notable that Ca-doped film had 5.45–5.55eV of optical band gap, while the band gap of Ca-free films was 5.80–5.85eV. This change in the band gap is ascribed to interaction of Ca with the BN sheets; first principle calculations on h-BN structure indicated that variation of inter-plane distance between the BN layers did not affect the band gap. This study highlights that PLD could prepare BN having short-range structure of h-BN sheets and being doped with electropositive cation which varies the optical band gap of the films.