Interpenetrated Lattices of Quaternary Chalcogenides Displaying Magnetic Frustration, High Na-Ion Conductivity, and Cation Redox in Na-Ion Batteries.

A series of quaternary selenides, NaxMGaSe4 (M = Mn, Fe, and mixed Zn/Fe), have been synthesized for the first time employing a high-temperature solid-state synthesis route through stochiometric or polychalcogenide flux reactions. Along with the selenides, a previously reported sulfide analogue, NaxFeGaS4, is also revisited with new findings. These compounds form an interpenetrated structure made up of a supertetrahedral unit. The electrochemical evaluations exhibit a reversible (de)intercalation of ∼0.6 and ∼0.45 Na-ions, respectively, from Na2.87FeGaS4 (1a) and Na2.5FeGaSe4 (2) involving Fe2+/Fe3+ redox when cycled between 1.5 and 2.5 V. Mössbauer spectroscopy of 1a shows the existence of a mixed oxidation state of Fe2+/3+ in the pristine compound and reversible oxidation of Fe2+ to Fe3+ during the electrochemical cycles. Na2.79Zn0.6Fe0.4GaSe4 possesses a reasonably high room temperature ionic conductivity of 0.077 ms/cm with an activation energy of 0.30 eV. The preliminary magnetic measurements show a bifurcation of FC-ZFC at 4.5 and 2.5 K, respectively, for 1a and Na3MnGaSe4 (4) arising most likely from a spin-glass like transition. The high negative values of the Weiss constants -368.15 and -308.43 K for 1a and 4, respectively, indicate strong antiferromagnetic interactions between the magnetic ions and also emphasize the presence of a high degree of magnetic frustration in these compounds.

A Comparative Study of Cationic Copper(I) Reagents Supported by Bipodal Tetramethylguanidinyl-Containing Ligands as Nitrene-Transfer Catalysts.

The bipodal compounds [(TMG2biphenN-R)CuI-NCMe](PF6) (R = Me, Ar (4-CF3Ph-)) and [(TMG2biphenN-Me)CuI-I] have been synthesized with ligands that feature a diarylmethyl- and triaryl-amine framework and superbasic tetramethylguanidinyl residues (TMG). The cationic Cu(I) sites mediate catalytic nitrene-transfer reactions between the imidoiodinane PhI = NTs (Ts = tosyl) and a panel of styrenes in MeCN, to afford aziridines, demonstrating comparable reactivity profiles. The copper reagents have been further explored to execute C-H amination reactions with a variety of aliphatic and aromatic hydrocarbons and two distinct nitrene sources PhI = NTs and PhI = NTces (Tces = 2,2,2-trichloroethylsulfamate) in benzene/HFIP (10:2 v/v). Good yields have been obtained for sec-benzylic and tert-C-H bonds of various substrates, especially with the more electron-deficient catalyst [(TMG2biphenN-Ar)CuI-NCMe](PF6). In conjunction with earlier studies, the order of reactivity of these bipodal cationic reagents as a function of the metal employed is established as Cu > Fe > Co ≥ Mn. However, as opposed to the base-metal analogues, the bipodal Cu reagents are less reactive than a similar tripodal Cu catalyst. The observed fluorophilicity of the bipodal Cu compounds may provide a deactivation pathway.

NaGaSe2: A Water-Loving Multifunctional Non-van der Waals Layered Selenogallate.

A missing member of well-known ternary chalcometallates, a sodium selenogallate, NaGaSe2, has been synthesized by employing a polyselenide flux and stoichiometric reaction. Crystal structure analysis using X-ray diffraction techniques reveals that it contains supertetrahedral adamantane-type Ga4Se10 secondary building units. These Ga4Se10 secondary building units are further connected via corners to form two-dimensional (2D) [GaSe2]∞- layers stacked along the c-axis of the unit cell, and the Na ions reside in the interlayer space. The compound has an unusual ability to absorb water molecules from the atmosphere or a nonanhydrous solvent to form distinct hydrated phases, NaGaSe2·xH2O (where x can be 1 and 2), with an expanded interlayer space, as verified by X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), desorption, and Fourier transform infrared spectroscopy (FT-IR) studies. The in situ thermodiffractogram indicates the emergence of an anhydrous phase before 300 °C with the decrease of interlayer spacings and reverting to the hydrated phase within a minute of re-exposure to the environment, supporting the reversibility of such a process. Structural transformation induced through water absorption results in an increase of Na ionic conductivity by 2 orders of magnitude compared to that of the pristine anhydrous phase, as verified by impedance spectroscopy. Na ions from NaGaSe2 can be exchanged in the solid-state route with other alkali and alkaline earth metals in a topotactic or nontopotactic way, leading to 2D isostructural and three-dimensional networks, respectively. Optical band gap measurements show a band gap of ∼3 eV for the hydrated phase, NaGaSe2·xH2O, which is in good agreement with the calculated band gap using a density functional theory (DFT)-based method. Sorption studies further confirm the selective absorption of water over MeOH, EtOH, and CH3CN with a maximum water uptake of 6 molecules/formula unit at a relative pressure, P/P0, of 0.9.

Crystal structure, electronic structure, and optical properties of the novel Li4CdGe2S7, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite.

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4-II-IV2-VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2-II-IV-VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

Discovery of an olivine-type lithium manganese thiophosphate, LiMnPS4, via a building block approach.

An olivine-type orthothiophospate LiMnPS4 has been synthesized for the first time through a building block approach by reacting preformed ternary lithium thiophospate with MnCl2. Diffuse reflectance measurements show an optical band gap of 2.36 eV, which is further confirmed by DFT calculations. Irreversible weak ferromagnetic ordering and metamagnetism are verified through preliminary magnetic measurements.

Sodium-Stuffed Open-Framework Quaternary Chalcogenide Built with (Cu2Ga6S18)16- Ribbons Cross-Linked by Unusual Linear Cu(I) Pillars.

A quaternary compound, Na15Cu3Ga6S18, the first member in the A-Cu-Ga-S (A = alkali metal) series, has been synthesized from a solid-state metathesis reaction between Na6Ga2S6 and CuCl as well as from a combination of Na2S, Ga, Cu, and S. The compound crystallizes in a monoclinic crystal system, space group C2/c, and represents a unique open-framework structure with channels filled with eight crystallographically distinct Na ions. The anionic framework is built up of infinite chains of corner-shared GaS4 tetrahedra fused together by an edge-shared dimer of CuS4 tetrahedra forming one-dimensional ribbons of (Cu2Ga6S18)16-, which are cross-linked by linearly coordinated S-Cu-S linkages resulting in a three-dimensional network with tunnels filled with Na atoms. Optical band gap measurements show that the compound has a direct band gap of 3.00 eV that is in good agreement with the theoretical band gap derived from density functional theory calculations. Band structure calculations further indicate that the states near the Fermi level are dominated by tetrahedral Cu+(d) and S(p) states resulting from the antibonding interactions, while s-d hybridization is prevalent in linear Cu+ coordination. Ionic conductivity measurements show that the compound has a room-temperature Na ion conductivity of 2.72 × 10-5 mS/cm with an activation energy of 0.68 eV, which corroborates well the nudged elastic band calculations.

Ternary alkali ion thiogallates, A5GaS4 (A = Li and Na), with isolated tetrahedral building units and their ionic conductivities.

Two new ternary thiogallates in the A5GaS4 (A = Li (i) and Na (ii)) series have been synthesized for the first time employing a gas passing route using oxide precursors and a high temperature solid state route using stoichiometric combinations of elements, respectively. Li5GaS4 crystallizes in the P21/m space group and the structure is built up of layers of corner sharing tetrahedra of LiS4 and GaS4 stacked along the a-axis and the octahedrally coordinated Li ions residing in the interlayer space. Na5GaS4 crystallizes in the Pbca space group and the structure consists of isolated (GaS4)5- tetrahedra held together by charge balancing sodium ions in distorted tetrahedral and octahedral coordination geometries. Measurements of ionic conductivity of the compounds showed room temperature ionic conductivities of 1.8 × 10-7 and 4.0 × 10-7 S cm-1 with activation energies of 0.54 and 0.28 eV, respectively, for I and II. Density functional theory calculations show close agreement in structural parameters with the measured data and predict band gaps of 2.75 eV (I) and 2.70 eV (II). Single point hybrid functional calculations result in band gaps of 3.95 and 3.65 eV correspondingly, in better agreement with the experimental value of ∼4.1 eV for both. Bond valence energy landscape maps suggest the absence of any suitable diffusion path for Li in Li5GaS4. On the other hand, BVEL maps of Na5GaS4 confirm that the tetrahedrally coordinated Na ions are responsible for ionic conduction, whereas the involvement of octahedrally coordinated Na ions in the conduction process could not be discerned.