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

Building-Block Approach to the Discovery of Na8Mn2(Ge2Se6)2: A Polar Chalcogenide Exhibiting Promising Harmonic Generation Signals with a High Laser-Induced Damage Threshold

A new polar quaternary chalcogenide, Na8Mn2(Ge2Se6)2, has been synthesized using the building-block approach by reacting preformed Na6Ge2Se6 and MnCl2 at 750 °C. The structure consists of layers of [Na(1)Mn(Ge2Se6)]3– stacked perpendicular to the c-axis and sodium ions occupying the interlayer space. An indirect bandgap of 1.52 eV has been calculated using density functional theory, which is expectedly underestimated compared to the observed optical bandgap of 1.95 eV derived from diffuse reflectance spectroscopic measurements in the UV/Vis/NIR region. Magnetic measurements confirm the paramagnetic nature of Na8Mn2(Ge2Se6)2 with an experimental magnetic moment of 5.8 μB in good agreement with the theoretical spin only moment of 5.92 μB for high spin Mn2+. Na8Mn2(Ge2Se6)2 exhibits a potentially wide region of transparency in the measured range of 2.5–25 µm. Na8Mn2(Ge2Se6)2 shows a modest second-harmonic generation (SHG) response but with a high laser-induced damage threshold (LIDT) of ~9x AgGaSe2. Third harmonic generation (THG) measurements indicate that Na8Mn2(Ge2Se6)2 displays a high THG coefficient (1.9x AgGaSe2) at λ = 1800 nm

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

Sodium-Stuffed Open-Framework Quaternary Chalcogenide Built with (Cu₂Ga₆S₁₈)¹⁶ˉ 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 x 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 theP21/mspace group and the structure is built up of layers of corner sharing tetrahedra of LiS4 and GaS4 stacked along thea-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 × 10-7S cm-1 with activation energies of 0.54 and 0.28 eV, respectively, forIandII. 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

High Sodium-Ion Conductivity in Interlocked Quaternary Chalcogenides Built with Supertetrahedral Building Units

Herein, we report the syntheses, structure, Na-ion conductivity, and theoretical investigation of two moisture stable quaternary compounds, Na3ZnGaQ4 (Q = S, Se). These compounds are synthesized using high-temperature solid-state synthesis routes employing polychalcogenide flux or by metathesis reactions. The crystal structure of these compounds is built up of a three-dimensional (3-D) network of corner-shared supertetrahedral (T2) units, where two such 3-D networks are interlocked. The d-block metal and the main group metal, Ga, occupy the same crystallographic site with a 1:1 ratio, making it a rare form of building unit. Band structure calculations show that both the compounds are wide band gap semiconductors with band gaps of 2.25 and 1.61 eV, respectively, for Na3ZnGaS4 (I) and Na3ZnGaSe4 (II), which are slightly underestimated compared to experimentally determined band gaps of 3.0 and 1.90 eV, respectively. I and II possess ionic conductivities of 3.74 x 10-4 and 0.12 mS/cm with activation energies of 0.42 and 0.38 eV, respectively, at 30 °C. Interestingly, I shows a significantly high ionic conductivity of 0.13 mS/cm at 30 °C upon exposure to air, which could be due to water adsorption on the surface or occlusion in the grain boundaries. Assuming the vacancy-Assisted diffusion mechanism for ionic conductance, this difference is consistent with the difference on vacancy formation energies in these compounds, as predicted by DFT calculations. The bond valence sum map indicates that in both structures, the lowest energy diffusion path is one dimensional and it is along the c axis of the unit cell

Ultralow Thermal Conductivity through the Interplay of Composition and Disorder between Thick and Thin Layers of Makovickyite Structure

Here we report the synthesis and characterization of three quaternary complex chalcogenides, Ag0.72Bi5.48Cu0.88Si9 (I), Ag0.70Bi5.30Cu1.3S9 (II), Ag0.34Bi4.54Cu1.98PbS9 (III). All the compounds in this homologous series crystallize in the C2/m space group and can be described as Pavonite structures. This structure type consists of alternating NaCl-like layers with varied thickness (nP), separated by a pair of square pyramids. All the compounds reported here are synthetic analogues of the n = 4 pavonite family and are known as makovickyite minerals. Compounds I-III possess complex crystal structures, consisting of mixed occupancies of Bi/Ag/Pb sites, as well as partially occupied Cu sites. These intrinsic assets lead to ultra-low lattice thermal conductivities, in the range of 0.75-0.42 Wm-1 K-1 from 300-500 K, and make these materials promising candidates for thermoelectric applications. All three structures exhibit very narrow indirect band gaps of less than 0.5 eV as confirmed by DRS measurements. Charge transport properties are consistent with n-type semiconducting behavior with moderate electrical conductivities and large Seebeck coefficients. Low temperature electrical resistivity and Seebeck coefficient measurements are also performed on II. A promising figure of merit, zT, of 0.09 for I and II, 0.11 for III can be achieved at 475 K

Building-Block Approach to the Discovery of Na₈Mn₂(Ge₂Se₆)₂: A Polar Chalcogenide Exhibiting Promising Harmonic Generation Signals with a High Laser-Induced Damage Threshold

A new polar quaternary chalcogenide, Na8Mn2(Ge2Se6)2, has been synthesized using the building-block approach by reacting preformed Na6Ge2Se6 and MnCl2 at 750 °C. The structure consists of layers of [Na(1)Mn(Ge2Se6)]3- stacked perpendicular to the c-axis and sodium ions occupying the interlayer space. An indirect bandgap of 1.52 eV has been calculated using density functional theory, which is expectedly underestimated compared to the observed optical bandgap of 1.95 eV derived from diffuse reflectance spectroscopic measurements in the UV/Vis/NIR region. Magnetic measurements confirm the paramagnetic nature of Na8Mn2(Ge2Se6)2 with an experimental magnetic moment of 5.8 μB in good agreement with the theoretical spin only moment of 5.92 μB for high spin Mn2+. Na8Mn2(Ge2Se6)2 exhibits a potentially wide region of transparency in the measured range of 2.5-25 µm. Na8Mn2(Ge2Se6)2 shows a modest second-harmonic generation (SHG) response but with a high laser-induced damage threshold (LIDT) of ~9x AgGaSe2. Third harmonic generation (THG) measurements indicate that Na8Mn2(Ge2Se6)2 displays a high THG coefficient (1.9 x AgGaSe2) at λ = 1800 nm

Unusual Atmospheric Water Trapping and Water Induced Reversible Restacking of 2D Gallium Sulfide Layers in NaGaS₂ Formed by Supertetrahedral Building Unit

Two new ternary materials NaGaS2 (1) and the Fe-doped phase of NaGaS2, NaFe0.135Ga0.865S2 (2), have been synthesized by employing polysulfide flux. Single crystal XRD analyses of 1 and 2 show that the structure is built up of adamantane-like Ga4S10 super tetrahedral fundamental building units. These admantane-like units are connected through their corners to form [GaS2]∞- layers that are stacked one over the other with Na ions residing in between the layers to balance the charge. Both the materials have the remarkable ability to absorb atmospheric water molecules and moisture from undried solvents as verified by TG analysis and FT-IR and XPS studies. The process of water absorption leads to stable distinct material NaGaS2·~H2O (1·H2O) and NaFe0.135Ga0.865S2·H2O (2·H2O) with restacked layers different from original crystal structure. This structural transformation is reversible as the transformed structures 1·H2O and 2·H2O can be returned to their original structures 1 and 2, respectively, upon heating. DFT calculation study reveals that a spontaneous exergonic hydration reaction takes place as outlined in NaGaS2 + H2O → NaGaS2H2O with the energy release, ΔE of -73.9 kJ mol-1. DFT calculation predicts an increase in the unit cell parameters of b and c directions and shrinkage along the a direction of hydrated phase 1·H2O with an overall volume increase of 36.6%. Structural transformation affects their physical properties as the pristine compound 1 possess Na+ ion conductivity of 2.88 x 10-7 S cm-1 at 22 ⁰C, whereas the hydrated compound 1·H2O displays ~40 times increased ion conductivity of 1.25 × 10-5 S cm-1 at the same temperature. DRS studies show very similar optical band gaps of ~4 eV for compounds 1 and 1·H2O, respectively, in reasonable agreement with the DFT(HSE) band gap estimation but more than 1 eV above the DFT(PBE)-predicted band gaps of ~2.4 eV. A sorption study indicates selective adsorption of water over MeOH, EtOH, and CH3CN with a maximum water uptake of 2.6 H2O per formula unit at P/Po= 0.9. A Karl Fischer titration study shows that NaGaS2 (1) is certainly capable of adsorbing water from wet methanol and can be useful as a fast desiccating agent