Enhancement of Low Temperature Superionic Conductivity by Suppression of Li Site Ordering in Li7Si2-xGexS7I.

Ge4+ substitution into the recently discovered superionic conductor Li7Si2S7I is demonstrated by synthesis of Li7Si2-xGexS7I, where x ≤ 1.2. The anion packing and tetrahedral silicon location of Li7Si2S7I are retained upon substitution. Single crystal X-ray diffraction shows that substitution of larger Ge4+ for Si4+ expands the unit cell volume and further increases Li+ site disorder, such that Li7Si0.88Ge1.12S7I has one Li+ site more (sixteen in total) than Li7Si2S7I. The ionic conductivity of Li7Si0.8Ge1.2S7I (x = 1.2) at 303 K is 1.02(3) × 10-2 S cm-1 with low activation energies for Li+ transport demonstrated over a wide temperature range by AC impedance and 7Li NMR spectroscopy. All sixteen Li+ sites remain occupied to temperatures as low as 30 K in Li7Si0.88Ge1.12S7I as a result of the structural expansion. This differs from Li7Si2S7I, where the partial Li+ site ordering observed below room temperature reduces the ionic conductivity. The suppression of Li+ site depopulation by Ge4+ substitution retains the high mobility to temperatures as low as 200 K, yielding low temperature performance comparable with state-of-the-art Li ion conducting materials.

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li6+xP1-xSixO5Cl.

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1-xSixO5Cl (0.3 < x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10-6 S cm-1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1-xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites.

Extended Condensed Ultraphosphate Frameworks with Monovalent Ions Combine Lithium Mobility with High Computed Electrochemical Stability.

Extended anionic frameworks based on condensation of polyhedral main group non-metal anions offer a wide range of structure types. Despite the widespread chemistry and earth abundance of phosphates and silicates, there are no reports of extended ultraphosphate anions with lithium. We describe the lithium ultraphosphates Li3P5O14 and Li4P6O17 based on extended layers and chains of phosphate, respectively. Li3P5O14 presents a complex structure containing infinite ultraphosphate layers with 12-membered rings that are stacked alternately with lithium polyhedral layers. Two distinct vacant tetrahedral sites were identified at the end of two distinct finite Li6O1626- chains. Li4P6O17 features a new type of loop-branched chain defined by six PO43- tetrahedra. The ionic conductivities and electrochemical properties of Li3P5O14 were examined by impedance spectroscopy combined with DC polarization, NMR spectroscopy, and galvanostatic plating/stripping measurements. The structure of Li3P5O14 enables three-dimensional lithium migration that affords the highest ionic conductivity (8.5(5) × 10-7 S cm-1 at room temperature for bulk), comparable to that of commercialized LiPON glass thin film electrolytes, and lowest activation energy (0.43(7) eV) among all reported ternary Li-P-O phases. Both new lithium ultraphosphates are predicted to have high thermodynamic stability against oxidation, especially Li3P5O14, which is predicted to be stable to 4.8 V, significantly higher than that of LiPON and other solid electrolytes. The condensed phosphate units defining these ultraphosphate structures offer a new route to optimize the interplay of conductivity and electrochemical stability required, for example, in cathode coatings for lithium ion batteries.

Polymorph of LiAlP2O7: Combined Computational, Synthetic, Crystallographic, and Ionic Conductivity Study.

We report a new polymorph of lithium aluminum pyrophosphate, LiAlP2O7, discovered through a computationally guided synthetic exploration of the Li-Mg-Al-P-O phase field. The new polymorph formed at 973 K, and the crystal structure, solved by single-crystal X-ray diffraction, adopts the orthorhombic space group Cmcm with a = 5.1140(9) Å, b = 8.2042(13) Å, c = 11.565(3) Å, and V = 485.22(17) Å3. It has a three-dimensional framework structure that is different from that found in other LiMIIIP2O7 materials. It transforms to the known monoclinic form (space group P21) above ∼1023 K. Density functional theory (DFT) calculations show that the new polymorph is the most stable low-temperature structure for this composition among the seven known structure types in the AIMIIIP2O7 (A = alkali metal) families. Although the bulk Li-ion conductivity is low, as determined from alternating-current impedance spectroscopy and variable-temperature static 7Li NMR spectra, a detailed analysis of the topologies of all seven structure types through bond-valence-sum mapping suggests a potential avenue for enhancing the conductivity. The new polymorph exhibits long (>4 Å) Li-Li distances, no Li vacancies, and an absence of Li pathways in the c direction, features that could contribute to the observed low Li-ion conductivity. In contrast, we found favorable Li-site topologies that could support long-range Li migration for two structure types with modest DFT total energies relative to the new polymorph. These promising structure types could possibly be accessed from innovative doping of the new polymorph.

Designing Three Fluorooxoborates with a Wide Transmittance Window by Anionic Group Substitution.

Three fluorooxoborates, K0.42Rb2.58B3O3F6 and M3B2PO5F4 (M = K, Cs), were designed and synthesized under the open system. One of the common features is that the title compounds consist of six-membered oxofluoride anions. The oxofluoride anions [B3O3F6]3- and [B2PO5F4]3- display structures similar to the boroxine [B3O6]3-. Anionic group substitution using the [BO2F2]3- and [PO4]3- units can improve the optical property of the [B3O6]3- anion and help to generate a wide transmittance window.

K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br): Syntheses, Characterizations, and Theoretical Studies of Noncentrosymmetric Halogen Borate-Carbonates with Short UV Cutoff Edges.

Two mixed halogen borate-carbonates K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br) have been successfully synthesized with the hydrothermal method. They crystallize in the space group P6̅2 c, and their structures feature 3D networks consisting of the K9O30Cl polyhedra. The isolated [B4O9]6- groups and [BO3]3- triangles are filled in the space of 3D networks. K9[B4O5(OH)4]3(CO3)X·7H2O (X = Cl, Br) show the largest band gaps (6.34 and 5.65 eV, respectively) among the borate-carbonate systems. In addition, we investigated the main-group-element borate halides based on the Inorganic Crystal Structure Database, compared the structures, and summarized which structures are beneficial to the formation of isomorphic compounds. Herein, the syntheses, structures, first-principle calculations, and optical properties were reported in the work.

Fluorooxoborates: Ushering in a New Era of Deep Ultraviolet Nonlinear Optical Materials.

Borates are one of the most important classes of functional materials, and several hundreds of artificial borates have been synthesized. The substitution of oxygen by fluorine leads to manifold classes of borates. Fluorooxoborates (also known as fluoroborates), in which the F atoms covalently connect with the B atoms, show additional compositional and structural diversity compared to classic borates. Recently, owing to the large polarizability anisotropy, large HOMO-LUMO gaps, and high hyperpolarizability of the oxyfluoride BOx F4-x building blocks, fluorooxoborates have received unprecedented attention in the search for new ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials. Specifically, some compounds have excellent NLO properties that are comparable or superior to KBe2 BO3 F2 , which is the only usable crystal that generates coherent light below 200 nm through a direct second harmonic generation (SHG) process. This Minireview illustrates recent progress on the synthesis, crystal structures, structure-properties relationships and applications of fluorooxoborates. This paper concludes by highlighting the outstanding opportunities offered by NLO fluorooxoborate crystals as an innovative avenue for DUV all solid-state coherent light generation.

K3 B6 O9 F3 : A New Fluorooxoborate with Four Different Anionic Units.

K3 B6 O9 F3 , a first fluorooxoborate containing four different units, that is, [BO3 ]3- , [BO4 ]5- , [BO3 F]4- , and [BO2 F2 ]3- , has been designed and synthesized by two different routes. The unique anionic 2∞ [B6 O9 F3 ]3- layers avoid forming terminal oxygen atoms, which are beneficial to enlarge the band gap. The structure-properties relationship of the title compound is discussed.

Mo6+ Cation Enrichment of the Structure Chemistry of Iodates: Syntheses, Structures, and Calculations of Ba(MoO2)2(IO3)4O, Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O, and Sr[(MoO2)6(IO4)2O4]·H2O.

The three metal iodates Ba(MoO2)2(IO3)4O (1), Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O (2), and Sr[(MoO2)6(IO4)2O4]·H2O (3) have been successfully synthesized by introducing second-order Jahn-Teller distorted Mo6+ cations by a mild hydrothermal method. Single-crystal X-ray diffraction (XRD) was used to determine the structures of the three title compounds. In compound 1, the [Mo2O11]10- dimers connect with the [IO3]- units by sharing oxygen atoms to form two-dimensional (2D) layers that are separated by the Ba2+ cations. For comparison, the [Mo2O11]10- dimers and the [IO3]- units are isolated in compound 2, and they are connected by the [BaO11]20- polyhedra forming a 3D network. For compound 3, the [MoO6]6- polyhedra link with each other by corner and edge sharing to build 2D corrugated layers with tunnels containing isolated [IO4]3- units. The [SrO9]16- polyhedra link the 2D corrugated layers to form a 3D network. The infrared (IR) spectra, the ultraviolet-visible-near-infrared (UV-vis-NIR) diffuse reflectance spectra, and thermal stabilities of compounds 1 and 2 are presented. In addition, the theoretical calculations are also carried out to evaluate their band gaps and density of states.

A Fluorooxosilicophosphate with an Unprecedented SiO2 F4 Species.

The large number and structural beauty of silicates are largely due to the variety of connection mode of SiO4 tetrahedra. SiO6 and SiF6 octahedra are also known and give rise for structural versatility of inorganic silicates. However, to date, the crystal structure of inorganic fluorooxosilicates with oxofluoride SiOx F4-x or SiOx F6-x species are unknown. Now, fluorine was successfully introduced into the silicophosphates, and the first fluorooxosilicophosphate K4 Si3 P2 O7 F12 with an unprecedented SiO2 F4 species was synthesized. The existence of Si-F bonds was verified by comprehensively experimental and theoretical work. Using ab initio and bond valence calculations, the oxofluoride SiOx F6-x species is shown to be stable when oxygen atoms connect to other atoms with strong covalent interactions. This work will contribute to the structural diversity of silicate chemistry by the exploration of the new fluorooxosilicates.

Experimental and theoretical studies on the linear and nonlinear optical properties of lead phosphate crystals LiPbPO4.

Phosphates with noncentrosymmetric (NCS) structures usually display short ultraviolet (UV) cut-off edges, but often exhibit weak powder second harmonic generation (SHG) intensities. Here we synthesized a NCS-phosphate, LiPbPO4, that achieves a desired balance between UV transparency, a cut-off edge of about 232 nm, and large SHG activity (about 3 × KDP). The three dimensional (3D) framework of LiPbPO4 consists of PO4 and LiO4 tetrahedra that are corner-shared, forming wide channels where lead cations reside. Local dipole moments for the PO4 and LiO4 tetrahedra as well as lead polyhedra in a unit cell were calculated. First-principles calculations have been performed for better understanding the structure-property relationships in LiPbPO4. In addition, to further explore spatial distribution of the electronic states dominating the NLO activity, the SHG density method was employed.

Superionic lithium transport via multiple coordination environments defined by two-anion packing.

Fast cation transport in solids underpins energy storage. Materials design has focused on structures that can define transport pathways with minimal cation coordination change, restricting attention to a small part of chemical space. Motivated by the greater structural diversity of binary intermetallics than that of the metallic elements, we used two anions to build a pathway for three-dimensional superionic lithium ion conductivity that exploits multiple cation coordination environments. Li7Si2S7I is a pure lithium ion conductor created by an ordering of sulphide and iodide that combines elements of hexagonal and cubic close-packing analogously to the structure of NiZr. The resulting diverse network of lithium positions with distinct geometries and anion coordination chemistries affords low barriers to transport, opening a large structural space for high cation conductivity.

Li6SiO4Cl2: A Hexagonal Argyrodite Based on Antiperovskite Layer Stacking.

A hexagonal analogue, Li6SiO4Cl2, of the cubic lithium argyrodite family of solid electrolytes is isolated by a computation-experiment approach. We show that the argyrodite structure is equivalent to the cubic antiperovskite solid electrolyte structure through anion site and vacancy ordering within a cubic stacking of two close-packed layers. Construction of models that assemble these layers with the combination of hexagonal and cubic stacking motifs, both well known in the large family of perovskite structural variants, followed by energy minimization identifies Li6SiO4Cl2 as a stable candidate composition. Synthesis and structure determination demonstrate that the material adopts the predicted lithium site-ordered structure with a low lithium conductivity of ∼10-10 S cm-1 at room temperature and the predicted hexagonal argyrodite structure above an order-disorder transition at 469.3(1) K. This transition establishes dynamic Li site disorder analogous to that of cubic argyrodite solid electrolytes in hexagonal argyrodite Li6SiO4Cl2 and increases Li-ion mobility observed via NMR and AC impedance spectroscopy. The compositional flexibility of both argyrodite and perovskite alongside this newly established structural connection, which enables the use of hexagonal and cubic stacking motifs, identifies a wealth of unexplored chemistry significant to the field of solid electrolytes.

The first lithium difluorophosphate LiPO2F2 with a neutral polytetrahedral microporous architecture.

Herein, we present the first lithium difluorophosphate LiPO2F2 exhibiting a unique polytetrahedral microporous architecture. The unique fluorine coordination environments and interlaced [LiO2]3- straight chains give rise to the neutral pore framework with a 10- membered ring channel along the crystallographic c axis. A variety of measurements have been adopted to systemically characterize LiPO2F2. This work contributes to the structural and functional diversity of phosphate chemistry by the exploration of fascinating difluorophosphates.

A new barium fluorooxoborate BaB5O8F·xH2O with large birefringence and a wide UV transparency window.

A new barium fluorooxoborate, BaB5O8F·xH2O (x ≈ 0.17), with two interesting independent three-dimensional B-O/F frameworks has been synthesized by a solid-state reaction in a closed system. Compared with the lead fluorooxoborate PbB5O8F, BaB5O8F·xH2O exhibits a comparable birefringence and a wider UV transparency window.

Ba3B10O17F2·0.1KF: the first mixed alkali/alkaline-earth metal fluorooxoborate with unprecedented double-layered B-O/F anionic arrangement.

The diverse oxyfluoride species and vast number of different topological arrangements lead to fluorooxoborates' structural beauty and richness. Herein, the first mixed alkali/alkaline-earth metal fluorooxoborate Ba3B10O17F2·0.1KF was successfully synthesized and systemically characterized. Its most important structural feature is an unprecedented double-layered B-O/F anionic arrangement.

K11RbB28O48: a new triple-layered borate with an unprecedented [B28O57] fundamental building block.

A new mixed-alkali borate, K11RbB28O48, has been obtained using the high-temperature solution method. It features a new [B28O57] fundamental building block. Meanwhile, these units further polymerize to form unusual triple-layered anionic groups. Thermal analysis, optical property measurement, and theoretical calculations were performed.

Synthesis of hierarchical-pore metal-organic framework on liter scale for large organic pollutants capture in wastewater.

Large organic pollutants in wastewater are harmful to human health and environmental ecosystem, and it is of great importance to remove them from waste water. In this report, hierarchical-pore metal-organic frameworks with different mesopore sizes (H-UiO-66-3.8 nm and H-UiO-66-17.3 nm) have been successfully synthesized at reflux condition. The resulting MOFs are characterized by N2 adsorption-desorption analysis and the results show that the samples have hierarchical-pore structures, which indicate the heat reflux method is efficient for the synthesis of H-UiO-66s on liter scale. In order to test the performance of H-UiO-66s for large organic pollutants capture, tetracycline (TC) was taken as an example to perform adsorption experiments. Remarkably, two kinds of H-UiO-66s exhibit significant improvement of adsorptive capacities compared with microporous UiO-66. The adsorption quantity of TC in H-UiO-66-17.3 nm is 666.67 mg·g-1, which is 430% higher than the value in original microporous UiO-66. In addition, the capture ability of H-UiO-66-17.3 nm surpasses various reported adsorbent materials, such as nanocomposite, activated carbon and NH2-MIL-101(Cr). Meanwhile, the H-UiO-66-3.8 nm and H-UiO-66-17.3 nm also show high adsorption capacities for Rhodamine B (RhB) and tannic acid (TA), indicating H-UiO-66s have wide potential application for capture various large organic pollutants. These results show that H-UiO-66s can be successfully synthesized on liter scale under reflux condition and these materials perform excellent adsorption capacities for various organic pollutants, leading the potential application in wastewater treatment.

Screening of Metal-Organic Frameworks for Highly Effective Hydrogen Isotope Separation by Quantum Sieving.

Separation of hydrogen isotopes is of great importance to produce highly pure hydrogen isotopes for numerous applications, which is however very difficult because of their almost identical thermodynamic properties. Adsorptive separation is considered as a simple, highly efficient, and cost-effective technique compared to the traditional methods, where the key is the suitable adsorbent. Herein, SIFSIX-3-Zn was screened out from the reported metal-organic frameworks (MOFs), exhibiting high selectivities for a D2/H2 mixture by quantum sieving effect. Advanced cryogenic thermal desorption spectroscopy confirms the calculation results, indicating that the selectivities for a 1:1 D2/H2 mixture at 20 K are larger than the values reported so far; especially, it shows a record value of 53.8 at 25 kPa. This demonstrates that this MOF is a promising candidate for highly effective hydrogen isotope separation.

Growth, Properties, and Theoretical Analysis of M2LiVO4 (M = Rb, Cs) Crystals: Two Potential Mid-Infrared Nonlinear Optical Materials.

Mid-Infrared nonlinear optical (Mid-IR NLO) crystals with excellent performances play a particularly important role for applications in areas such as telecommunications, laser guidance, and explosives detection. However, the design and growth of high performance Mid-IR NLO crystals with large NLO efficiency and high laser-damage threshold (LDT) still face numerous fundamental challenge. In this study, two potential Mid-IR NLO materials, Rb2LiVO4 (RLVO) and Cs2LiVO4 (CLVO) with noncentrosymmetric structures (Orthorhombic, Cmc21) were synthesized by high-temperature solution method. Thermal analysis and powder X-ray diffraction demonstrate that RLVO and CLVO melt congruently. Centimeter sized crystals of CLVO have been grown by the top-seeded solution growth method. RLVO and CLVO exhibit strong second harmonic generation (SHG) effects (about 4 and 5 times that of KH2PO4, respectively) with a phase-matching behavior at 1.064 µm, and a wide transparency range (0.33-6.0 µm for CLVO). More importantly, RLVO and CLVO possess a high LDT value (~28 × AgGaS2). In addition, the density functional theory (DFT) and dipole moments studies indicate that the VO4 anionic groups have a dominant contribution to the SHG effects in RLVO and CLVO. These results suggest that the title compounds are promising NLO candidate crystals applied in the Mid-IR region.