Fast Ion-Conducting Thioboracite with a Perovskite Topology and Argyrodite-like Lithium Substructure

Na2B12Si6Se18: a novel B12-cluster-containing quaternary selenoborate framework material

We report the successful synthesis of the new quaternary, three-dimensional Na2Si6B12Se18 material using a solid-state synthesis route. This compound is characterized by a unique framework structure that features the rare icosahedral [B12]10+ cation which is connected via Si2Se6 polyhedral units into a 3D framework structure with Na+ ions located inside the channels. The Na2Si6B12Se18 compound crystallizes in the cubic crystal system in the space group Im3̄, with lattice parameter a = 11.91110(10) Å and unit cell volume of 1689.88(4) Å3. The 3D framework of the [Si6B12Se18]2- anion contains B12 icosahedra with six of their edges bridged by tetrahedral SiSe4 groups. The B12 icosahedral units are connected by the SiSe4 tetrahedral units which bridge between each B12 icosahedron. Ultraviolet-Visible (UV-vis) spectroscopy revealed that Na2Si6B12Se18 exhibits semiconducting behavior with an estimated bandgap of 2.1(1) eV. In situ powder X-ray diffraction (PXRD) studies were conducted to investigate the phase formation of this compound as a function of temperature to obtain information of its thermal stability and structural evolution.

Crystal structures of the alkali aluminoboracites A4B4Al3O12Cl (A = Li, Na)

Single crystals of alkali aluminoboracites, A 4B4Al3O12Cl (A = Li, Na), were grown using the self-flux method, and their isotypic cubic crystal structures were determined by single-crystal X-ray diffraction. Na4B4Al3O12Cl is the first reported sodium boracite, and its lattice parameter [13.5904 (1) Å] is the largest among the boracites consisting of a cation-oxygen framework reported so far. For both crystals, structure models refined in the cubic space group F 3c, which assume that all cubic octant subcells in the unit cell are equivalent, converged with R1 factors of ∼0.03. However, the presence of weak hhl reflections with odd h and l values indicates that refinements in the space group F23, which presume a checkerboard-like ordering of two types of subcells with slightly different atomic positions, are more appropriate.

Effect of Relaxations on the Conductivity of La1/2+1/2x Li1/2-1/2x Ti1-x Al x O3 Fast Ion Conductors

Perovskite-type solid-state electrolytes, Li_3x La_2/3-x TiO₃ (LLTO), are considered among the most promising candidates for the development of all-solid-state batteries based on lithium metal. Their high bulk ionic conductivity can be modulated by substituting part of the atoms hosted in the A- or B-site of the LLTO structure. In this work, we investigate the crystal structure and the long-range charge migration processes characterizing a family of perovskites with the general formula La_1/2+1/2x Li_1/2-1/2x Ti_1-x Al _x O₃ (0 ≤ x ≤ 0.6), in which the charge balance and the nominal A-site vacancies (n _A = 0) are preserved. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) investigations reveal the presence of a very complex nanostructure constituted by a mixture of two different ordered nanoregions of tetragonal P4/mmm and rhombohedral R3̅c symmetries. Broadband electrical spectroscopy studies confirm the presence of different crystalline domains and demonstrate that the structural fluctuations of the BO₆ octahedra require to be intra- and intercell coupled, to enable the long-range diffusion of the lithium cation, in a similar way to the segmental mode that takes place in polymer-ion conductors. These hypotheses are corroborated by density functional theory (DFT) calculations and molecular dynamic simulations.