Sodium Selenotetrelates with Isolated TtSe4-Tetrahedra (Tt = Si, Ge, Sn): Synthesis, Crystal Structures, Thermal Behavior, DFT Modeling, and Na Ion Conductivities

Selenotetrelate compounds Na4TtSe4 (Tt = Si, Ge, Sn) were synthesized by solid-state reactions. A new modification of Na4SiSe4 (Na4SiSe4-cP72), which crystallizes in the cubic space group P4̅3n (no. 218) with a = 12.130(1) Å and V = 1784.453(5) Å3, and a new modification of Na4SnSe4 (Na4SnSe4-tI216), which crystallizes in the tetragonal space group I41/acd (no. 142) with a = 14.4053(4) Å, c = 28.5751(8) Å and V = 5929.7(3) Å3, were discovered. All of the title compounds exhibit moderate to good sodium ion conductivities, as revealed by electrochemical impedance spectroscopy. The formation reaction of Na4SiSe4 was further investigated by high-temperature X-ray powder diffraction of the ball-milled reaction mixture. Density functional-based quantum chemical calculations were performed to compare the different modifications of Na4SiSe4 and Na4SnSe4 energetically. Further modifications of Na4SiSe4 and Na4GeSe4 seem plausible, as revealed by density functional theory modeling. The stability of the hypothetic modifications was examined by phonon dispersion calculations

The Stacking Faulted Nature of the Narrow Gap Semiconductor Sc2_{2}Si2_{2}Te6_{6}

Crystals of Sc$_{2}$Si$_{2}$Te$_{6}$ have been grown and its crystal, micro- and electronic structures were investigated. The layered character of the title compound exhibits stacking faults that impede a full structural characterization by single crystal X-ray diffraction due to diffuse scattering. Based on high resolution transmission electron micrographs and diffraction patterns, the stacking faulted nature of the real structure of Sc$_{2}$Si$_{2}$Te$_{6}$ has been revealed. Different stacking models were derived from the idealized, faultless structure and the stacking disorder was quantitatively analyzed by Rietveld refinement of powder X-ray diffraction patterns. An energetic comparison of the stacking models by density functional theory is in line with the experimental observations. Further, the bonding situation was investigated by electronic structure calculations. Sc$_{2}$Si$_{2}$Te$_{6}$ is a narrow gap semiconductor with an indirect band gap of 0.65 eV

Strong Lewis and Brønsted Acidic Sites in the Borosulfate Mg3[H2O→B(SO4)3]2

Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg-3[H2O -> B(SO4)(3)](2) is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid-base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF3 and B(C6F5)(3) revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Bronsted acidity than comparable silicates or phosphates. Additionally, Mg-3[H2O -> B(SO4)(3)](2) was characterised by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations

Surface Accumulation of Cerium, Self-Assembling Peptide, and Fluoride on Sound Bovine Enamel

The accumulation of caries-preventive compounds on sound enamel is crucial in order to improve the inhibition of carious lesion initiation. The aim of this research was to investigate the initial accumulation of cerium, oligopeptide p11-4, and fluoride from NaF or amine fluoride (AmF) on sound enamel in vitro by means of energy dispersive X-ray spectroscopy (EDX). Polished bovine enamel specimens (n = 120 from 60 teeth) were fabricated. Out of these, 12 specimens each were treated with CeCl3 (cerium(III) chloride heptahydrate 25%), oligopeptide p11-4 (Curodont Repair, Credentis), NaF (10,000 ppm F−), AmF (amine fluoride, Elmex Fluid, CP-GABA GmbH, 10,000 ppm F−), or Aqua demin (control). After rinsing with water, the surface elemental composition (Ce, N, F, Ca, P, O, Na, Mg) was measured (EDX; EDAX Octane Elect detector, APEX v2.0), expressed in atomic percent (At%) and analyzed (non-parametric statistics, α = 0.05, error rates method). Another 12 specimens per treatment group were fabricated and used for analyzing accumulation in cross-sections with EDX linescans and two-dimensional EDX-mappings. The surface median atomic percent of cerium (At%Ce) was 0.8 for CeCl3, but no Ce was found for any other group. N, specifically for oligopeptide p11-4, could not be detected. Fluorine could only be detected on fluoridated surfaces. The median atomic percent of fluorine (At%F) was 15.2 for NaF and 17.0 for AmF. The Ca/P ratio increased significantly compared to the control following the application of NaF and AmF (p < 0.001), but decreased significantly for CeCl3 (p < 0.001). In cross-sectioned specimens of the CeCl3-group, 12.5% of the linescans revealed cerium at the enamel surface, whereas 83.3% of the NaF linescans and 95.8% of the AmF linescans revealed fluorine at the enamel surface. Following the application of oligopeptide p11-4, no traces of N were detectable. In the depth of the samples, no signal was detected for any of the corresponding elements exceeding the background noise. Cerium and fluorine (from both NaF and AmF), but not the oligopeptide p11-4, precipitated on sound enamel

Designing 3D topological insulators by 2D-Xene (X = Ge, Sn) sheet functionalization in the GaGeTe-type structures

State-of-the-art theoretical studies anticipate a 2D Dirac system in the "heavy'' analogues of graphene, free-standing buckled honeycomb-like Xenes (X = Si, Ge, Sn, Pb, etc.). Herewith we regard a 2D sheet, which structurally and electronically resembles Xenes, in a 3D periodic, rhombohedral structure of layered AXTe (A = Ga, In; X = Ge, Sn) bulk materials. This structural family is predicted to host a 3D strong topological insulator with Z(2) = 1;(111) as a result of functionalization of the Xene derivative by covalent interactions. The parent structure GaGeTe is a long-known bulk semiconductor; the "heavy'', isostructural analogues InSnTe and GaSnTe are predicted to be dynamically stable. Spin-orbit interaction in InSnTe opens a small topological band gap with inverted gap edges that are mainly composed of the In-5s and Te-5p states. Our simulations classify GaSnTe as a semimetal with topological properties, whereas the verdict for GaGeTe is not conclusive and urges further experimental verification. The AXTe family structures can be regarded as stacks of 2D layered cut-outs from a zincblende-type lattice and are composed of elements that are broadly used in modern semiconductor devices; hence they represent an accessible, attractive alternative for applications in spintronics. The layered nature of AXTe should facilitate the exfoliation of their hextuple layers and manufacture of heterostructures

First principles calculations on structure, bonding, and vibrational frequencies of SiPsub 2

Pyrite type SiP2 is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP2 is seen as a model compound with molecular [P–P] entities and [SiP6] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P–P and Si–P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP2 and to compare and contrast the different methods of calculation

Synthesis‐controlled polymorphism and optical properties of phyllosilicate‐analogous borosulfates M[B2(SO4)4] (M = Mg, Co)

Increased synthetic control in borosulfate chemistry leads to the access of various new compounds. Herein, the polymorphism of phyllosilicate-analogous borosulfates is unraveled by adjusting the oleum (65 % SO3) content. The new polymorphs beta-Mg[B-2(SO4)(4)] and alpha-Co[B-2(SO4)(4)] both consist of similar layers of alternating borate and sulfate tetrahedra, but differ in the position of octahedrally coordinated cations. The alpha-modification comprises cations between the layers, whereas in the beta-modification cations are embedded within the layers. With this new synthetic approach, phase-pure compounds of the respective polymorphs alpha-Mg[B-2(SO4)(4)] and beta-Co[B-2(SO4)(4)] were also achieved. Tanabe-Sugano analysis of the Co(2+)polymorphs reveal weak ligand field splitting and give insights into the coordination behavior of the two-dimensional borosulfate anions for the first time. DFT calculations confirmed previous in silico experiments and enabled an assignment of the polymorphs by comparing the total electronic energies. The compounds are characterized by single-crystal XRD, PXRD, FTIR, and UV/Vis/NIR spectroscopy, thermogravimetric analysis (TGA), and density functional theory (DFT) calculations

Strong Lewis and Brønsted acidic sites in the borosulfate Mg3[H2O→B(SO4)3]2

Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg(3)[H(2)O→B(SO(4))(3)](2) is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid–base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF(3) and B(C(6)F(5))(3) revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Brønsted acidity than comparable silicates or phosphates. Additionally, Mg(3)[H(2)O→B(SO(4))(3)](2) was characterised by X‐ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations