Structures and physical properties of new semiconducting polyselenides Ba2CudeltaAg4-deltaSe5 with unprecedented linear Se(3)4- units

The title compounds were prepared from the elements in evacuated silica tubes at 650 degrees C, followed by slow cooling. Ba2Ag4Se5 forms a new structure type, space group C2/m, with a=16.189(2) A, b=4.5528(6) A, c=9.2500(1) A, beta=124.572(3) degrees, and V=561.4(1) A3 (Z=2). A maximum of 44% of the Ag atoms may be replaced with Cu atoms without changing the structure type. The crystal structure is composed of Ag4Se(5)4- layers, interconnected via the Ba2+ cations. The Ag atoms show irregular [3+1] coordination by the Se atoms, and the Ba atoms are located in capped square antiprisms formed by Se atoms. Most intriguing is the unprecedented occurrence of linear Se(3)4- units. According to the formulation (Ba2+)2(Ag+)4Se(3)4-(Se2-)2, this selenide is electron-precise with eight positive charges equalizing the eight negative charges. Electronic structure calculations indicated the presence of a band gap, as was experimentally confirmed: the electrical conductivity measurement revealed a gap of 0.6 eV for Ba2CuAg3Se5.     

Heavy atom analogues of 1,2,3-dithiazolylium salts: preparation, structures and redox chemistry

Synthetic routes to salts of the benzo[1,2,3]thiatellurazolylium cation [2c](+) and its selenium analogue [2b](+) are described. Access to the cation frameworks involves the intermediacy of N,N,S-trisilylated 2-aminobenzenethiol. The latter reacts smoothly with selenium and tellurium halides ECl4 (E = Se, Te) to afford the desired heterocyclic benzo cations [2b](+) and [2c](+) as their chloride salts. Anion exchange provides the corresponding GaCl4(-), OTf(-) and TeCl5(-) salts of [2c](+), all of which have been characterized by X-ray crystallography. While the gallate salts of the sulfur and selenium cations [2a](+) and [2b](+) crystallize as ion-paired cations and anions, salts of [2c](+) adopt solid-state structures that display strong association of the cations via short intermolecular Te-N' bonds. However, crystallization of [2c](+) salts in dichloroethane in the presence of GaCl3 leads to cleavage of the dimers and the formation of a Lewis acid adduct at nitrogen. Reduction of the benzo cations [2a,b](+) affords the respective radicals 2a,b, both of which have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Attempts to generate the corresponding radical 2c have been unsuccessful, although a material of nominally correct elemental composition can be generated by chemical reduction. The energetics of association of [2a,b,c](+) in solution has been probed by means of density functional theory calculations using the polarized continuum model. The results suggest that the dimeric nature of the Te-centered cation is retained in solution. The strength of the interaction is, however, less than in N-alkylated tellurodiazolylium salts.     

Structure and physical properties of the new telluride BaAg2Te2 and its quaternary variants BaCuδAg2-δTe2

The new materials BaCu_δAg_2-δTe₂ (0?δ?2) were prepared from the elements at 800 °C in evacuated silica tubes. BaAg₂Te₂ crystallizes in the α-BaCu₂S₂ type, space group Pnma, with lattice parameters a=10.8897(3) Å, b=4.6084(1) Å, c=11.8134(3) Å (Z=4). The structure consists of a three-dimensional network of vertex- and edge-condensed AgTe₄ tetrahedra, which includes the Ba²⁺ cations in linear channels running along the short b-axis. Half of the Ag atoms participate in an Ag atom zigzag chain extended parallel to the channels. BaAg₂Te₂ is a p-type semiconductor with large Seebeck coefficient. Within the series BaCu_δAg_2−δTe₂, the electrical conductivity increases and the Seebeck coefficient decreases strongly with increasing Cu content.     

A New Sodium Thioborate Fast Ion Conductor: Na3B5S9

We report a new sodium fast-ion conductor, Na₃B₅S₉, that exhibits a high Na ion total conductivity of 0.80 mS cm⁻¹ (sintered pellet; cold-pressed pellet=0.21 mS cm⁻¹). The structure consists of corner-sharing B₁₀S₂₀ supertetrahedral clusters, which create a framework that supports 3D Na ion diffusion channels. The Na ions are well-distributed in the channels and form a disordered sublattice spanning five Na crystallographic sites. The combination of structural elucidation via single crystal X-ray diffraction and powder synchrotron X-ray diffraction at variable temperatures, solid-state nuclear magnetic resonance spectra and ab initio molecular dynamics simulations reveal high Na-ion mobility (predicted conductivity: 0.96 mS cm⁻¹) and the nature of the 3D diffusion pathways. Notably, the Na ion sublattice orders at low temperatures, resulting in isolated Na polyhedra and thus much lower ionic conductivity. This highlights the importance of a disordered Na ion sublattice—and existence of well-connected Na ion migration pathways formed via face-sharing polyhedra—in dictating Na ion diffusion.