Nanotube Structure of AsPS4-xSex (x = 0, 1).

Single-wall nanotubes of isostructural AsPS4-xSex (x = 0, 1) are grown from solid-state reaction of stoichiometric amounts of the elements. The structure of AsPS4 was determined using single-crystal X-ray diffraction and refined in space group P1¯. The infinite, single-walled AsPS4 nanotubes have an outer diameter of ≈1.1 nm and are built of corner-sharing PS4 tetrahedra and AsS3 trigonal pyramids. Each nanotube is nearly hexagonal, but the ≈3.4 Å distance between S atoms on adjacent nanotubes allows them to easily slide past one another, resulting in the loss of long-range order. Substituting S with Se disrupted the crystallization of the nanotubes, resulting in amorphous products that precluded the determination of the structure for AsPS3Se. 31P solid-state NMR spectroscopy indicated a single unique tetrahedral P environment in AsPS4 and five different P environments all with different degrees of Se substitution in AsPS3Se. Optical absorption spectroscopy revealed an energy band gap of 2.7 to 2.4 eV for AsPS4 and AsPS3Se, respectively. Individual AsPS4 microfibers showed a bulk conductivity of 3.2 × 10-6 S/cm and a negative photoconductivity effect under the illumination of light (3.06 eV) in ambient conditions. Thus, intrinsic conductivity originates from hopping through empty trap states along the length of the AsPS4 nanotubes.

Polysulfide chalcogels with ion-exchange properties and highly efficient mercury vapor sorption.

We report the synthesis of metal-chalcogenide aerogels from Pt(2+) and polysulfide clusters ([S(x)](2-), x = 3-6). The cross-linking reaction of these ionic building blocks in formamide solution results in spontaneous gelation and eventually forms a monolithic dark brown gel. The wet gel is transformed into a highly porous aerogel by solvent exchanging and subsequent supercritical drying with CO(2). The resulting platinum polysulfide aerogels possess a highly porous and amorphous structure with an intact polysulfide backbone. These chalcogels feature an anionic network that is charged balanced with potassium cations, and hosts highly accessible S-S bonding sites, which allows for reversible cation exchange and mercury vapor capture that is superior to any known material.

Molecular germanium selenophosphate salts: phase-change properties and strong second harmonic generation.

A new series of germanium chalcophosphates with the formula A(4)GeP(4)Q(12) (A = K, Rb, Cs; Q = S, Se) have been synthesized. The selenium compounds are isostructural and crystallize in the polar orthorhombic space group Pca2(1). The sulfur analogues are isostructural to one another but crystallize in the centrosymmetric monoclinic space group C2/c. All structures contain the new molecular anion [GeP(4)Q(12)](4-); however, the difference between the sulfides and selenides arises from the change in crystal packing. Each discrete molecule is comprised of two ethane-like P(2)Q(6) units that chelate to a central tetrahedral Ge(4+) ion in a bidentate fashion. The selenides were synthesized pure by stoichiometric reaction of the starting materials, whereas the sulfides contained second phases. The band gaps of the molecular salts are independent of the alkali metal counterions and have a value of 2.0 eV for the selenides and 3.0-3.1 eV for the sulfides. All A(4)GeP(4)Se(12) compounds melt congruently, and the potassium analogue can be quenched to give a glassy phase that retains its short-range order as shown by Raman spectroscopy and powder X-ray diffraction. Interestingly, K(4)GeP(4)Se(12) is a phase-change material that reversibly converts between glassy and crystalline states and passes through a metastable crystalline state upon heating just before crystallizing into its slow-cooled form. Initial second harmonic generation (SHG) experiments showed crystalline K(4)GeP(4)Se(12) outperforms the other alkali metal analogues and exhibits the strongest second harmonic generation response among reported quaternary chalcophosphates, ~30 times that of AgGaSe(2) at 730 nm. A more thorough investigation of the nonlinear optical (NLO) properties was performed across a range of wavelengths that is almost triple that of previous reports (λ = 1200-2700 nm) and highlights the importance of broadband measurements. Glassy K(4)GeP(4)Se(12) also exhibits a measurable SHG response with no poling.

Germanium selenophosphates: the incommensurately modulated 1/∞[Ge(4-x)P(x)Se12(4-)] and the molecular [Ge2P2Se14]6-.

The new germanium selenophosphates K(4)Ge(4-x)P(x)Se(12) (1) and Rb(6)Ge(2)P(2)Se(14) (2) are reported. The former is a one-dimensional metastable compound synthesized using the polychalcogenide flux method that crystallizes in the monoclinic space group P2(1)/c with lattice parameters a = 6.7388(7) Å, b = 13.489(1) Å, c = 6.3904(6) Å, and ß = 91.025(8)°. At a glance, a mixed Ge(4+)/P(5+) tetrahedral site and disordered Se position are found among the corner sharing tetrahedra that make up the polymeric anion. After careful examination, the structure was found to be incommensurately modulated and a single q-vector of q = 0.4442(6)a* + 0.3407(6)c* was determined after annealing single crystals below their decomposition point for 30d. The latter compound contains the new discrete molecular anion [Ge(2)P(2)Se(14)](6-) and crystallizes in P1 with lattice parameters a = 7.2463(8) Å, b = 9.707(1)Å, c = 11.987(1)Å, α = 79.516(9)°, ß = 89.524(9)°, and γ = 68.281(9)°. Both compounds are semiconductors with band gaps of 1 and 2 being 1.9 eV and 2.2 eV, respectively.

Arsenic-containing chalcophosphate molecular anions.

We report five new discrete molecular arsenic-based chalcophosphates, K(7)As(3)(P(2)Se(6))(4) (1), K(6)As(2)(P(2)Se(6))(3) (2), Cs(6)As(2)(P(2)Se(6))(3) (3), and Cs(5)As(P(2)Q(6))(2) [Q = Se (4a) and S (4b)]. Each of the compounds contains unique complex anions comprised of common building blocks that have condensed to produce these anions. Phosphorus forms well-known [P(2)Q(6)](4-) moieties in all of the compounds that are bridged by arsenic trigonal pyramids in 1 and 2 and distorted octahedra in 3, 4a, and 4b. Although 2 and 3 have the same molecular formula, the structural difference between the two salts is attributed to the size of the alkali metal. The influence of flux basicity also seems to play a role in the formation of the molecular anion in 4a and 4b, which has been observed with other trivalent main-group elements at the octahedral position but only with the highly basic cesium alkali metal as the counterion. All structures were determined by single-crystal X-ray diffraction and are discussed along with phase-purity powder X-ray diffraction, thermal analyses, electronic absorption, and Raman spectroscopy.