ABSTRACT
We report on a new layered semiconductor Tl8Sn10Sb16Se48 with an indirect band gap of 0.45 eV. The novel structure is made of alternating layers of SnSe2-type [Sn5Sb2Se14] and SnSe-type [Tl4Sb6Se10]. The material exhibits two-dimensional (2D) electron variable range hopping at low temperatures, indicating an absence of interlayer coherency of the electronic state. Theoretical calculations unveil a 2D confinement for electrons in the [Sn5Sb2Se14] sheet and confirm the heterostructure nature. This unique electronic structure is attributed to the weak interlayer coupling and structure distortion in the electron-poor [Tl4Sb6Se10] layer that energetically impedes electron propagation.
ABSTRACT
The quaternary Cs2S/Bi/As/S system was studied in an attempt to introduce two different asymmetric but isoelectronic building units, namely, [Bi(III)(x)S(y)] and [As(III)(x)S(y)], in a single structure. Reactions with a comparatively lower equivalent of arsenic in the Cs2S/Bi/As/S mixture led to the crystalline compound Cs3Bi(AsS4)2. The structure features tetrahedral [As(V)S4](3-) connected to Bi(III) centers to give infinite (1/∞)[Bi(AsS4)2(3-)] chains. When the basicity was raised in these low arsenic fluxes by increasing the Cs2S fraction, the crystalline compound Cs9Bi(AsS4)4, also featuring [As(V)S4](3-) anions, was formed. On the other hand, arsenic-rich mixtures of Cs2S/Bi/As/S led to the formation of the glassy phase Cs2BiAs3S7, which contains As(III) species. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and pair distribution function (PDF) analysis indicate the presence of As(III)-containing [AsnS2n+1] fragments in the glass structure. Several glasses in the series Cs(n-1)BiAs(n)S(2n+1) were also prepared using solid-state fusion reactions. The band gaps of the Cs(n-1)BiAs(n)S(2n+1) glasses are in the range of 1.51-1.81 eV, while that of the crystalline compound Cs3Bi(AsS4)2 is ~2.33 eV. The thermal and optical behaviors of these compounds are correlated with their structures and building units.
ABSTRACT
The ability to fabricate flexible filtration membranes that can selectively separate particles of different sizes is of considerable interest. In this article, we describe a facile, reproducible and simple one-step method to produce pores in polydimethylsiloxane (PDMS) membranes. We embedded micron-sized NaHCO(3) particles in 50 micron thick PDMS films. After curing, the membranes were immersed in concentrated HCl acid. Pores were generated in the membrane by the evolution of CO(2) gas from the reaction of NaHCO(3) and HCl. High resolution Scanning Electron Microscope images clearly reveal the presence of openings on the surface and the cross-section of the membranes. Fluorescence and back-scattered electron imaging of porous PDMS membrane with embedded gold nanoparticles and comparison with non-porous PDMS membranes provided unambiguous evidence of pores in the membrane. Transport studies of molecular fluoresceinate ions, ions (sodium and chloride) and 240 nm polystyrene nanoparticles through these membranes demonstrate passable pores and existence of channels within the body of the membrane. Mechanically stretching the porous PDMS membrane and comparing the flow rates of fluoresceinate ions and the polystyrene beads through the stretched and unstretched membranes allowed a direct proof of the modulation of transport rate in the membranes. We show that stretching the membranes by 10% increases the flow rate of fluorescein molecules by 2.8 times and by a factor of approximately ~40% for the polystyrene nanoparticles.
ABSTRACT
Eu(3)(AsS(4))(2) and A(x)Eu(3-y)As(5-z)S(10) (A = Li, Na) are the members of a new thioarsenate family. They feature As(5+) and As(3+) centers, respectively. The rhombohedral Eu(3)(AsS(4))(2) features a new structure type consisting of eight-coordinate Eu(2+) centers and AsS(4)(3-) anions, whereas the monoclinic A(x)Eu(3-y)As(5-z)S(10) (Li(0.73)Eu(3)As(4.43)S(10) and Na(0.66)Eu(2.86)As(4.54)S(10)) belong to the rathite sulfosalt family and are comprised of apparent [As(10)S(20)](10-) segments linked with Eu(2+) ions to give a three-dimensional network. They appear to be alkali-metal-stabilized derivatives of the putative parent phase "Eu(3)As(5)S(10)".
ABSTRACT
The reaction of Mn and Cd in alkali metal polythioarsenate fluxes afforded four new compounds featuring molecular anions. K(8)[Mn(2)(AsS(4))(4)] (I) crystallizes in the monoclinic space group P2/n with a = 9.1818(8) A, b = 8.5867(8) A, c = 20.3802(19) A, and beta = 95.095(2) degrees. Rb(8)[Mn(2)(AsS(4))(4)] (II) and Cs(8)[Mn(2)(AsS(4))(4)] (III) both crystallize in the triclinic space group P1 with a = 9.079(3) A, b = 9.197(3) A, c = 11.219(4) A, alpha = 105.958(7) degrees, beta = 103.950(5) degrees, and gamma = 92.612(6) degrees for II and a = 9.420(5) A, b = 9.559(5) A, c = 11.496(7) A, alpha = 105.606(14) degrees, beta = 102.999(12) degrees, and gamma = 92.423(14) degrees for III. The discrete dimeric [Mn(2)(AsS(4))(4)](8-) clusters in these compounds are composed of two octahedral Mn(2+) ions bridged by two [AsS(4)](3-) units and chelated each by a [AsS(4)](3-) unit. Rb(8)[Cd(2)(AsS(4))(2)(AsS(5))(2)] (IV) crystallizes in P1 with a = 9.122(2) A, b = 9.285(2) A, c = 12.400(3) A, alpha = 111.700(6) degrees, beta = 108.744 degrees, and gamma = 90.163(5) degrees. Owing to the greater size of Cd compared to Mn, the Cd centers in this compound are bridged by [AsS(5)](3-) units. The [Cd(2)(AsS(4))(4)](8-) cluster is a minor component cocrystallized in the lattice. These compounds are yellow in color and soluble in water.
ABSTRACT
Through a flame-melting/rapid-cooling process, metastable forms of solid state compounds can be discovered. We describe here an example where both slow and rapid crystallizations of a stoichiometric "KInSnSe(4)" melt give rise to kinetic forms of KInSnSe(4). These forms (alpha- and beta-) convert to the thermodynamically stable gamma-form upon heating below the melting point.
ABSTRACT
Four cubic compounds are reported that contain the supertetrahedral cluster [M(4)Sn(4)S(17)](10)(-) where M = Mn, Fe, Co, Zn. The cluster features a central quadruply bridging sulfide ion (mu(4)-S) that holds together four divalent M atoms in a tetrahedral arrangement. This core is capped with four tridentate [SnS(4)](4)(-) fragments to complete the structure.
ABSTRACT
Two noncentrosymmetric quaternary tin chalcoarsenates, Cs(2)SnAs(2)S(9) (1) and Cs(2)SnAs(2)Se(9) (2), were synthesized by the polychalcoarsenate flux method. Compound 1 crystallizes in the orthorhombic space group Pmc2(1) with a = 7.386(3) A, b = 14.614(5) A, c = 14.417(5) A, and Z = 4. Compound 2 crystallizes in the monoclinic space group P2(1) with a = 7.715(5) A, b = 17.56(1) A, c = 7.663(5) A, beta = 115.86(1) degrees, and Z = 2. Both structures contain the same tin-centered molecular cluster anions [Sn[AsQ(2)(Q(2))][AsQ(Q(2))(2)]](2)(-) (Q = S, Se) separated by Cs cations. The Sn(4+) ion is in a distorted octahedral environment coordinated by two different pyramidal-shaped tridentate ligands, [AsQ(2)(Q(2))](3)(-) and [AsQ(Q(2))(2)](3)(-). These compounds absorb visible light at energies above 1.98 and 1.45 eV for 1 and 2, respectively. Differential thermal analysis revealed that 1 melts at 350 degrees C and on cooling gives a glass. The glass recrystallizes at 268 degrees C upon subsequent heating. Compound 2 melts at 258 degrees C.
ABSTRACT
Two low-dimensional compounds, KSnAsS(5) and K(2)SnAs(2)S(6), were prepared using liquid polythioarsenate salts, and the results differ from those obtained with the well studied thiophosphate flux. KSnAsS(5) crystallizes in the orthorhombic space group Pbam with cell parameters of a = 8.136(2) A, b = 13.784(4) A, c = 7.428(2) A. KSnAsS(5) features the unusual pyramidal species [AsS(2)(S(2))](3-). K(2)SnAs(2)S(6) crystallizes in the trigonal space group P3 macro with cell parameters a = 6.717(5) A, b = 7.204(8) A, gamma = 120 degrees. The compounds were obtained by controlling the Lewis basicity of the K(2)S/As(2)S(3)/S flux. The optical, thermal, and spectroscopic properties of the compounds are reported.
