Double molybdates of silver and monovalent metals

The Ag2MoO4–Cs2MoO4 system was studied by powder X-ray diffraction, the formation of a new double molybdate CsAg3(MoO4)2 was established, its single crystals were obtained, and its structure was determined. CsAg3(MoO4)2 (sp. gr. P3¯, Z = 1, a = 5.9718(5), c = 7.6451(3) Å, R = 0.0149) was found to have the structure type of Ag2BaMn(VO4)2. The structure is based on glaserite-like layers of alternating MoO4 tetrahedra and Ag1O6 octahedra linked by oxygen vertices, which are connected into a whole 3D framework by Ag2O4 tetrahedra. An unusual feature of the Ag2 atom environment is its location almost in the centre of an oxygen face of the Ag2O4 tetrahedron. Caesium atoms are in cuboctahedral coordination (CN = 12).We determined the structures of the double molybdate of rubidium and silver obtained by us previously and a crystal from the solid solution based on the hexagonal modification of Tl2MoO4, which both are isostructural to glaserite K3Na(SO4)2 (sp. gr. P3¯m1). According to X-ray structural analysis data, both crystals have nonstoichiometric compositions Rb2.81Ag1.19(MoO4)2 (a = 6.1541(2), c = 7.9267(5) Å, R = 0.0263) and Tl3.14Ag0.86(MoO4)2 (a = 6.0977(3), c = 7.8600(7) Å, R = 0.0174). In the case of the rubidium compound, the splitting of the Rb/Ag position was revealed for the first time am ong molybdates. Both structures are based on layers of alternating MoO4 tetrahedra and AgO6 or (Ag, Tl)O6 octahedra linked by oxygen vertices. The coordination numbers of rubidium and thallium are 12 and 10

Single-crystal structures of A 2SiF6 (A = Tl, Rb, Cs), a better structure model for Tl3[SiF6]F, and its novel tetragonal polymorph

We report on the syntheses and single-crystal structure determinations of the compounds A 2SiF6 (A = Tl, Rb, Cs). In comparison to the previous powder-based structure models we achieved more precise atom positions and distances. The compounds crystallize in the K2PtCl6 structure type, space group Fm 3 ‾ $‾{3}$ m (No. 225, cF36) with a = 8.4749(10) Å, V = 608.7(2) Å3, Z = 4 at T = 100 K for Tl2SiF6, a = 8.3918(10) Å, V = 591.0(2) Å3, Z = 4 at T = 100 K for Rb2SiF6, and a = 8.8638(11) Å, V = 696.4(3) Å3, Z = 4 at T = 200 K for Cs2SiF6. For the compound Tl3[SiF6]F we present a previously unknown tetragonal modification and correct the crystal structure of its trigonal modification to hexagonal. The tetragonal one crystallizes in the (NH4)3[SiF6]F structure type, space group P4/mbm (No. 127, tP22) with a = 8.0313(8), c = 5.8932(6) Å, V = 380.13(7) Å3, Z = 2, T = 298 K, and the crystal structure of the hexagonal modification is best described in space group P63 mc (No. 186, hP22) with a = 7.8248(4), c = 6.8768(4) Å, V = 364.64(4) Å3, Z = 2, T = 100 K.

THE SOURCES OF RADIATION IN THE SHORT-WAVE RANGE ON THE BASIS OF II-VI HETEROLAYERS

The possibility of obtaining zinc selenide and zinc sulfide layers of hexagonal modification by isovalent substitution method is shown. They are characterized by intensive luminescence which is formed by the dominant annihilation of bound excitons for α-ZnSe and recombination on donor-acceptor pairs for α-ZnS. The resulting radiation covers the violet wavelength range. Quantum radiation efficiency reaches η = 10–12% for α-ZnSe and η = 5–8% for α-ZnS. The radiation is characterized by high temperature stability and repeatability of characteristics and parameters.

Raman Spectroscopy Study on Na2/3Mn1-xFexO2 Oxides

The structural properties of sodium manganates and iron substituted sodium manganates with compositions Na2/3Mn1-xFexO2 (x=0, 1/3 and 2/3) were studied by Raman spectroscopy. The Raman spectroscopy allows distinguishing between layered phases with orthorhombic (Cmcm space group) and hexagonal (P63/mmc space group) distortion. It has been found that the crystal structure and the composition of Na2/3MnO2 display a strong dependence on the history of the thermal treatment. The orthorhombic distorted modification is stabilized at high temperatures (1000 oC). At lower quenching temperature, there is a phase separation into an orthorhombic and a hexagonal modification, concomitant with an increase in the oxidation state of Mn. When Fe substitutes for Mn, the hexagonal modification is stabilized. In order to understand the origin of the Raman spectra of Na2/3Mn1-xFexO2, we have used Na2/3Co2/3Mn1/3O2 as a standard for hexagonal structure, where Co3+ and Mn4+ are statistically distributed in the transition metal layers.