S. F. Solodovnikov

Phase formation and structure of high-pressure gas hydrates and modeling of tetrahedral frameworks with uniform polyhedral cavities

During the last decade, a number of high-pressure gas hydrates have been prepared and structurally characterized. One of the most interesting results obtained is the formation of compounds having tetrahedral water frameworks with uniform polyhedral cavities (e.g., clathrates THF·6H2O and 2Ar·6H2O), unprecedented among ambient pressure hydrates. In order to predict, reveal and perform effective investigations of such gas hydrates at high pressures it is plausible to know all possible tetrahedral frameworks of this kind. The solution of this problem demands to elaborate methods of a topological design of the tetrahedral frameworks represented as space-filling packings of uniform polyhedra with trivalent vertices (simple polyhedra). There are two related approaches to solve this problem: tiling of 3D space into symmetrically equal polyhedra (stereohedra) and generation of periodic four-connected nets. At present, the complete set of tetrahedral frameworks built of uniform simple 14-hedra (23 packings) was obtained and more than 800 frameworks were constructed from larger simple stereohedra. This article gives a discussion of structural and energetic characteristics of the frameworks generated, as well as the possibilities of using these results for interpretation of experimental structural data and a deliberate synthesis of clathrate compounds possessing new structures. Experimental and theoretical data show a high probability of finding a whole series of novel high-pressure gas hydrates with the tetrahedral water frameworks built of simple stereohedra. Of these, the most probable structures are those with 14-hedral cavities, these structures having the stoichiometry of six water molecules per cavity. In our opinion, the formation of such hydrates could be expected for guest molecules with van der Waals diameters from 5.8 to 7.2 Å. However, these hydrates cannot be excluded for substantially smaller guests as well, provided that the water framework cavities include two guest molecules. Packing polymorphism (different space filling arrangements of the same stereohedron) revealed in the search for tilings of 3D space into stereohedra offers experimental discovery of this phenomenon, which can be promoted by such delicate effects as the guest-guest interaction. The set of the derived water frameworks as stereohedra space-fillings gives opportunities to select starting structure models in the course of a diffraction study of polycrystalline high-pressure gas hydrate samples.

Rubidium dimolybdate, Rb2Mo2O7, and caesium dimolybdate, Cs2Mo2O7

The crystal structures of dirubidium heptaoxodimolybdate, Rb(2)Mo(2)O(7), and dicaesium heptaoxodimolybdate, Cs(2)Mo(2)O(7), in the space groups Ama2 and P2(1)/c, respectively, have been determined for the first time by single-crystal X-ray diffraction. The structures represent two novel structure types of monovalent ion dimolybdates, A(2)Mo(2)O(7) (A = alkaline elements, NH(4), Ag or Tl). In the structure of Rb(2)Mo(2)O(7), Mo atoms are on a twofold axis, on a mirror plane and in a general position. One of the Rb atoms lies on a twofold axis, while three others are on mirror planes. Two O atoms attached to the Mo atom on a mirror plane are located on the same plane. Rubidium dimolybdate contains a new kind of infinite Mo-O chain formed from linked MoO(4) tetrahedra and MoO(6) octahedra alternating along the a axis, with two terminal MoO(4) tetrahedra sharing corners with each octahedron. The chains stack in the [001] direction to form channels of an approximately square section filled by ten-coordinate Rb ions. Seven- and eight-coordinate Rb atoms are located between chains connected by a c translation. In the structure of Cs(2)Mo(2)O(7), all atoms are in general positions. The MoO(6) octahedra share opposite corners to form separate infinite chains running along the c axis and strengthened by bridging MoO(4) tetrahedra. The same Mo-O polyhedral chain occurs in the structure of Na(2)Mo(2)O(7). Eight- to eleven-coordinate Cs atoms fill the space between the chains. The atomic arrangement of caesium dimolybdate has an orthorhombic pseudosymmetry that suggests a possible phase transition P2(1)/c-->Pbca at elevated temperatures.

New structure type in the morphotropic series of A 2 + M 2 2+ (MoO4)3: crystal structure of Rb2Cu2(MoO4)3

Isostructural compounds A2Cu2(MoO4)3 (A = Rb, Cs) were synthesized. The new structure type of the compounds was established for the rubidium-containing compound as an example (a =27.698,b =5.102,c =19.292 å, Β = 707.26‡,Z = 8, space group C2/c, R = 0.016). The structure is characterized by pairs of infinite wolframite-like ribbons of CuO(4+2) octahedra stretching along [010] and additionally bridged by MoO4 tetrahedra. The tetrahedra located between the ribbons weakly interact with each other at distances Mo-O of 2.545 and 2.853 å. There are four such quasione-dimensional copper-molybdenum-oxygen radicals per unit cell; the radicals are united into a single structure by rubidium ions having coordination numbers (CN) 9 and 10.

Binary molybdates K4M2+(MoO4)3 (M2+=Mg, Mn, Co) and crystal structure of K4Mn(MoO4)3

AbstractBinary molybdates K4M2+ (MoO4)3 (M2+=Mg, Mn, Co) isostructural to triclinic \ga-K4Zn(WO4)3 were synthesized, and optimal conditions for their spontaneous crystallization were found. It was established by XRPA and DTA that at 530°C the structure of the compound with cobalt undergoes a transition to the orthorhombic structure of K4Zn(MoO4)3. The structure of K4Mn(MoO4)3 was determined from single crystal diffraction data (a=7.613, b=9.955, c=10.156 Å,α=92.28,β=106.66,γ=105.58°, Z=2, space group

A NEW TYPE OF MIXED FRAMEWORK IN THE CRYSTAL STRUCTURE OF BINARY MOLYBDATE Nd2Zr3(MoO4)9

P\bar 1

A correlation between the structure and some physical properties of binary molybdates (Tungstates) of uni- and bivalent metals

, R=0.030). In this compound, Mn has a higher coordination number (CN=5+1) than that of Zn inα-K4Zn(WO4)3 (CN=4+1). The main structural feature is pairs of MnO6 octahedra linked by the bridging MoO4 tetrahedra into ribbons stretching along the a axis. The structure is compared with related structures of binary molybdates and other members of the alluaudite family.

A morphotropic series of the cluster complexes [Ln(DMF)8][Re6Q7Br7] (Q = S, Se): Synthesis, structure, and thermal transformations

Cs2Pb(MoO4)2 crystals were prepared by crystallization from their own melt, and the crystal structure has been studied in detail. At 296 K, the molybdate crystallizes in the low-temperature α-form and has a monoclinic palmierite-related superstructure (space group C2/m, a = 2.13755(13) nm, b = 1.23123(8) nm, c = 1.68024(10) nm, β = 115.037(2)°, Z = 16) possessing the largest unit cell volume, 4.0066(4) nm3, among lead-containing palmierites. The compound undergoes a distortive phase transition at 635 K and incongruently melts at 943 K. The electronic structure of α-Cs2Pb(MoO4)2 was explored by using X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy methods. For α-Cs2Pb(MoO4)2, the photoelectron core-level and valence-band spectra and the XES band representing the energy distribution of Mo 4d and O 2p states were recorded. Our results allow one to conclude that the Mo 4d and O 2p states contribute mainly to the central part and at the top of the valence band, respectively, with also significant contributions throughout the whole valence-band region of the molybdate under consideration.

Synthesis and crystal structure of a new octahedral rhenium cluster compound Cs3Re6Se7Br7.H2O

Single crystals of binary molybdate Nd2ZrMoO4)9 were grown. The crystal structure of this compound was investigated by X-ray diffraction analysis (CAD-4 diffractometer, MoKα radiation, 2844 reflections. R = 0.0230), and a new type of structure was found. The crystals are trigonal with cell dimensions a = 9.804(1), c = 58.467(12) å, V= 4867(1) å3, Z = 6, dcalc = 4.098 g/cm3, space group R3−c. The structure involves polyhedra of three types: MoO4 tetrahedra, ZrO6 octahedra, and NdO9 tricapped trigonal prisms linked by their common vertices into an original three-dimensional framework.