Assembly, phase transitions, order and elemental distribution in microporous tin(IV) chalcogenides

The optimized syntheses of 15 members of the family of orthorhombic microporous layered TMA2Sn3SxSe7-x materials are reported, where 0 <= x <= 7 and TMA+ is the organic templating cation tetramethylammonium. From the single crystal XRD structure of the end-member TMA2Sn3Se7, it has been established that the open-framework of these materials comprises four different kinds of building unit. Three primary units based upon distorted trigonal bipyramidal SnSe5 link together through sharing a common axial Se atom and two shared equatorial edges, to produce Sn3Se 7 broken-cube clusters, referred to as the secondary unit. Further connection of these clusters produce a planar honeycomb arrangement of 24-atom ring micropores, referred to as the tertiary unit. Parallel stacking of these anionic Sn 3Se72- layers produces the microporous layered tin(IV) selenide unit cell referred to as the crystallographic quaternary structure. Two crystallographically distinct and charge-balancing TMA + templating cations are located between and within the layers. The chalcogenide distribution within the framework of the family of TMA 2Sn3SxSe7-x materials has been established at three different length scales. Powder X-ray diffraction establishes that the distribution of chalcogenide elements within the microporous sheets is random and obeys Vegard's law for a lattice solid solution. UV-Vis diffuse reflectance spectroscopy has determined the organization of chalcogenides at the level of the broken-cube clusters, while FT-Raman and NMR spectroscopies define the local distribution around the trigonal bipyramidal sites. These latter techniques show site selectivity of the chalcogenide arrangement at the length scale of the primary and secondary units. A series of phase transformations has been unveiled from X-ray diffraction studies of the time evolution of materials that form in the hydrothermal synthesis of microporous tin(IV) selenides. The single crystal XRD structures of three of four identified phases have been solved and compared. It has been determined that at 150°C the material initially crystallizes in the orthorhombic polymorph of the microporous layered TMA2Sn3Se 7. Overtime, this metastable phase is observed to transform into a monoclinic polymorph of TMA2Sn3Se7 which subsequently converts to a novel tetragonal phase. Single crystal XRD shows that the structure of the tetragonal material has a unique three-dimensional open-framework structure formulated as TMA2Sn5Se10O. The basic-building unit of this structure is a new kind of oxygen-centered Sn4Se 10O2- adamantanoid cluster which are linked together through all four of the terminal selenides, through tetrahedral Sn(IV)Se 4 sites, to form a zinc blende type of expanded lattice. A model for the mode of formation and interconversion of these two- and three-dimensional open structures is proposed. The templated syntheses of TMA2Sn3S7 and TBA2Sn4S9 microporous layered tin(IV) sulfides have been carried out under both microgravity (&micro;G) and Earth (1G) conditions in order to elucidate the influence of the gravitational field on the self-assembly and crystal growth processes of this class of materials. It has been determined that the long range ordering of the porous layers and the population of defects, but not the short range ordering within the layers is influenced by gravity. Bulk and surface crystallinity, smoothness of crystal faces, optical quality, crystal habits, registry of the porous layers, and accessible void volume to adsorbates have been found to be improved in the space-grown crystals.