ALPHA-LA4TI9SI4O30 AND BETA-LA4TI9SI4O30 - SYNTHESIS AND STRUCTURE OF THE 2ND MEMBER (M=2) OF NOVEL LAYERED OXOSILICATES CONTAINING (110) RUTILE SHEETS - ELECTRICAL PROPERTY AND BAND-STRUCTURE CHARACTERIZATION OF THE MIXED-VALENCE TITANIUM(III/IV) OXOSILICATE SERIES, LA4TI(SI2O7)(2)(TIO2)(4M) (M=1,2)

Single crystals of two mixed-valence lanthanum titanium(III/IV) oxosilicates, alpha- and beta-La4Ti9Si4O30, were grown by a high-temperature, solid-state reaction employing BaCl2 molten salt. Their structures were determined by single crystal X-ray diffraction methods. Both phases crystallize in a monoclinic lattice, C2/m (No. 12), with a = 13.545(2) Angstrom, b = 5.571(1) Angstrom, c = 15.1888(9) Angstrom, beta = 110.922(7)degrees, and V = 1105.1(3) Angstrom(3) for alpha-La4Ti9Si4O30 and a = 13.536(2) Angstrom, b = 5.750(1) Angstrom, c = 14.252(1) Angstrom, beta = 95.387(8)degrees, and V = 1104.4(3) Angstrom(3) for beta-La4Ti9Si4O30 with Z = 2. These phases are polymorphs which represent the second member (m = 2) of the layered oxosilicate series, La4Ti(Si2O7)(2)(TiO2)(4m). The two-dimensional framework can be viewed as a rutile lattice, TiO2, that is sliced by closed-shell, nonmagnetic silicate slabs, La4Ti(Si2O7)(2), along the (110) plane at various octahedra thicknesses, m. The physical and electronic properties of this layered compound series were characterized by measuring their electrical resistivities and calculating their electronic band structures with the extended Huckel tight-binding (EHTB) method. The bulk resistivity showed semiconducting behavior with small gap energies. When the temperature is lowered, the m = 1 and 2 phases exhibit a sharp resistivity increase below similar to 20 and similar to 80 K, respectively. The EHTB calculations reveal that the d electrons of the La4Ti(Si2O7)(2)(TiO2)(4m) (m = 1, 2) phases reside not in the silicate slabs but in the (110) rutile layers, and the bottom portions of their d-block bands are partially filled. The observed sharp resistivity transition requires a localization of the electrons in the partially filled rutile band. The nonmetallic state of these quasi-two-dimensional compounds is probably caused by a bipolaron formation not be electron-electron repulsion or random potential. In this paper, the results from the EHTB calculations and the bond valence sum analysis are contrasted with regard to charge distribution.