Structural and Optical Properties of Ba3(Nb6-xTax)Si4O26 (x=0.6, 1.8, 3.0, 4.2, 5.4)
Winnie K. Wong-Ng, Guangyao Liu, Weifang Liu, Yuqi Yang, Shouyu Wang, Yucheng Lan, James A. Kaduk
Structure and optical properties have been successfully determined for a series of titanium- and cobalt-containing layered alkaline-earth silicate compounds, Ba3(Nb1-xTax)Si4O26 (x=0.6, 1.8, 3.0, 4.2, 5.4). The structure of this solid solution was found to be hexagonal P-62m (No. 189), with Z=1. With x increases from 0.6 to 5.4, the lattice parameter a increases from 8.98804(8) Å to 9.00565(9) Å, and c decreases from 7.83721(10) Å to 7.75212(12) Å. As a result, the volume decreases from 548.304(11) Å3 to 544.479(14) Å3, respectively. The (Nb/Ta)O6 distorted octahedra formcontinuous chains along the c-axis. These (Nb/Ta)O6 chains are in turn linked with the Si2O7 groups to form distorted pentagonal channels in which Ba ions were found. These Ba2+ ions have full occupancy and a 13-fold coordination environment with neighboring oxygen sites. Another salient feature of the structure is the linear Si-O-Si chains. When x in Ba3(Nb1-xTax)Si4O26 increases, the bond valence sum (VBS) values of the Ba sites increase slightly (2.09 to 2.20), indicating the size of the cage becoming progressively smaller (over-bonding). While SiO4 cages are also slightly smaller than ideal (range form 4.16 to 4.19), the (Nb/Ta)O6 octahedral cages are slightly larger than ideal (range from 4.87 to 4.90), giving rise to an under-bonding situation. The band gaps of the solid solution members were measured to be between 3.39 and 3.59 eV, and the x=3.0 member was modeled by Density functional theory (DFT) technique to be 3.07 eV. The band gaps of these materials indicate they are potential candidates for UV photocatalyst.