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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.
Journal of Applied Physics


Ba3(Nb1-xTax)Si4O26 (x=0.1, 0.3, 0.5, 0.7, 0.9), X-ray powder reference diffraction patterns, photocatalysts
Created December 5, 2019, Updated May 2, 2020