Crystal Structures and Optical Properties of Group 14 Chalcogenides

Abstract

The crystal structures and physical properties of several new tetrel and chalcogenide compounds have been investigated in this thesis. The crystal structures, syntheses, and electronic structures of the three centrosymmetric compounds Ba7Ge2Se17 (Pnma, a = 12.6518(1) Å, b = 20.0692(2) Å, c =12.3067(9) Å), Ba6Ge2Se12 (P21/c, a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and β = 90.303(1) °), and Ba8Sn4Se17 (C2/c, a = 47.2887(2) Å, b = 12.6224(5) Å, c = 25.7173(1) Å, and β = 104.729(1) ⁰) have been reported. The experimentally determined band gaps of these semiconductors are 1.51 eV, 1.75 eV, and 1.92 eV. The non-centrosymmetric compounds (NCS) γ-Sr2GeSe4 and α-Pb2GeSe4 have already been reported, but the new substitution variants Sr1.31Pb0.69GeSe4 (Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr0.21Pb1.79GeSe4 (I4̅3d, a = 14.6177(3) Å) have been introduced here for the first time. The experimental band gaps of γ-Sr2GeSe4, Sr1.3Pb0.7GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 are 2.00 eV, 1.65 eV, 1.45 eV and 1.42 eV. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr2GeSe4, and Sr1.3Pb0.7GeSe4 (approximated as the supercell “Sr1.5Pb0.5GeSe4”), were studied both theoretically using first-principle calculations and experimentally. The theoretical calculation found the effective NLO susceptability, deff, of γ-Sr2GeSe4 and “Sr1.5Pb0.5GeSe4” to be 10.8 pm V-1 and 8.8 pm V-1 respectively at the static limit. The experimental deff’s of γ-Sr2GeSe4, Sr1.3Pb0.7GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 are 2.6 pm V-1, 2.3 pm V-1, 0.68 pm V-1, and 0.79 pm V-1. The final non-centrosymmetric compound discussed in this thesis is BaCuSiTe3 (Pc, a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, and β = 122.022(1) °,) which contains [Si2Te6]6- polyhedra. The theoretical deff of BaCuSiTe3 was calculated to be 3.4 pm V-1 at the static limit and experimentally found to be 0.52 pm V-1.