Menezes, Luke2023-12-042023-12-042023-12-042023-11-22http://hdl.handle.net/10012/20128The crystal structures and physical properties of several new group 14 and 15 chalcogenides are discussed in this thesis. The thesis discusses the chalcoantimonates TlLa2Sb3Se9 and La12+zSb9-ySe38-z (simplified as La12Sb9Se38). TlLa2Sb3Se9 crystallizes in an ordered variant of the KLa2Sb3S9 structure type (space group = P212121). The thermoelectric properties of TlLa2Sb3Se9 were enhanced through p-type doping by replacing La3+ with Ca2+. The largest thermoelectric figure-of-merit was 0.078 at 623 K in the TlLa0.95Ca0.05Sb3Se9 sample. The La12Sb9Se38 (Pm3 ̅) structure type features La3+/Sb3+ disorder and S2-/S22- disorder, making it possible to produce nonstoichiometric compounds within a narrow phase width. The low thermal conductivities of samples with the nominal compositions La12.17Sb8.5S38 and La12.17Sb8.5S37.75 were around 1 W m-1 K-1. The latter half of this document focusses on Si, Ge, and Sn selenides. Ba6Ge2Se12 (P21/c) and Ba7Ge2Se17 (Pnma) adopt new structure types—both possess positional disorder confirmed via a single crystal, Rietveld, and pair distribution function models. The Ba6Ge2Se12 structure contains disordered Se22- dumbbells which may align for quasi-infinite 1D chains, whereas the Ba7Ge2Se17 structure contains disordered [GeSe5]4- anions. The thermal conductivities of Ba6Ge2Se12 and Ba7Ge2Se17 range from 0.3 – 0.4 W m-1 K-1. Substituting Si into the Ge compound Ba6Ge2Se12 compounded produced the new compound Ba6Si2Se12 (P1 ̅). Up to 75% of the Si atoms in the Ba6Si2Se12 structure may be replaced with Ge while preserving the triclinic structure. The Si4+/Ge4+ disorder and the positional disorder in the Se22- dumbbells were studied using powder X-ray diffraction patterns collected using synchrotron radiation. The ultra-low thermal conductivity of Ba6Si2Se12 ranges from 0.3 to 0.5 W m-1 K-1. The final chapters discuss Sr compounds as well as Ba compounds. Sr8Ge4Se17 (P1 ̅) and Ba8Sn4Se17 (C2/c) share stoichiometries but adopt different structure types. The Ba8Sn4Se17 unit cell may be regarded as a 2 × 1 × 4 supercell of the Sr8Ge4Se17 unit cell. The structures of these two compounds were finalized using Rietveld refinements on powder X-ray diffraction data collected using synchrotron radiation, as no disorder was observed in these structures. Despite not having structural disorder, the ultra-low thermal conductivity of Ba8Sn4Se17 was found to be as low as 0.3 W m-1K 1 due to its complex structure. The final compound discussed is the noncentrosymmetric compound Sr6Ge3OSe11 (P3m1). This chapter explores partial isovalent substitution to design noncentrosymmetric structures by promoting the alignment of [GeOSe3]4- anions. The second-order nonlinear susceptibilities (dil) of Sr6Ge3OSe11 were calculated to be d15 = -12.9 pm V-1, d22 = -15.4 pm V-1, d33 = 15.0 pm V-1 and deff = 17.0 pm V-1. Size-dependent second harmonic generation intensity experiments revealed that Sr6Ge3OSe11 is phase matchable at 1064 nm with an intensity equal to 0.62 × KH2PO4.enchalcogenidesthermal conductivitycrystal structuresnonlinear opticsDFTsolid state chemistryDoctoral Thesis