Investigation of Crystal Structures and Ultra-Low Thermal Conductivities in Novel Group 14 and 15 Chalcogenides

Abstract

The 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.