Properties-Driven Synthesis, Structural Insights, and Discovery of Complex Intermetallics and Alloys
Understanding the intimate relationship between the structure and physical properties of a material is one of the central goals of solid state chemistry. As subtle structural changes can have wide ranging effects on electrical and magnetic properties, it is necessary to fully, and correctly, characterize and elucidate the structure. This dissertation presents the results from four studies aimed at synthesizing, discovering, and structurally characterizing new complex intermetallic phases and alloys. The first study describes an investigation of structural phase transitions in the new series of magnetocaloric alloys (MnNiSi)1-x(FeCoGe)x (x = 0.36, 0.37, 0.38, 0.39, 0.40). These compounds exhibit coupled magneto-structural transitions, where ferromagnetic ordering coincides with a structural phase transition. Our results indicate the high temperature hexagonal phase (space group P63/mmc) transforms to a low temperature orthorhombic phase (space group Pnma) under ambient pressure; however, low pressure studies (< 1 x 10-2 torr) reveal anomalous behavior wherein the high temperature phase persists below the expected phase transition temperature. The second describes a structural study on compounds adopting the Yb3Rh4Sn13-structure type. Single crystal X-ray diffraction experiments revealed the compounds to be highly disordered, with weak, unindexed reflections present. A new structural model was developed for Lu3Ir4Ge13 to account for the disorder. Furthermore, a correlation was found between the electrical resistivity and crystallographic disorder, which was developed into a general rule for predicting electrical properties for this class of material. The third study describes the synthesis and characterization of Er1.33Pt3Ga8. This compound was synthesized using the self-flux method. The structure was initially modeled using space group R3 ̅m with lattice parameters a = 4.3163(2) Å and c = 38.547(17) Å. However, single crystal X-ray diffraction revealed weak, unindexed, supercell reflections. Crystal structure refinements using the modulated structure approach were used to account for these reflections. The synthesis, average structure, and modulated structure are presented. The final study describes the design, synthesis, and characterization of the new series of intergrowth compounds Pr8Fe8.22Sb6Te12, Pr4Mn1.81Sb2Te6, and Ln4Mn2-xSb4+xTe6 (Ln = Ce, Pr, Sm; x = 0.18, 0.36, 0.60, respectively). These compounds are composed of LnTe3 type layers separated by transition metal-antimony containing layers along the c¬-axis. These compounds represent a new class of complex layered materials, and provide insight into strategies aimed at discovering other low dimensional compounds. Additionally, the synthesis and characterization of La(Sb1-xBix)Te (x ~ 0.5) is reported.