Synthesis, Characterization and Properties of [(SnSe)1+δ]m(MoSe2)n and New Rare Earth (LaSe1-x)1.17(VSe2-y)n (n = 2-4) and [(EuSe)1+δ]1(VSe2)n (n = 1-3) Ferecrystal Systems
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Solid state synthesis of layered, rotationally disordered intergrowths consisting of rock salt (MX) and hexagonal (TX2) constituents in various sequences [(MX)1+δ]m[TX2]n is carried out by developing structural and compositional prototypes of the desired product, using fine control of the elemental reactants and then annealing at low temperature to facilitate self-assembly. (M = Sn, La, Eu; T = V, Mo.) The remarkable rotational disorder in these systems - in contrast to traditional misfits - and their proven applications in thermal, electrical and thermoelectric disciplines make them a useful group of materials for demonstrating control of reaction pathways of solid state reactions using low temperatures and short times. The synthesized materials are structurally characterized using X-ray diffraction (XRD), X-ray reflectivity (XRR), and Scanning Transmission Electron Microscopy (STEM). Electrical characterization is carried out on patterned samples using the Van der Pauw method of resistivity and the Hall effect method. Composition of the samples is determined using wavelength dispersive electron probe microanalysis (EPMA). Time domain thermoreflectance is used to determine the cross plane thermal conductivity. The family of [(SnSe)1.05]m(MoSe2)n (m = n = 1, 2, 3, 4), which possess the same composition but different unit cell thicknesses, shows that there is no correlation between c-axis unit cell thickness and cross plane thermal conductivity. The family of structural isomers [(SnSe)1.05]4[MoSe2]4, [(SnSe)1.05]3[MoSe2]3[(SnSe)1.05]1[MoSe2]1, [(SnSe)1.05]3[MoSe2]2[(SnSe)1.05]1[MoSe2]2, [(SnSe)1.05]2[MoSe2]3[(SnSe)1.05]2[MoSe2]1,[(SnSe)1.05]2[MoSe2]1[(SnSe)1.05]1[MoSe2]2[(SnSe)1.05]1[MoSe2]1 and [(SnSe)1.05]2[MoSe2]2[(SnSe)1.05]1[MoSe2]1[(SnSe)1.05]1[MoSe2]1 have the same c-axis lattice thickness and absolute composition but have different numbers of [(SnSe)1.05]/[MoSe2] interfaces. Thermal conductivity studies carried out on these showed no correlation with the interface density. (LaSe1-x)1.17(VSe2-y)n (n = 2, 3, 4) feature a family of compounds that self-assemble at higher than usual temperatures. They form non-stoichiometric moieties with unique structural proclivities including La vacancies and V interstitials compared to other ferecrystals or previous misfits. The designable electrical properties show evidence of charge transfer. (EuSe)1+δ(VSe2)n (n = 1, 2, 3) is a family of materials that complements the investigation of Ln-based ferecrystals. They show evidence of multiple M oxidation states. These compounds highlight the use of rational design of structure and composition to tune properties. This dissertation includes previously published and unpublished co-authored material.