Chemistry and Biochemistry
Bayer School of Natural and Environmental Sciences
Jeffry D. Madura
Antiferromagnetism, Diamond-like semiconductor, Laser damage threshold, Lithium ion conductivity, Nonlinear optic, Second harmonic generation
In this work, several new lithium sulfides were synthesized and investigated for applications in nonlinear optics, solid-state batteries, and magnetoelectronics. Chapter 1 provides an overview of diamond-like semiconductors (DLSs). The multinary semiconductors were synthesized via high temperature solid-state and polychalcogenide flux methods. Single crystal X-ray diffraction was used to solve and refine the structures. All products were characterized using synchrotron powder diffraction with Rietveld refinements. Chapter 2 is focused on magnetic properties of I2-II-IV-VI4 DLSs. Divalent ions are directed to specific locations within the wurtz-kesterite structure to generate antiferromagnetic ordering. The indirect-gap (Eg = 1.4 eV) semiconductor Li2FeGeS4 has a magnetic structure that is commensurate with the nuclear structure according to neutron powder diffraction, while Li2FeSnS4 has a direct bandgap (Eg = 1.9 eV) and an incommensurate magnetic structure. Both compounds undergo a conventional metamagnetic transition, with similar magnetic phase diagrams. Chapter 3 outlines the impacts that the class of chalcogenide-based materials have made on Li+ ion conductivity. Li2 II GeS4 DLSs have wurtz-kesterite, wurtz-stannite, and lithium cobalt(II) silicate structures that exhibit variations in cation ordering patterns, which yield different Li+ diffusion pathways. Ion conductivities of ~10 7-10-5 S/cm at 100 °C have been observed in Li2MnGeS4, Li2CoGeS4, Li2FeGeS4, and Li2CdGeS4. Chapters 5 and 6 demonstrate that quaternary DLSs are the logical successors of the ternary chalcopyrites that transformed nonlinear optical (NLO) applications for the infrared. Second harmonic generation (SHG) has been discovered in DLSs containing transition metals, namely Li2MnGeS4, Li2FeGeS4, Li2FeSnS4 and Li2CoSnS4. The wide-gap Li2MnGeS4 exhibits a very high laser damage threshold (LDT) ( 16 GW/cm2) and a SHG susceptibility of 15 pm/V. Li2CdGeS4 exhibits the highest SHG susceptibility ((2) = 51.1 pm/V) and satisfies several criteria for practical infrared NLO applications, including a wide region of optical transparency (0.5-22 μm), high thermal stability (m.p. ~890 °C), and environmental stability. Most notably, Li2CdGeS4 has a wide bandgap (Eg = 3.15 eV) that imparts a large LDT 4 GW/cm2. Although it is commonly accepted that high LDTs and strong optical nonlinearity are mutually exclusive, Li2CdGeS4 simultaneously exhibits an exceptional laser damage threshold and high SHG that exceeds some benchmark materials.
Brant, J. (2015). Synthesis, Structure Elucidation and Physicochemical Characterization of Li2-Ii-Iv-S4 Diamond-Like Semiconductors: Study of Nonlinear Optical Properties, Magnetism, and Lithium Ion Conductivity (Doctoral dissertation, Duquesne University). Retrieved from https://dsc.duq.edu/etd/346