| Using first-principles electronic-structure calculations (i.e., the linearized-augmented-planewave method), we have studied the materials properties and stability of a number of transition metal-metalloid compounds. The classes of systems examined include: (1) B1-structure (NaCl-type) compounds of early transition-metal carbides and nitrides; (2) several platinum and iridium silicides; and (3) some a spinel-structure cuprous sulfide, CuIr2S4.;1. The nitrides and carbides we studied have remarkable physical and chemical properties, including extraordinary hardness and strength, high melting temperatures, superconductivity, and superior chemical stability. Attempts at systematic improvements of the remarkable properties of these materials are often frustrated by structural instabilities driven by elastic or phonon instabilities, as well as intrinsic non-stoichiometry and the formation of vacancies and defect structures. The instability of MoN at high pressures is demonstrated, in contrast to conjectures in the literature concerning its metastability. We also find a dynamic instability (zone boundary phonon) in MoC despite its elastic stability.;2. Platinum and iridium silicide compounds have been used in infrared detectors for a number of years. The optical properties of these compounds are explored, and suggestions for increasing their effectiveness are made based on theoretical predictions of anisotropic absorption and structurally dependent absorption in the infrared region. The Fermi surfaces and their relationship to the observed optical properties are discussed. An extensive study of the equilibrium properties of these Pt/Ir-Si compounds is also presented.;3. Spinel-structure chalcogenide compounds have a wide array of interesting and unusual properties. We have studied the as-yet-not-understood metal-insulator transition (MIT) in CuIr2S 4. First-principles calculations have failed to reproduce the insulating state. Possible explanations are explored including relativistic effects (spin-orbit interactions), relaxations of internal coordinates, and LDA errors. |
