| Ab initio pseudopotential total energy and band-structure calculations with a plane wave basis set are used to investigate the structural energetics and stability of three different systems: the binary Al-rich compounds in the Al-Co system, the ternary NiSnM (M = Ti, Zr, Hf) compounds, and the TX compounds (T = Ti, Zr, V, Nb; and X = C, N, O) in the rocksalt and related structures. Technical issues arising in the application of the plane wave pseudopotential method to these compounds, specifically the transferability and plane wave convergence of the pseudopotentials, the broadening schemes for finite k-point sampling, and the implementation of the conjugate gradients algorithm for metallic systems, are discussed. A new variational formulation of density functional perturbation theory for computing linear response functions of metallic systems is presented. For the Al-Co system, first principles results for a variety of real and hypothetical compounds are discussed in connection with recent pair potential calculations and trends in the electronic density of states. For the NiSnM compounds, the relation between the formation of a gap at the Fermi level and structural stability is investigated. Using first principles information in combination with a tight-binding analysis and substructure decomposition, it is concluded that the formation of a gap in the electronic spectrum is not directly correlated with structural stability in this intermetallic system. For the TX compounds, trends in the calculated rocksalt lattice constants, band structures, and zone center optical phonon frequencies are used to investigate the special case of NbN, which has the highest superconducting transition temperature in this class of compounds. The polymorphism and metastability of this compound are examined with particular emphasis on the structure of its recently discovered new superconducting phase. Total energy calculations strongly suggest that this new phase cannot have the experimentally reported NbO structure type. Various approaches to the identification of an alternative structure are discussed. |
