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We present a first-principles investigation of the structures and the dynamical stability of the austenite and martensite phases of binary Ti 3 Nb alloys, used as a model system for the superelastic and superplastic gum-metal alloy. For the body-centered cubic high-temperature β phase, structural models are constructed by optimizing the chemical decoration of a large supercell and by a cluster expansion method. The energetically most favorable structure is found to be elastically stable but dynamically unstable in the harmonic approximation. At finite temperature anharmonic phonon-phonon interactions treated in a self-consistent phonon approximation stabilize the structure already at room temperature. For the low-temperature α , ω, and α phases stable structure models have been constructed. For the hexagonal α phase a model is generated by optimizing the chemical decoration of a supercell based on the hexagonal B h lattice. The hexagonal ω structure may be derived from the body-centered cubic β phase using the (111) plane collapse model. The structure of the orthorhombic α phase may be viewed as produced by a strain-induced transformation of the body-centered cubic β phase, albeit with a different chemical decoration. The relaxed structures of the α , ω, and α phases were found to be both elastically and dynamically stable in the low-temperature limit. The martensitic temperatures for the β → α , β → ω, and β → α transitions were estimated by comparing the Helmholtz free energies as a function of temperature.

In the present work we report on the synthesis, crystal structure, and physical properties (resistivity, magnetization, heat capacity) in combination with density functional theory (DFT) calculations of the electronic structure and phonon properties for the intermetallic compound LaPtSi. LaPtSi crystallizes in its own noncentrosymmetric structure type (space group I 4 1 md; a = 0.42502(1) nm and c = 1.4525(5) nm), which is an ordered ternary derivative of the centrosymmetric α-ThSi 2 -structure. The weakly correlated compound LaPtSi (Sommerfeld value γ = 6.5 mJ/molK 2 ) exhibits superconductivity below T c = 3.35 K and appears to be a fully gapped, weakly coupled s-wave BCS superconductor. The experimental observations are supported by DFT calculations which show that, despite a substantial spin-orbit splitting of the Fermi surfaces, a spin-singlet pairing is prevalent.

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Based on density functional theory the noncentrosymmetric superconductor Mo3Al2C in its well established β-Mn type (P4132) crystal structure is investigated. In particular, its thermodynamical and dynamical stabilities are studied by calculating lattice vibrations within the harmonic approximation. It is found that the fully stoichiometric compound is dynamically unstable. However, compounds with carbon vacancies, i.e., Mo3Al2C1−x, can be dynamically stabilized for vacancy concentrations x > 0.09. By means of a simple thermodynamical model we estimate x ∼ 0.13 − 0.14 for Mo3Al2C1−x at the experimental preparation temperatures. The influence of the carbon vacancy concentration on the electronic structure is investigated.