Although the theory of lattice dynamics was established six decades ago, its accurate implementation for polar solids using the direct (or supercell, small displacement, frozen phonon) approach within the framework of density-function-theory-based first-principles calculations had been a challenge until recently. It arises from the fact that the vibration-induced polarization breaks the lattice periodicity, whereas periodic boundary conditions are required by typical first-principles calculations, leading to an artificial macroscopic electric field. The article reviews a mixed-space approach to treating the interactions between lattice vibration and polarization, its applications to accurately predicting the phonon and associated thermal properties, and its implementations in a number of existing phonon codes.
A systematic study of stacking fault energy (γ(SF)) resulting from induced alias shear deformation has been performed by means of first-principles calculations for dilute Ni-base superalloys (Ni(23)X and Ni(71)X) for various alloying elements (X) as a function of temperature. Twenty-six alloying elements are considered, i.e., Al, Co, Cr, Cu, Fe, Hf, Ir, Mn, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Sc, Si, Ta, Tc, Ti, V, W, Y, Zn, and Zr. The temperature dependence of γ(SF) is computed using the proposed quasistatic approach based on a predicted γ(SF)-volume-temperature relationship. Besides γ(SF), equilibrium volume and the normalized stacking fault energy (Γ(SF) = γ(SF)/Gb, with G the shear modulus and b the Burgers vector) are also studied as a function of temperature for the 26 alloying elements. The following conclusions are obtained: all alloying elements X studied herein decrease the γ(SF) of fcc Ni, approximately the further the alloying element X is from Ni on the periodic table, the larger the decrease of γ(SF) for the dilute Ni-X alloy, and roughly the γ(SF) of Ni-X decreases with increasing equilibrium volume. In addition, the values of γ(SF) for all Ni-X systems decrease with increasing temperature (except for Ni-Cr at higher Cr content), and the largest decrease is observed for pure Ni. Similar to the case of the shear modulus, the variation of γ(SF) for Ni-X systems due to various alloying elements is traceable from the distribution of (magnetization) charge density: the spherical distribution of charge density around a Ni atom, especially a smaller sphere, results in a lower value of γ(SF) due to the facility of redistribution of charges. Computed stacking fault energies and the related properties are in favorable accord with available experimental and theoretical data.
Icosahedral short-range order, of which Al atoms are caged in the center of icosahedra with Cu and Zr atoms being the vertices, has been evidenced in the Cu46Zr46Al8 glassy structure by ab initio molecular dynamics simulation. These Al-centered clusters distribute irregularly in the three-dimensional space and form a “backbone” structure of the Cu46Zr46Al8 glass alloy. It is suggested that this kind of local structural feature is attributed to the requirement of efficient dense packing and the chemical affinity between Zr–Zr, Zr–Al, and Cu–Zr atoms. Our calculated results are found to be in good agreement with the experimental data.
Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4. 5 × 10 −6 , 6.9 × 10 −6 , 1.3 × 10 −5 , and 3.8 × 10 −5 S/cm at 30 • C, 50 • C, 70 • C, and 90 • C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30-90 • C. The structure of amorphous LLTO was predicted by the ab-initio molecular dynamics (AIMD) simulations and analyzed by partial pair distribution functions, coordination numbers and Voronoi tessellation. It is observed that the local environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacement curves and the conductivities were evaluated from the Nernst-Einstein's relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volumes.
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