Density functional study of the electronic structure and lattice dynamics of SrCl2.

A theoretical study of the structural, electronic, optical and lattice dynamical properties of SrCl(2) in the cubic fluorite structure has been performed using first-principles calculations. The calculated ground state and elastic properties are in good agreement with the experiments. The calculated band gap is underestimated within the generalized gradient approximation for the exchange and correlation functional. GW calculations have been performed in order to improve the band gap and good agreement with the experiment is obtained. The phonon dispersion relations are discussed in detail in addition to the ground state and elastic properties. Also, the optical properties are computed with DFT corrected by the GW approximation.

Dynamical stabilization of the body centered cubic phase in lanthanum and thorium by phonon-phonon interaction.

A recently developed self-consistent ab initio lattice dynamical method has been applied to the high temperature body centered cubic (bcc) phase of La and Th, which are dynamically unstable at low temperatures. The bcc phase of these metals is found to be stabilized by phonon-phonon interactions. The calculated high temperature phonon frequencies for La are found to be in good agreement with the corresponding experimental data.

Entropy driven stabilization of energetically unstable crystal structures explained from first principles theory.

Conventional methods to calculate the thermodynamics of crystals evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the body-centered cubic (bcc) crystal structure when it appears as a high-temperature phase of many metals. A method for calculating temperature dependent phonon spectra self-consistently from first principles has been developed to address this issue. The method combines concepts from Born's interatomic self-consistent phonon approach with first principles calculations of accurate interatomic forces in a supercell. The method has been tested on the high-temperature bcc phase of Ti, Zr, and Hf, as representative examples, and is found to reproduce the observed high-temperature phonon frequencies with good accuracy.

Anomalous thermal expansion in alpha-titanium.

We provide a complete quantitative explanation for the anisotropic thermal expansion of hcp Ti at low temperature. The observed negative thermal expansion along the c axis is reproduced theoretically by means of a parameter free theory which involves both the electron and phonon contributions to the free energy. The thermal expansion of titanium is calculated and found to be negative along the c axis for temperatures below approximately 170 K, in good agreement with observations. We have identified a saddle point van Hove singularity near the Fermi level as the main reason for the anisotropic thermal expansion in alpha-titanium.