Phases of Ca from first principles

Structure and stability under pressure of cubic and hexagonal diamond crystals of C, BN and Si from first principles

Application has been made of first-principles total-energy band-structure theory to find the equilibrium states under constant pressure of the super-hard cubic diamond (cd) and hexagonal diamond (hd) structures of carbon (C), boron nitride (BN) and medium-hard silicon (Si). The absolute stability of the equilibrium state is found by determinations of the breakdown under pressure of several deformations of lattice parameters around the equilibrium state. The calculations show that the hd structures are much stronger than the cd structures. Thus the γ angle of the hd structure of both C and BN is stable for pressures greater than 20 Mbar while the γ angle of the cd structures breaks down at 13 and 11 Mbar respectively. Also the bulk moduli B of the hd structure of C and BN are substantially greater than the B values of the cd structure above 2 Mbar; the B values of hd structures of C and BN are 20% greater than cd structures at p = 20 Mbar. However the cd structures have greater stability relative to the hd structures as shown by a lower Gibbs free energy at pressures up to 20 Mbar. Comparison is made with the pressure dependences of the medium-hard crystals of Si in the same structures, which show notably different behavior.

High-pressure phases of calcium: density-functional theory and diffusion quantum Monte Carlo approach

The phase diagram of Ca is examined using a combination of density-functional theory (DFT) and diffusion quantum Monte Carlo (DMC) calculations. Gibbs free energies of several competing structures are computed at pressures near 50 GPa. Existing disagreements for the stability of Ca both at low and room temperature are resolved with input from DMC. Furthermore, DMC calculations are performed on 0 K crystalline structures up to 150 GPa and it is demonstrated that the widely used generalized gradient approximation of DFT is insufficient to accurately account for the relative stability of the high-pressure phases of Ca. The results indicate that the theoretical phase diagram of Ca needs a revision.

The role of electron correlations in the binding properties of Ca, Sr, and Ba

In order to apply wavefunction-based correlation methods to solids it is necessary to have reliable Hartree-Fock (HF) results for the infinite system of interest. We performed Hartree-Fock calculations for the group 2 heavy alkali-earth metals Ca, Sr, and Ba. For that, basis sets of valence-double-ζ quality have been optimized for the periodic systems. In all cases small-core pseudopotentials were used to deal with the scalar-relativistic effects. We determine the cohesive energies, the equilibrium volumes and the bulk moduli of the systems at the Hartree-Fock level and compare them with experimental data as well as the results of density functional theory calculations. Relativistic effects in the case of Ba are estimated by using a non-relativistic pseudopotential. The comparative HF versus the density functional theory (DFT) study of the electronic structures of Ca, Sr, and Ba has been performed.