Stability of simple cubic calcium at high pressure: A first-principles study

Abstract

The origin of the temperature dependence of the stability of the simple cubic ͑sc͒ structure of Ca in the pressure range 32-109 GPa is investigated by the use of plane-wave density-functional calculations and first-principles molecular-dynamics ͑MD͒ simulations based on localized basis set method employed in the SIESTA code. Constant-pressure MD simulations are performed on the competing sc and I4 1 / amd structures in this pressure range. The results are analyzed to recover details of the structures and dynamics at 300 K of sc and the recently predicted 0 K lowest-enthalpy I4 1 / amd structure of Ca. The structure at 300 K appears to be an almost pure sc structure and not an average of sc and the lowest enthalpy I4 1 / amd structure. The stability of the I4 1 / amd structure at0Kissuggested to partially result from differences in Coulombic core interactions. The enthalpy difference between I4 1 / amd and sc is much less at 300 K than at 0 K also indicates that the sc structure is becoming more stable with increasing temperature.

Existence of the β-tin structure in Sr: First evidence from computational approach

“…In brief, sX-LDA functional treats d electron differently compared to other functionals, 22,36,[42][43][44][45] PBE and LDA, see Fig. 4.…”

Atomistic Study of Lithium Ion Dynamics in Li12Si7

“…The formation energy of Li vacancy in the Li 12 Si 7 alloy E f (Vac Li ) was calculated using the equation (1): Ab initio molecular dynamics (MD) simulations were performed within the framework of DFT as implemented Spanish Initiative for Electronic with Thousands of Atoms (SIESTA) code [49][50][51][52] . All calculations were performed using the 152 atoms supercell with one Li vacancy.…”

Calcium with the β-tin structure at high pressure and low temperature

“…On the other hand, high-pressure room temperature experiments confirmed that Ca-III adopts the nearly sc structure (8). To overcome this discrepancy, Teweldeberhan et al (16) used diffusion quantum Monte Carlo calculations, Errea et al (18) used ab initio methods including anharmonic effects to stabilize the sc phase at room temperature, and Yao et al (17) provided a detailed analysis of ab initio molecular dynamics simulations that could account for the stability of the sc structure at room temperature. The remaining major discrepancy is that Yao et al (14) and Oganov et al (13) predicted that Ca with I4 1 ∕amd symmetry (β-tin structure) has a much lower zero-temperature enthalpy than sc, but in spite of many experimental efforts, the presumably more stable β-tin structured Ca has never been discovered.…”

“…The pressure-induced electronic transition from a normal metal to a superconductor occurs in the Ca-III phase region above 44 GPa (10,11), and the T c increases with pressure from 2 K to 10 K within the Ca-III region (6). Ca-III is also a very active test case of modern theories and experiments, where a number of paradoxes arise (12)(13)(14)(15)(16)(17)(18). Based on density-functional theory (DFT) calculations, Ca-III with sc structure has imaginary phonon frequencies, indicating dynamic instability (19)(20)(21).…”