First-principles study of the structural properties of alkali metals

Total Energy Calculations for Structural Phase Transformations

The structural integrity and physical properties of crys talline solids are frequently limited or enhanced by the occurrence of phase transformations. Martensitic trans formations involve the collective displacement of atoms from one ordered state to another. Modern methods to determine the microscopic electronic changes as the atoms move are now accurate enough to evaluate the very small energy differences involved. Extensive first principles calculations for the prototypical Martensitic transformation from body-centered cubic (bcc) to close- packed 9R structure in sodium metal are described. The minimum energy coordinate or configuration path be tween the bcc and 9R structures is determined as well as paths to other competing close-packed structures. The energy barriers and important anharmonic interactions are identified and general conclusions drawn. The calcu lational methods used to solve the Schrödinger equa tion include pseudopotentials, fast Fourier transforms, efficient matrix diagonalization, and supercells with many atoms.

Comparison of density functional approximations and the finite-temperature Hartree-Fock approximation in warm dense lithium

We compare the behavior of the finite-temperature Hartree-Fock model with that of thermal density functional theory using both ground-state and temperature-dependent approximate exchange functionals. The test system is bcc Li in the temperature-density regime of warm dense matter (WDM). In this exchange-only case, there are significant qualitative differences in results from the three approaches. Those differences may be important for Born-Oppenheimer molecular dynamics studies of WDM with ground-state approximate density functionals and thermal occupancies. Such calculations require reliable regularized potentials over a demanding range of temperatures and densities. By comparison of pseudopotential and all-electron results at T=0 K for small Li clusters of local bcc symmetry and bond lengths equivalent to high density bulk Li, we determine the density ranges for which standard projector augmented wave (PAW) and norm-conserving pseudopotentials are reliable. Then, we construct and use all-electron PAW data sets with a small cutoff radius that are valid for lithium densities up to at least 80 g/cm{3}.

An embedded-atom-method model for alkali-metal vibrations

We present an embedded-atom-method (EAM) model that accurately describes the vibrational dynamics in the alkali metals Li, Na, K, Rb and Cs. The bulk dispersion curves, frequency-moment Debye temperatures and temperature-dependent entropy Debye temperatures are all in excellent agreement with experimental results. The model is also well suited for studying surface vibrational dynamics in these materials, as illustrated by calculations for the Na(110) surface.

Ab initio thermodynamics of lithium oxides: from bulk phases to nanoparticles

Lithium ion batteries are nowadays key devices for energy storage, and a great research effort is under way to develop and apply new materials. Recently, new approaches have been proposed that rely on the reversible formation of either Li2O or Li2O2 at the electrodes. The details of their formation and dissolution are, however, still unclear. As a first step towards the understanding of these processes, bulk lithium oxides, their surfaces and their nanoparticles have been here investigated by density functional theory and ab initio thermodynamics. At a pressure of 1 atmosphere of oxygen, Li2O2 is the stable bulk phase below 5 K, where a transition to Li2O takes place. Wulff's construction predicts an octahedral shape for Li2O nanoparticles and the form of a hexagonal prism for Li2O2. By taking into account the effect of the surfaces, a size-dependent phase diagram is calculated. At an oxygen pressure of 1 atmosphere and a temperature of 300 K, Li2O is the stable phase for particles with a diameter larger than approximately 2.5 nm. This size-dependent oxidation behavior of lithium should be taken into account in the design of nanostructured oxygen cathodes.

First principles investigation of noncovalent complexation: a [2.2.2]-cryptand ion-binding selectivity study

A first principles methodology, aimed at understanding the roles of molecular conformation and energetics in host-guest binding interactions, is developed and tested on a system that pushes the practical limits of ab initio methods. The binding behavior between the [2.2.2]-cryptand host (4,7,13,16,21,24-hexaoxa-1,10-diaza-bicyclo[8.8.8]hexacosane) and alkali metal cations (Li(+), Na(+), and K(+)) in gas, water, methanol, and acetonitrile is characterized. Hartree-Fock and density functional theory methods are used in concert with crystallographic information to identify gas phase, energy-minimized conformations. Gas phase free energies of binding, with vibrational contributions, are compared to solution-state binding constants through relative binding selectivity analysis. Calculated relative binding free energies qualitatively correlated with solution state experiments only after gas phase metal desolvation is considered. The B3LYP exchange-correlation functional improves theoretical correlations with experimental relative binding free energies. The relevance of gas phase calculations towards understanding binding in condensed phases is discussed. Natural bond orbital methods highlights previously unidentified intramolecular and intermolecular M(+)(222) chemistries, such as an intramolecular n'(O)-->sigma*(CH) hydrogen bond.

On the constitution of sodium at higher densities

Using density functional theory the atomic and electronic structures of sodium are predicted to depart substantially from those expected of simple metals for r(s)<2.48 ( p>130 GPa). Newly predicted phases include those with low structural symmetry and semimetallic electronic properties (including zero-gap semiconducting limiting behavior), and even those that raise the possibility of superconductivity, all at currently achievable pressures. Important differences emerge between sodium and lithium at high densities, and these are attributable to corresponding differences in their respective cores.