Atomic compressibility and reversible insertion of atoms intosolids

Abstract

We developed a theoretical model to investigate the compressibility ofatoms. Atoms are confined inside a spherical cavity, simulatednumerically by a finite repulsive potential barrier. The energy levels andwavefunctions of confined atoms are determined by solving, for differentcavity radii, the relativistic Dirac-Fock equations, including formally therepulsive barrier. The changes in the atomic size and in the ground-stateenergy level allow one to define a positive isotropic pressure exerted onthe confined atom. The model is applied to atomic caesium and it isdemonstrated quantitatively that the remarkable compressibility ofcaesium originates from a purely atomic mechanism, namely thepressure-induced collapse of the 5d orbital. We propose that thismechanism can also drive, at an atomic level, a reversible insertion ofatoms into solids. Applications to lithium-ion batteries are brieflydiscussed at the end of this paper.