Varying Electronic Configurations in Compressed Atoms: From the Role of the Spatial Extension of Atomic Orbitals to the Change of Electronic Configuration as an Isobaric Transformation.

Roberto Cammi,N W Ashcroft,Roald Hoffmann,Martin Rahm

doi:10.1021/acs.jctc.0c00443

Abstract

A quantum chemical model for the study of the electronic structure of compressed atoms lends itself to a perturbation-theoretic analysis. It is shown, both analytically and numerically, that the increase of the electronic energy with increasing compression depends on the electronic configuration, as a result of the variable spatial extent of the atomic orbitals involved. The different destabilization of the electronic states may lead to an isobaric change of the ground-state electronic configuration, and the same first-order model paves the way to a simple thermodynamical interpretation of this process.

Highlights

A characteristic feature of compression, and a source of inherent interest in it, is that the ground-state electronic configuration of an atom may change as the pressure increases
In their ground states their valence electrons enter preferably 3d orbitals, not 4s. This is not a theoretician’s dream; there is direct experimental information on this, not for atoms, but for the extended elemental solids. This is what we learned in a recent paper[1] where we presented a quantum chemical method for the study of the electronic structure in compressed atoms
We suggested that both the variable destabilization of the electronic configurations and the associated transition pressures can be traced to varying destabilization of the atomic orbitals of the compressed atoms, a consequence of their variable spatial extension

Summary

INTRODUCTION

A characteristic feature of compression, and a source of inherent interest in it, is that the ground-state electronic configuration of an atom (and in atoms in bulk matter, or in compounds) may change as the pressure increases. We complete our XP-PCM first-order theory by determining an analytic form of the pressure experienced by the compressed atom, in a manner analogous to the analytical forms that we have derived above for the orbital energies and for the total electronic energy. In the chemical pressure (CP) method, the definition of macroscopic (or internal) pressure p is equivalent to the pressure defined with the XP-PCM model (as we can see by comparison of eq 4 of ref 17f with eq 3 of our paper I) In both the CP and XP-PCM methods the internal pressure is computed as the negative of the derivative of the electronic energy with respect to the volume occupied by the material system (i.e., the unit cell volume in the case of the CP method and the volume of the cavity for XP-PCM method). The XP-PCM decomposition of the pressure through eq 15 has a different aim: analyzing the Pauli repulsive interaction ensuing on compression and its effect on atomic energy levels and configuration energies and, correspondingly, atomic volumes and electronegativities

PRESSURE-INDUCED CONFIGURATIONAL CHANGE IN THE SCANDIUM ATOM

CONCLUSIONS

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