High Pressure Electrides: A Predictive Chemical and Physical Theory

Abstract

Electrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ. With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE. By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between the surrounding He and the contained atoms or molecules) atoms and an interstitial space, we are able to semiquantitatively explain and predict the propensity of various elements to form HPEs. The slopes in energy of various orbitals with pressure (s > p > d) are essential for identifying trends across the entire Periodic Table. We predict that the elements forming HPEs under 500 GPa will be Li, Na (both already known to do so), Al, and, near the high end of this pressure range, Mg, Si, Tl, In, and Pb. Ferromagnetic electrides for the heavier alkali metals, suggested by Pickard and Needs, potentially compete with transformation to d-group metals.

Structural and electronic properties of the alkali metal incommensurate phases

“…What differentiates these crystalline electride phases is the almost perfect localization of electrons in interstitial sites: the ELF maxima values are close to unity, they coincide with NNM's at the centers of the interstices, and their basins form well-defined chemical entities integrating to a pair of electrons. These Lewis pairs can be regarded as "pseudoanions," and the crystal structures related to conventional ionic compounds where the position of these pseudoanions is occupied by anions [62].…”

Pressure-stabilized lithium caesides with caesium anions beyond the −1 state

“…These unusual compounds contain alkali metal anions as well as alkali cations embedded in organic molecules such as cryptands or crown ethers [3][4][5][6][7] . The unique oxidation states are related to another class of materials called electrides 6,[8][9][10][11][12] . When electrons are detached from the embedded alkali metal cations, they either fill the interstitial sites forming electrides or, if there are alkali metal atoms available, bind loosely to them.…”

Perspective: Role of structure prediction in materials discovery and design

“…101 These experimental developments will further stimulate first-principles DFT calculations of solids at extreme pressures and temperatures. For example, there has been much interest in predicting the stability of "electride" structures at high pressures in which some of the higher energy electrons occupy interstitial regions within the crystal and behave as anions, 40,100,[102][103][104][105] see Fig. 7.…”