High-Pressure Electrides: A Quantum Chemical Perspective

It has long been assumed that all matter will assume simple closed-packed lattices and become metallic under pressure, in accordance with the Thomas-Fermi-Dirac (TFD) model. However, this model struggles to explain pressure driven complex structural transitions that have been observed in elements, including sodium, challenging our conventional understanding of compressed matter. Moreover, in stark contrast to the TFD picture, first-principles calculations suggest that various elements and compounds become electrides under pressure. Electrides, characterized by concentrations of charge density at interstitial regions, can be thought of as ionic compounds where electrons behave as the anions. Though ambient-pressure molecular electrides have been extensively studied via experiments and computations, high-pressure electrides (HPEs) are not well understood. The identification and characterization of HPEs has been, to date, purely based on theory including topological analysis of the electron density and the electron localization function. Here, we review these theoretical analyses tools and suggest guidelines that can be used to classify systems as electrides. Moreover, we describe models used to rationalize the electronic structure of HPEs, drawing parallels with ambient pressure molecular systems, and urge for the development of experimental techniques that provide evidence for the theoretically calculated charge localization.