High-pressure electrides: From design to synthesis

Abstract

Electrides are unique ionic compounds that electrons serve as the anions. Many electrides with fascinating physical and chemical properties have been discovered at ambient condition. Under pressure, electrides are also revealed to be ubiquitous crystal morphology, enriching the geometrical topologies and electronic properties of electrides. In this Review, we overview the formation mechanism of high-pressure electrides (HPEs) and outline a scheme for exploring new HPEs from pre-design, CALYPSO assisted structural searches, indicators for electrides, to experimental synthesis. Moreover, the evolution of electronic dimensionality under compression is also discussed to better understand the dimensional distribution of anionic electrons in HPEs.

Electrides: a review

“…It is also possible to create electrides using high-pressure to squeeze electrons off the valence shell and into structurally active roles. 66,67 The oldest-known and most straight-forward examples of high-pressure electrides are the high-pressure alkali metals, that were long noted to have surprising properties. It was expected that a material would approach an ideal metal as pressure increased, 68 whereas alkali metals were observed to begin as almost ideal metals under ambient conditions and become semimetallic or even insulating under pressure.…”

Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium

“…Inorganic electrides form a class of emerging materials that has attracted considerable attention since the synthesis of the first room temperature stable inorganic electride of [Ca 24 Al 28 O 64 ] 4+ (e – ) 4 in 2003 . Recent first-principles calculations predict that there may be approximately 200 potential inorganic electride materials. − In particular, high pressure has been proved to be an efficient way to enhance the electron localization in the lattice interstice and to synthesize novel electrides. − Another aspect that should be noted is that these potential inorganic electrides mainly contain elemental metals and alloys. In other words, electron localization is a very important phenomenon and cannot be ignored, especially in metals and alloys under compression.…”

Recent Advances and Applications of Inorganic Electrides

“…Although a large number of electrides have been discovered at ambient conditions, pressure becomes an irreplaceable tool to stabilize electrides with intriguing properties, ,, indicating that pressure is able to modify the valence electron activity of atoms and to overcome reaction barrier. − For instance, the first high-pressure Na-hP4 electride is insulating, different from the well-known free-electron-like behavior in metals or well-accepted pressure-induced metallization . On the other hand, theoretical calculations play an important role in understanding the formation mechanism, , disclosing electron properties, − and providing an effective strategy for performance improvement of electrides. − Alternatively, high-throughput screening , and unbiased structural prediction, − especially for their combination with pressure, accelerate the discovery of electrides and electride-based new materials, not only consisting of unprecedented prototype structures but also exhibiting interesting properties. − …”

“…The electronic properties of electrides are closely related to the topologies of interstitial electrons. , To date, a variety of topologies in electrides have been identified from zero-dimensional (0D) cavities, ,, 1D-linked channels, ,− 2D planes, ,,− to 3D configurations. , For instance, Ca 2 N, consisting of interconnected interstitial electrons (i.e., anionic electrons form a 2D electron gas), demonstrates metallicity at ambient pressure . Under compression, its dimensionality of interstitial electrons reduces from 2D to 1D, and further to 0D.…”

“…Although a large number of electrides have been discovered at ambient conditions, pressure becomes an irreplaceable tool to stabilize electrides with intriguing properties, 4,5,11 indicating that pressure is able to modify the valence electron activity of atoms and to overcome reaction barrier. 12−16 For instance, the first high-pressure Na-hP4 electride is insulating, different from the well-known free-electron-like behavior in metals or wellaccepted pressure-induced metallization.…”