Electrides and Their High-Pressure Chemistry

Recently, electrides were discovered in many systems (especially those containing alkali and alkali earth metals) at high pressures. An electride can be defined as an ionic compound where the role of an anion is played by a strongly localized electron density. High-pressure emergence of electrides is due to the Pauli expulsion of valence electrons from the core, while some electrides are better described as originating from multicenter covalent bonds. Not being bound directly to a nucleus, these localized electrons are chemically active, making electrides the strongest reducing agents known, able to interact even with such an extremely inert element as helium. The Discovery of Electrides In classical chemical systems, electron density is peaked on nuclei. However, in the 1970–1980s, a new type of compounds was established, in which bare electrons, not bound to any particular nucleus, are concentrated in the interstitial space and behave as anions. Such compounds were named electrides. X. Dong (B) Center for High Pressure Science and Technology Advanced Research, Beijing 100193, China e-mail: [email protected] A.R. Oganov Skolkovo Innovation Center, Skolkovo Institute of Science and Technology, 3 Nobel Street, Moscow 143026, Russia e-mail: [email protected] A.R. Oganov State University of New York, Stony Brook, NY 11794-2100, USA A.R. Oganov Moscow Institute of Physics and Technology, 9 Institutskiy Lane, Dolgoprudny, Moscow Region 141700, Russia A.R. Oganov International Center for Materials Design, Northwestern Polytechnical University, Xi’an 710072, China © Springer International Publishing AG 2017 G.G.N. Angilella and A. La Magna (eds.), Correlations in Condensed Matter under Extreme Conditions, DOI 10.1007/978-3-319-53664-4_6 69 [email protected] 70 X. Dong and A.R. Oganov Fig. 6.1 Structure of the organic electride Cs(15-crown-5)2e. Electron-trapping cavities and channels are shown in pink, 15-crown-5 molecules in blue, and cesium cations in yellow. Inset shows the ball-and-stick representation of the Cs(15-crown-5)2 “sandwich,” with the Cs+ ion drawn to scale [1] (color figure online) The first clear report of an electride we could find dates back to 1981 [2], followed by many other discoveries of chemically complex compounds (at first, only organic, but then also inorganic [3]). In 1990, Dye [1, 4, 5] discovered that an organic crown ether or cryptand can trap electrons as counterions, in alkali metal complexes, such as Cs(18-crown-6)2e or Cs(15-crown-5)2e. In such electrides as shown in Fig. 6.1, the localized electrons are usually single electrons occupying the cavities, and there is a weak spin–spin interaction between electrons occupying neighboring cavities. So, electrides known at normal pressure are paramagnetic or antiferromagnetic. Since high pressure favors spin pairing, such spin-polarized electrides will obviously give way to spin-paired electrides under compression. Another insight into the nature of high-pressure electrides comes from a quantum-mechanical consideration of the interaction between valence and core electrons. In 2008, Rousseau and Ashcroft [6] modeled the effects of core electrons as a hard potential v(r) = V0θ(rc − r), where rc is the ionic radius, θ is the Heaviside step function and V0 a hard screening potential. Using this potential, they wrote the Hamiltonian and the Schrödinger equation: