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

Abstract

Main group elements usually assume a typical oxidation state while forming compounds with other species. Group I elements are usually in the þ 1 state in inorganic materials. Our recent work reveals that pressure may make the inner shell 5p electrons of Cs reactive, causing Cs to expand beyond the þ 1 oxidation state. Here we predict that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond À 1. Although Li and Cs only form alloys at ambient conditions, we demonstrate that these metals form stable intermetallic Li n Cs (n ¼ 1-5) compounds under pressures higher than 100 GPa. Once formed, these compounds exhibit interesting structural features, including capped cuboids and dimerized icosahedra. Finally, we explore the possibility of superconductivity in metastable LiCs and discuss the effect of the unusual anionic state of Cs on the transition temperature.

Novel lithium-nitrogen compounds at ambient and high pressures

“…This structure is an interesting example of Li-N compounds which can be viewed as a combination of a single Li atom and a slightly distorted Li 12 N icosahedral group (with N atom inside the Li 12 icosahedron). A similar Li 12 Cs icosahedron is present in the Pnna structure of Li 3 Cs compound 29 , where neighboring icosahedra share Li-Li edges. However, the Li 12 N icosahedra are isolated and do not share Li atoms with each other in our Li 13 N compound.…”

“…The Li-N bond lengths in the Li 12 N icosahedron are 1.934, 1.951 and 2.011 Å, i.e. nearly identical, and Li-Li distances are also nearly identical, ranging from 2.026 to 2.113 Å (maximum difference 4.3%, to compare with 22.3% in Li 3 Cs 29 ).…”

Mixing unmixables: Unexpected formation of Li-Cs alloys at low pressure

“…The most significant conclusion following from this first part of our experimental study is that the formation of Li-Cs crystalline alloys does occur unambiguously in a pressure regime much lower than theoretically predicted, that is, >50 GPa ( 3 ). The observed structures are likely not related to the predicted Li-rich Cs alloys ( 4 ).…”

“…Zhang and Zenger predicted that intermetallic crystalline structures can be stabilized as a result of a pressure-induced increase in charge transfer from Li to Cs. A more recent study also predicted that under pressure, electrons can be transferred from Li in Li-rich Li-Cs alloys, causing Cs to become anionic with a formal charge larger than −1 ( 4 ). These proposals are not unreasonable because pressure-promoted electron transfer in solids is not uncommon.…”

Predicted novel insulating electride compound between alkali metals lithium and sodium under high pressure

“…Subsequent experimental study by in situ synchrotron powder x-ray diffraction [22] demonstrated that the Li-Cs alloy could be synthesized at very low pressure (>0.1 GPa), and analysis of the valence charge density also showed that electrons are donated from Cs to Li, resulting in a charge state of -1 for Li. Interestingly, Cs can also obtain electrons from Li and become anionic with a formal charge much beyond -1 at high pressures, as Botana et al [23] reported in the stable LinCs (n=2-5) compounds under pressures above 100 GPa using first principles method within DFT scheme. This phenomenon can partially explained by tracking the variation of electronegativity between Li and Cs with pressure [24] .…”