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

Main group elements usually assume a typical oxidation state while forming compounds with other species. Group I elements are usually in the +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 +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 -1. Although Li and Cs only form alloys at ambient conditions, we demonstrate that these metals form stable intermetallic LinCs (n=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.