We show how pressure fundamentally alters the repulsive nature of the nonreactive Li-Cs mixture, converting it from strongly phase separating at ambient pressure to strongly long-range ordering at high pressures. The ordered phases found via a global space group optimization within the density-functional theory are ${\mathrm{Li}}_7\mathrm{Cs}$ in the $Cmmm$ structure, LiCs in the $B2$ structure, and ${\mathrm{Li}}_7\mathrm{Cs}$ in the $C2/m$ structure. These structures are remarkably stabilized by a pressure-induced increase in charge transfer from Cs to Li unit, an unusual effect concerning two elements from the same group (isovalent). These high-pressure phases exhibit interesting behaviors: (i) LiCs ($B2$) has its $\mathrm{Cs}(5p)$ core states nearly merged with the valence $\mathrm{Cs}(6s)$ states, indicating that core states can become valence states at high pressures; (ii) ${\mathrm{Li}}_7\mathrm{Cs}$ ($Cmmm$) structure exhibits an interesting 1D electronic structure within a 3D crystal structure.

• Received 3 March 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.245501