Altered reactivity and the emergence of ionic metal ordered structures in Li-Cs at high pressures.

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 Li7Cs in the Cmmm structure, LiCs in the B2 structure, and Li7Cs 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 Cs(5p) core states nearly merged with the valence Cs(6s) states, indicating that core states can become valence states at high pressures; (ii) Li7Cs (Cmmm) structure exhibits an interesting 1D electronic structure within a 3D crystal structure.