Electronic origin of melting T–P curves of alkali metals with negative slope and minimum

Abstract Group I elements—alkali metals Li, Na, K, Rb and Cs—are examples of simple metals with one s electron in the valence band. Under pressure these elements display unusually complex structural behaviour transforming from close-packed to low symmetry open-packed structures. Unexpectedly complex form was found for melting curves of alkalis under compression with an initial increase in the melting temperature in accordance with Lindemann criterion and a further decrease to a very low melting point. To understand complex and low symmetry crystal structures in compressed alkalis, a transformation of the electron energy levels was suggested which involves an overlap between the valence band and outer core electrons. Within the model of the Fermi sphere–Brillouin zone interaction one can understand the complex melting curve of alkalis.