“…In fact, as a consequence of the induced s-p orbital mixing, 1 pressure makes them depart from their simplicity at ambient conditions to become metals with rather complex behavior. 2 Not only superconductivity arises under pressure in lithium with a large T c , 3,4 but, even more strikingly, both lithium and sodium become semiconductors when further compressed, 5,6 confirming the predictions made by Neaton and Ashcroft. 7,8 Moreover, contradicting common sense, the melting curve of the alkalies shows a negative slope of the melting temperature as a function of pressure in a wide pressure range, [9][10][11][12][13] principally, due to the softening of a transverse phonon mode induced by compression.…”