An electronic-driven liquid-liquid phase transition in rubidium is revealed by picosecond acoustic measurements combined with ab initio calculations. Picosecond acoustics were used to measure the melting line up to 10 GPa, finding the maximum in the melting curve at 7 GPa and 555 K. We observe the onset of a continuous liquid-liquid phase transition beginning around the melting maxima through until 16 GPa. Sound velocity shows a softening similar to that reported for liquid caesium, caused by a

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... change in the bulk modulus during a crossover from the low-density to the high-density liquid. Guided by the ab initio calculations, we relate the changes in the thermoelastic properties to the progressive localization of the valence electrons in the pressure range of 6-16 GPa. At high pressure rubidium forms an electride liquid quantified by the appearance of interstitial quasiatoms localized in the valence electron density.