Abstract
The velocity of longitudinal ultrasonic waves, ν1,(P), in polycrystalline cesium was measured at 293 ±1K in the pressure interval 0–5 GPa. v 1(P) alterations at BCC-FCC phase transition at 2.3 GPa and at the electronic-structure transformation at 4.3 GPa were obtained. Decrease of v 1(P) to 4.3 GPa after a maximum at ∼3.8 GPa were found, that gave evidence to the appearance of a corresponding soft mode in the FCC-Cs phonon spectrum. The peculiarities of dependence v 1(P) correlate with s-conduction electrons promotion to the empty d-band in accordance with the theoretically predicted continuous electronic s-d transformation in Cs.