High-pressure vibrational properties of dense rubidium

At ambient conditions, alkali metals adopt the body centered cubic structure, while if compressed up to tens
of GPa and above, they exhibit complex low-symmetry modifications, due to the density-driven transition of the
valence electrons from the s state to states of higher angular momentum. These high-pressure, low-symmetry
phases, whose unit cells may include up to tens of atoms, allow rich Raman activity, which was previously
observed only in lighter alkalis Na and Li. Here we report an extensive study of the optical phonons of highly
dense Rb up to 100 GPa in diamond anvil cells, conducted by challenging experimental Raman spectroscopy
measurements and ab initio computer simulations. The relative (relative to the normal condition value) density
behavior of Raman frequencies of Rb is compared to that of Na and Li, once the frequencies of the two light
alkali elements have been rescaled by (M_Na/M_Rb )^0.5 and (M_Li/M_Rb)^0.5, respectively, where M_Na, M_Li, and M_Rb are
the atomic masses of the here considered alkali elements. Importantly, while the rescaled density behaviors of
Na and Li agree with each other, Rb significantly differs, which highlights the different nature of the valence
electron transition being of the s-d and of the s-p type in heavy and light alkali metals, respectively, a result that
calls for further similar investigations of K and Cs.