A new structural phase transition in cesium has been found around 72 GPa using high-pressure powder x-ray-diffraction techniques with a diamond-anvil cell and an imaging plate. The crystal structure of the high-pressure phase Cs(VI) is either hexagonal close packed or double hexagonal close packed. The transition from the open-packed Cs(V) structure to the close-packed Cs(VI) structure implies significant influence of the 5p core repulsion to the cohesion of Cs(VI), as well as a possible occurrence of the 5p-5d overlap.

• Received 21 September 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.2014