Systematics in the melting behavior of the alkali metals from DAC measurements

Abstract

The alkali metals K, Rb and Cs undergo a series of phase transitions that are related to a s-to-d electron transfer. These solid-solid transitions strongly affect the melting behavior in the vicinity of the transitions. An externally-heated diamond-anvil cell was used to measure melting temperatures of the alkali metals, Na, K, Rb and Cs at pressures to 150 kbar. Melting was determined visually by changes of the optical properties of the metals. Pressures were obtained from the known pressure and temperature shifts of the ruby line which can be used for accurate measurements to about 650 K. The melting temperature of sodium rises continuously with pressure. The slopes of the melting curves potassium and rubidium change appreciably at the bcc-fcc transitions at 110 and 90 kbar, respectively. An extremely steep melting curve of cesium is observed after the electronic transition at 42 kbar.