High pressure behaviour of Caesium

The high-pressure behaviour of Cs is believed to result from pressure-driven s-to-d electron transfer, which results in a number of structural phase transitions and unusual optical and melting behaviour. In addition, the s-to-d transition changes the bonding character of Cs from the non-directional bonding of a nearly-free electron metal at ambient pressure to more complex directional bonding at high pressures. As a result, Cs transforms from simple structures to more complex open-packed structures on pressure increase. It is only recently that the structure of Cs-V has been reported, and that Cs-III, long supposed to be fcc, has been shown to have a complex orthorhombic structure with 84 atoms per unit cell. Calculations of the electron density in Cs-IV and in the predicted high-pressure complex phases of Li and Na, suggest that in each structure there should be interstitial electron density between the atomic sites. Indeed, it may be possible to regard Cs-IV as 'Cesium electride' Cs+e-.