Electronic Phase Transition of Cesium Metal under High Pressure

The isostructural phase transition of cesium metal, CsII←→CsIII, under pressure is discussed in terms of the crystalline energy as a function of volume. Using the augmented plane wave (APW) method and the quantum defect method, the energy bands are calculated at symmetry points in the Brillouin zone for the ran e of relative volume V/V_0 (V_0 : the volume at normal pressure) from 1.30 to 0.24. As the volume decreases, there appears a pronounced tendency of d-states to fall lower relative to s-states, which rise significantly at V/V_0≤0.6. Besides overall rise of the band energy with decreasing volume, a small humped portion is found at 0.4≤V/V_0≤0.5, this being attributed to the shift of electrons into the vicinity of X. The transition beyond this portion is accompanied by a significant volume change. Thus the CsII←→CsIII transition is interpreted as an electronic one. A calculation using the Green\u27s function method is also carried out and the results are compared with those by the APW method