The Absence of CDW Order in PbSb, and its Unexpected Softness

We have performed an ab-initio calculation of the electronic structure of PbSb. Since Sb usually takes 3- valence state in a compound, Pb in PbSb formally takes 3+ valence state, which is very unusual. The stable valences of Pb are 2+ and 4+, and Pb3+ state is unstable and may have large charge fluctuation. This type of “valence-skip” fluctuation can induce charge-density-wave or superconductivity. However, our calculation shows that the valence of Pb is 2+ or smaller. We have also found that PbSb is one of the “softest metals” except for the alkali metal elements.