Melting in alkali-metal–lead alloys: KPb and CsPb

Inelastic neutron-scattering measurements have been made on KPb and CsPb alloys at various temperatures in the solid and liquid states. In the thermodynamic data CsPb exhibits two transitions near the melting point at 869 and 920 K. In the intermediate phase, quasielastic broadening is observed, indicating that the phase is a plastic crystal characterized by rapid reorientations of structural units. The results are well reproduced by a simple model describing independent jump reorientations of ${\mathrm{Cs}}_4$ ${\mathrm{Pb}}_4$ units between the four orientations found in the crystal at room temperature. KPb on the other hand has a single melting transition at 862 K, and no quasielastic broadening is observed below this temperature, indicating that translational and orientational order disappear simultaneously. The model for jump reorientations in CsPb also explains previous neutron-diffraction results for the intermediate phase.