Low-frequency collective modes in the superionic phase of lead fluoride studied by quasielastic cold neutron scattering

The dynamic structure factor $S(Q,\ensuremath{\omega})$ for lead fluoride ${\mathrm{PbF}}_{2}$ in the normal and the superionic states at $T=293$ and 823 K has been investigated by cold neutron scattering. In the superionic state on a constant Q scale, two symmetrical peaks at low energies over a limited range of wave-vector transfers are observed. This structure is similar to the long-wavelength collective excitations (Brillouin doublet) specific to some liquid metals. Such an observation could support the idea of liquidlike behavior at the level of the anion sublattice in the superionic state. However, the energies of these excitations are about one order of magnitude smaller and the corresponding Q values are rather high compared to those of the collective modes observed in the simple classical liquids. Therefore these excitations are of different nature from those characteristic for a classical liquid.