Evolution of the neutron quasi-elastic scattering through the ferroelectric phase transition in 93%PbZn$_{1/3}$Nb$_{2/3}$O$_3$ - 7% PbTiO$_3$

We show that the neutron diffuse scattering in relaxor ferroelectric (1-x)PbZn$_{1/3}$Nb$_{2/3}$O$_{3}$ - x PbTiO$_{3}$ (x=0.07) consists of two components. The first component is strictly elastic but extended in q-space and grows below 600 K. The second component, that was not reported before for the (1-x)PbZn$_{1/3}$Nb$_{2/3}$O$_{3}$ - x PbTiO$_{3}$ (x=0.07) relaxor ferroelectrics, is quasi-elastic with a line-width that has a similar temperature dependence as the width of the central peak observed by Brillouin spectroscopy. The temperature dependence of the susceptibility of the quasi-elastic scattering has a maximum at the ferroelectric transition.