  We study the ultrafast phonon response of mixed-valence perovskite
Cs$_2$Au$_2$I$_6$ using pump-probe spectroscopy under high-pressure in a
diamond anvil cell. We observed a remarkable softening and broadening of the Au
- I stretching phonon mode with both applied pressure and photoexcitation.
Using a double-pump scheme we measured a lifetime of the charge transfer
excitation into single valence Au$^{2+}$ of less than 4 ps, which is an
indication of the local character of the Au$^{2+}$ excitation. Furthermore, the
strong similarity between the pressure and fluence dependence of the phonon
softening shows that the inter-valence charge transfer plays an important role
in the structural transition.
