Short small-polaron lifetime in the mixed-valence perovskite   Cs$_2$Au$_2$I$_6$ from high-pressure pump-probe experiments
  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. 
M. Trigo, J. Chen, M. P. Jiang, W. L. Mao, S. C. Riggs, M. C. Shapiro,   I. R. Fisher, D. A. Reis
cond-mat.str-el cond-mat.mtrl-sci