Short small-polaron lifetime in the mixed-valence perovskite Cs$_2$Au$_2$I$_6$ from high-pressure pump-probe experiments

TL;DR

This study uses high-pressure pump-probe spectroscopy to investigate ultrafast phonon dynamics in the mixed-valence perovskite Cs$_2$Au$_2$I$_6$, revealing rapid charge transfer and phonon softening linked to structural changes.

Contribution

It provides the first measurement of charge transfer excitation lifetime and demonstrates the role of inter-valence charge transfer in pressure-induced structural transitions.

Findings

Charge transfer excitation lifetime < 4 ps

Phonon softening and broadening under pressure and photoexcitation

Inter-valence charge transfer influences structural transition

Abstract

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.