Unusual terahertz-wave absorptions in delta/alpha-mixed-phase FAPbI(3) single crystals: interfacial phonon vibration modes

Abstract

The terahertz (THz)-wave absorption properties in organic-inorganic hybrid perovskite (OHP) materials are investigated with the in-depth development of OHP-based THz applications. In the THz range from 0.5 to 3 THz, OHPs typically show several interesting phonon modes such as transverse, longitudinal, and halogen self-vibrations. To modulate these frequencies, the density changes in defect-incorporated structures and element mixtures were tested and confirmed. In the literature, the origin of phonon modes in OHP materials have been mostly explained. However, we found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. FAPbI(3) single crystals, organic-inorganic hybrid perovskites, of the delta-, delta/alpha-mixed-, and alpha-phases were prepared. We intriguingly found that the delta/alpha-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the delta- or alpha-phases, although the delta/alpha-mixed-phase sample was confirmed to be formed by a physical combination of the delta- and alpha-phases without the creation of any new chemical states. Our theoretical study performed with ab initio calculations provides an explanation for these unusual THz-wave absorption behaviors; they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI(3).