Unusual terahertz-wave absorptions in δ/α-mixed-phase FAPbI3 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 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, density changes defect-incorporated structures element mixtures were tested confirmed. literature, origin OHP have been mostly explained. However, we found new vibration formamidinium (FA)-based structures. FAPbI single crystals, organic–inorganic perovskites, δ -, / α -mixed-, -phases prepared. We intriguingly that -mixed-phase exhibited significant THz-wave peaks at 2.0 2.2 not related any either - or -phases, although sample was confirmed be formed by a physical combination without creation chemical states. Our theoretical study performed ab initio calculations provides an explanation for unusual behaviors; they originate novel excited seamless interfaces mixed phase .