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Observation of strain-induced phonon mode splitting in the tetragonal hybrid halide perovskite

We report the structural and low-frequency vibrational properties of methylammonium lead-iodide (MAPbI3) perovskites studied by terahertz spectroscopy. The characteristic low-frequency mode splitting is observed in room-temperature tetragonal MAPbI3 films grown on Si, and the frequencies of split modes are found to coincide with those of orthorhombic phases. The strain arising from the symmetry and lattice mismatch against the substrates is responsible for the local distortion of inorganic cages and results in the mode-splitting. The use of a mesoporous TiO2 interlayer effectively reduces the strain at the perovskite-substrate interface and induces larger grain growth and the improvement of structural ordering in the tetragonal MAPbI3 films.