Lattice Dynamics and Structural Phase Transitions in Two-Dimensional Ferroelectric Methylhydrazinium Lead Bromide Investigated Using Raman and IR Spectroscopy

Abstract

Two-dimensional (2D) lead halide perovskites are promising photovoltaic, light-emitting, nonlinear optical, and ferroelectric materials. Their key optoelectronic properties and polar order of organic cations are strongly affected by the dynamic response of the ionic lattice. Herein, we report polarized Raman and IR spectroscopy studies of methylhydrazinium lead bromide (MHy2PbBr4) exhibiting ferroelectric, photoluminescence, and multiple nonlinear optical phenomena. We determined the wavenumbers of longitudinal optical (LO) and transverse optical (TO) modes and their symmetries. These studies reveal that the dielectric response of MHy2PbBr4 is dominated by vibrations of the inorganic sublattice, which exhibits a large LO-–TO splitting (up to 44 cm–1). We also performed temperature-dependent Raman spectroscopy studies to monitor the effect of temperature on lattice dynamics and the effect of structural phase transitions on phonon modes. Raman spectroscopy data reveal that Raman bands are very narrow at 80 K, indicating a well-ordered structure. However, the increase of temperature leads to a very pronounced broadening of bands, indicative of a strong increase of lattice anharmonicity due to increased motional freedom of MHy+ cations and the inorganic network. Although former X-ray diffraction analysis showed that MHy2PbBr4 undergoes an order–disorder phase transition at 351 K, this transition leads to weak wavenumber shifts and broadening of Raman bands, more pronounced for modes related to the inorganic subnetwork. Furthermore, we do not observe any clear anomalies at 371 K, where the second phase transition into another disordered phase occurs. Our results indicate, therefore, that in contrast to many 2D lead halide perovskites, the dynamics of MHy+ cations and the inorganic network in MHy2PbBr4 do not exhibit pronounced slowing down at the order–disorder phase transition temperature but rather decrease gradually over a very large temperature range.