Hybrid organic–inorganic halide perovskites exhibit strongly coupled structural and dynamical degrees of freedom in which lattice distortions, molecular reorientation, and compositional disorder collectively govern phase stability and functional response. A central unresolved question is why some hybrid perovskites undergo well-defined cooperative phase transitions, whereas others evolve into frustrated, glassy states without long-range symmetry breaking.
Here, we present a comparative, multi-technique investigation of phase behavior in mixed-halide and mixed-cation hybrid perovskites using temperature-dependent Raman spectroscopy (down to 10 cm-1), Brillouin light scattering, broadband dielectric spectroscopy, differential scanning calorimetry, and X-ray diffraction. In mixed-halide MAPbBr3-xClx systems, compositional disorder disrupts cooperative octahedral tilting, producing suppressed phase transitions and glassy freezing characterized by strong phonon broadening, frequency dispersion, and absence of thermodynamic discontinuity. Similarly, DMA–MA mixed perovskites exhibit dipolar glass behavior governed by Vogel–Fulcher-type freezing of molecular reorientation.
In contrast, fully DMA-based DMAPbBr3-xClx perovskites display robust first-order orthorhombic–hexagonal phase transitions across the entire composition range, with transition temperatures tunable from ~251 K to ~318 K by halide substitution. Raman and Brillouin measurements reveal pronounced phonon and acoustic softening approaching the transition, accompanied by discontinuous elastic anomalies, while dielectric spectroscopy shows step-like permittivity changes and DSC confirms finite latent heat. These results demonstrate that the larger DMA cation stabilizes a pre-distorted bioctahedral framework, strengthens N–H···X hydrogen bonding, and reduces the free-energy barrier for cooperative ordering, thereby suppressing glassy frustration even under strong halide disorder.
By correlating lattice dynamics, elastic response, dipolar reorientation, and thermodynamic signatures, we establish a unified mechanistic picture linking organic-cation chemistry to macroscopic phase behavior. This disorder-to-order crossover provides general design principles for controlling phase stability, lattice softness, and dynamic response in hybrid perovskites, with direct implications for thermally robust optoelectronic and ferroelastic materials.
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Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (No. RS-2023-00219703) and by the support from South Korea - Learning & Academic research institution for Master's⋅PhD students, and Postdocs(LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2024-00443714).