Phase transitions, dielectric response and lattice dynamics of dimethylammonium mixed lead halide perovskites

초록

Hybrid organic-inorganic perovskites exhibit rich structural dynamics that govern their stability and optoelectronic performance. Here we map the temperature-composition phase behavior of dimethylammonium lead mixed halides, DMAPbBr3-xClx with x = 0, 0.5, 1.5, 2, 2.5 and 3, by combining temperature-dependent powder Xray diffraction, Raman and Brillouin spectroscopy, dielectric spectroscopy and differential scanning calorimetry. All compositions undergo a first-order transition between an orthorhombic (P212121) low-temperature phase and a hexagonal (P63/mmc) high-temperature phase. The transition temperature increases monotonically with Cl content at 251, 256, 265, 283, 306, and 318 K for x = 0, 0.5, 1.5, 2, 2.5, and 3, respectively. Distinct experimental signatures include: (i) abrupt hardening/softening anomalies of low-frequency Raman modes (down to 10 cm-1) associated with octahedral twists and Pb-X stretching; (ii) softening of the longitudinal acoustic phonon mode and a discontinuous jump at transition temperatures in the Brillouin spectra, evidencing elastic modulus renormalization; (iii) step-like increases in the real part of the dielectric permittivity (epsilon 1), reflecting increased DMA reorientational polarizability and (iv) sharp DSC endotherms that quantify latent heat and confirm the first-order character. We integrate these datasets to develop a temperature-composition phase diagram for DMAPbBr3-xClx and clarify how halide substitution stiffens the lattice and strengthens hydrogen-bond network to tune the transition temperature. The results provide mechanistic insight into cation-framework coupling in DMA-based perovskites and valuable insights for optimizing their structural properties to enhance device performance.

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