Expanding the horizons of the thermodynamic landscape and optoelectronic properties of soft 2D hybrid perovskites MHy2PbX4

Abstract

Compared to their three-dimensional analogues, the layered perovskites A₂BX₄ (A = organic cation; X = Br, I, Cl) display higher anisotropy, elasticity and sensitivity to external stimuli. In this study, we employ a combination of diffraction, calorimetric, dielectric, vibrational and optical measurements to uncover the rich phase diversity and atypical pressure-induced transformations of a layered perovskite analogue, methylhydrazinium tetraiodoplumbate (CH₃NH₂NH₂Pbl₄, MHy₂PbI₄). At ambient pressure, MHy₂PbI₄ exists in four paraelectric phases I–IV, depending on temperature. The combined structural and spectroscopic measurements revealed that phase II (space group Pmcn) is exceptionally responsive to pressure stimuli and transforms to phase V (Pmmn) at 70 MPa; further transitions of MHy₂PbI₄ at 1.15 and 3.00 GPa led to the polar phases VI (Pmn2₁) and VII (P2₁). Phase VI strikingly reassembles the ambient-pressure ferroelectric phase of MHy₂PbBr₄. The transition from phase V to VI reduces the cation disorder and deforms the PbI₆ octahedra which leads to a two-fold increase of the free exciton (FX) and the suppression of bound-exciton (BX1 and BX2) luminescence band intensities as well as to red shifts of absorption (dE/dp = −58(2) meV GPa⁻¹) and emission signals (dE/dp = −63(2) meV GPa⁻¹). Above 3.0 GPa, a new phase VII displays negative area compressibility with β_a = −22.4(8) and β_b = −6.2(6) TPa⁻¹ along the polyanionic sheets. The structure of phase VII revealed the underlying mechanism involving pressure-triggered penetration of MHy⁺ cations into the voids between the highly deformed octahedra. Overall, the construction of the p–T phase diagram for MHy₂PbI₄ demonstrates the high structural elasticity of this hybrid perovskite, its pressure-induced polar phases isostructural with ferroelectric analog MHy₂PbBr₄ and unique mechanical responses to external stimuli.