Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy₂PbBr₄

The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy2PbBr4, comprising a very small spacer cation (methylhydrazinium, MHy+). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy2PbBr4 undergoes an unusual Pmn21 → P21 phase transition near 4 GPa, associated with the extrusion of some MHy+ cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb–NH2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa–1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn21 phase show the band gap narrowing, followed by its widening in the high-pressure P21 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb–Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure–strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.高压的应用可通过改变原子间距离调控材料的物理化学性质。压力还可诱导结构相变，形成具备增强或全新功能特性的新相。本文针对一款二维（2D）钙钛矿材料MHy₂PbBr₄——其包含尺寸极小的间隔阳离子甲基肼阳离子（methylhydrazinium，MHy⁺）——开展了互补性的高压单晶X射线衍射、拉曼光谱及光学研究。该晶体具备备受青睐的铁电性，以及卓越的多重线性与非线性光学（NLO）性能。单晶X射线衍射结果表明，MHy₂PbBr₄在约4 GPa附近发生了罕见的Pmn2₁→P2₁相变，该相变伴随部分MHy⁺阳离子从层间空间挤出至无机片层内部的空位中，此现象此前未在任何二维杂化钙钛矿中被报道。抗衡阳离子的迁移显著增强了Pb–NH₂相互作用，实现了前所未有的沿垂直于多阴离子层方向的正线性压缩率三倍提升，同时在层内实现了-22.39 TPa⁻¹的大幅负线性压缩率。拉曼光谱数据证实，该相变与晶体结构的强烈畸变以及氢键网络的重构密切相关；而常压相Pmn2₁在受压过程中的吸收光谱显示其带隙先窄化，随后在高压相P2₁中带隙展宽。自由激子（free-exciton，FE）光致发光（PL）的压力依赖性也呈现出从红移到蓝移的类似变化。此外，高压诱导的相变使得PL强度大幅增强，其中归因于自陷激子（self-trapped excitons，STEx）的宽带发射现象尤为显著。我们将吸收光谱与PL光谱中观测到的现象归因于：常压相下Pb–Br键的缩短，以及高压相下无机层的畸变加剧与PbBr₆八面体的倾斜。总体而言，针对这款包含极小间隔阳离子的二维杂化化合物的研究，整体上深化了我们对高压效应对二维杂化钙钛矿性能影响的理解，且展现出与含大有机阳离子的同类材料截然不同的受压行为。研究揭示，结构-应变机制可用于设计具备非常规结构、力学与光电子特性的新型高压相。