Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy₂PbBr₄

Abstract

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, MHy 2 PbBr 4 , 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 MHy 2 PbBr 4 undergoes an unusual Pmn2 1 → P2 1 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− NH 2 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 Pmn2 1 phase show the band gap narrowing, followed by its widening in the highpressure P2 1 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 PbBr 6 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.

Broadband yellow and white emission from large octahedral tilting in (110)-oriented layered perovskites: imidazolium-methylhydrazinium lead halides

“…The broadband emissions with large Stoke shifts are usually attributed to self-trapped excitons (STEx). 27,[68][69][70][71][72] Stability of STEx states increases with the enhancement of exciton-phonon coupling, and numerous studies showed that the intensity of STEx PL and Stokes shift increase with increasing distortion of the inorganic layers (decreasing y link ). 27,68,72,73 Since the high level of structural distortions is typical of (110)-oriented perovskites, 36,46,53 these compounds often show STEx related broadband emission, including the white-light one.…”

Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal

“…However, in our powder X-ray diffraction (PXRD) measurements using silicone oil as the PTM, the second-order Birch–Murnaghan equation of state demonstrates B 0 of 27.9(9) GPa with B ′ of 4.0 in phase II, which is almost 2 times larger than that in the PTM of the ME mixture, implying that the [Ni­(en) 3 ]­(ox) single crystal is extremely sensitive to the hydrostatic environment. Such character has been observed in most of the soft materials, such as coordination polymers , and hybrid perovskites. − …”

Highlighting Recent Research from Latin America in Chemistry of Materials

“…Studies of 2D halide perovskites were undertaken by a number of different groups, making advances in their design broadly. For example, Solis-Ibarra and co-workers showed that the broadband emission of these materials could be enhanced through a combination of halide bonds and halide mixing while the application of pressure can induce phase transitions and, in turn, modulate optical properties as shown by a multicountry collaboration lead by Katrusiak …”

“…Thus, it is not surprising that other recent manuscripts highlight materials with potential energy and light-emitting/phosphor applications. Organic, hybrid, − and inorganic materials have been the focus of these studies, with a recent review on dual-state emission in organic fluorophores . Studies of 2D halide perovskites were undertaken by a number of different groups, making advances in their design broadly.…”

“…For example, Solis-Ibarra and coworkers showed that the broadband emission of these materials could be enhanced through a combination of halide bonds and halide mixing 12 while the application of pressure can induce phase transitions and, in turn, modulate optical properties as shown by a multicountry collaboration lead by Katrusiak. 11 Finally, while the manuscripts highlighted thus far emphasize a range of applications, many other papers reflect a deep appreciation for fundamental studies that correlate atomic and nanoscale structure to properties. These manuscripts are highly collaborative, bringing together material synthesis, analytical characterization, and computational tools to answer questions across a diversity of material classes.…”