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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 βa = −22.4(8) and βb = −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. [ABSTRACT FROM AUTHOR]

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I–II), T2 = 339 K (phase II–III), and T3 = 280 K (phase III–IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4. [ABSTRACT FROM AUTHOR]

The common property of all materials is their elasticity, directly related to the chemical components and interactions in their structure. However, the progress in constructing better devices is irreversibly tied to the understanding and designing of static or dynamic structural responses of strained materials. Such structural-property relationships have been explained for polymeric frameworks involving flexible 1,6-hexanediamine (HDA) linkers, with discrete HDA conformers. In the structure of ambient-pressure polymer Cd(HDA)2(NO3)2 the HDA linkers are conformationally disordered. This tetragonal phase α is stable down to 190 K, when the HDA linkers order in different conformations, which triggers a ferroelastic transition to the triclinic phase δ; the flash-cooling of phase α overcools it to 100 K. High pressure induces a ferroelastic transition to triclinic phase β at 1.10 GPa, followed by an isostructural transition to phase γ at 2.00 GPa. All four phases α–δ differ in the configurations of HDA conformers and Cd-coordination involving nitrate linkers. The unusual convex-shaped monotonic compression of Cd(HDA)2(NO3)2 phases α, β and γ has been explained by the mechanism of buckling-sticks: the increasing strain and mounting energy accumulated in the buckling HDA linkers is released by their conversions to shorter conformers. An analogous conformational transition takes place in Cd2(HDA)3(NO3)4 at 1.50 GPa. In another polymer Cu(HDA)2(MeCN)2·2BF4 the conformational changes induce its very high compressibility and partial amorphization above 1.20 GPa, caused by the non-coordinated conversions of the HDA conformers. [ABSTRACT FROM AUTHOR]

Understanding and control of the spatiotemporal stimuli-responsiveness of flexible metal–organic frameworks are crucial for the development of novel adsorbents for gas storage and separation technologies. Herein, we report two isostructural pillared-layer dynamic frameworks differing only in one atom that bridge a benzenocarboxylate linker. Through a synthetic approach, we switch the stepwise CO2-induced transformation into a continuous one. Our findings are proved by equilibrium and time-resolved in situ powder X-ray diffraction collected during CO2 adsorption at 195 K. Finally, we use high-pressure single and multi-gas adsorption experiments to show the superiority of continuous breathing in CH4 storage and CH4/CO2 separation at 298 K. This report demonstrates that the desirable mechanism of flexible frameworks can be readily achieved through single-atom exchange enabling efficient gas separation and storage. [ABSTRACT FROM AUTHOR]

Phenyl-substituted perylene diimide (PTCDI-Ph) derivatives are prospective organic, electronic and optoelectronic materials. In this study, the effects of modified intermolecular interactions and molecular conformations on the energy gap and the optical properties of PTCDI-Ph were explored at high pressure and various temperatures by calorimetry, X-ray diffraction, UV-vis absorption, and photoluminescence spectroscopy. Below 498 K, PTCDI-Ph transforms from phase I to phase II, which is stable down to 93 K at least. Above 3.0 GPa, phase II transforms into phase III. The compressed intermolecular distance between perylene cores linearly narrows the band gap, due to the intensified π–π stacking. The negative linear compressibility of the crystal along [100] coincides with the negative thermal expansion in this direction. Owing to the strong pressure and temperature dependence of the luminescence spectrum, the PTCDI-Ph derivatives constitute a promising class of highly stimuli-responsive elastic organic materials with broad potential for tailoring their optoelectronic properties through physical and chemical modifications. [ABSTRACT FROM AUTHOR]