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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]

An unrelenting focus on improving stability and optoelectronic performance of lead halide three-dimensional hybrid organic-inorganic perovskites (3D HOIPs, ABX3 formula) leads to the usage of many chemical engineering techniques, including X-site alloying. Formerly we have shown the usefulness of this approach in 3D HOIPs comprising methylhydrazinium (MHy+); this time we prove it again in the layered (A2BX4) perovskites. Herein we report the synthesis, crystal structure and physicochemical characterization of MHy2PbBr4-xIx in a wide concentration range (x = 0.25, 0.3, 0.4, 1.1, 2.35, 2.5, 3.1). Reported compounds crystallise in the Ruddlesden-Popper phase with corner-sharing [PbX6]4- (X = Br, I) octahedra, forming (010) layers, separated by MHy+. An abundant temperature-driven polymorphism is observed, starting from the Br-rich compounds with a sequence of Pnmm, Pnma and P21/c phases on cooling, through the region with the P[1 with combining macron] low-temperature phase, and ending with the I-rich compounds with an incommensurately modulated Pnma(00γ)0s0 superspace group. We also report, for the first time in the MHy-based perovskites, the 2D → 3D transition to Pm3 with combining macron]m cubic symmetry and a 3D perovskite phase with a dominant presence of iodine. With the use of Raman spectroscopy, the dynamics of MHy+ cations in particular phases is thoroughly analysed, revealing the positional disorder in room-temperature and high-temperature zones and its freezing at low temperatures. Linear optical studies demonstrate the decrease of the energy band gap (2.91 eV to 2.31 eV) and a strong change in photoluminescence colour (bluish-green to yellow-green) along with increasing iodine contribution. [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]

2D Ruddlesden‐Popper metal‐halide perovskites feature particularly strong excitonic effects, making them a fascinating playground for studying exciton physics. A complete understanding of the properties of this quasi‐particle is crucial to fully exploit the tremendous potential of 2D perovskites (2DP) in light emission applications. Despite intense investigations, some of the exciton properties remain elusive to date, for example, the energy‐ordering of the exciton states within the so‐called fine structure manifold. Using optical spectroscopy, it demonstrates that in the archetypical 2DP (PEA)2PbI4, in contradiction to theoretical predictions, the energy of the bright out‐of‐plane exciton state is higher than that of two in‐plane states. Having elucidated the order of exciton fine structure, it determines the g‐factor of the dark exciton transition, together with the values of the electron and hole g‐factors in the direction parallel to the c‐axis of the crystal. In this way, it provides for the first time, a complete picture of the exciton fine structure in (PEA)2PbI4 2DP. [ABSTRACT FROM AUTHOR]

Lattice symmetrization is a term coined for the intentional doping of methylammonium lead iodide (MAPbI3) with larger organic cations to lower the transition temperature to the cubic phase, whose stability is much preferred over the tetragonal one to avoid undesired lattice strain harmful for device operation, which could appear due to the cubic-tetragonal phase transition (PT) present near 330 K in undoped MAPbI3. The following case study of three-dimensional (3D) organic cation-alloyed perovskites of formula MA1−xMHyxPbI3 (MHy+ = methylhydrazinium; x < 0.25) unveils the complex impact of MHy+ doping on the stability of crystal phases. For low doping of x ≤ 0.115 the cubic-tetragonal PT temperature strongly decreases on doping, as expected. The MHy+-induced lattice symmetrization worked to the point that at room-temperature (RT) the cubic phase could be observed at a doping of x = 0.057, the lowest among so far used organic dopants. By contrast, the temperature of the tetragonal-orthorhombic PT increases on doping, opposite to what was observed for analogous doped MAPbI3 systems. Unexpectedly, however, beyond x > 0.2 the tendency reverses, as the temperatures of tetragonal-to-cubic PTs shift strongly to higher values. Significant changes in PT mechanism at high doping conditions are inferred from large thermal hystereses, Raman scattering data showing the presence of two unique MA+ cations in the orthorhombic phase, and dielectric spectroscopy demonstrating dipolar relaxation for low-doped systems, and its suppression for highly-doped ones. The substitution of MA+ with MHy+ leads to a weak widening of the band gap while retaining efficient emission and extended absorption, suitable for optoelectronic applications. [ABSTRACT FROM AUTHOR]

At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials. [ABSTRACT FROM AUTHOR]