High-Pressure Study of Perovskite-Like Organometal Halide: Band-Gap Narrowing and Structural Evolution of [NH₃‑(CH₂)₄‑NH₃]CuCl₄

Searching for nontoxic and stable perovskite-like alternatives to lead-based halide perovskites for photovoltaic application is one urgent issue in photoelectricity science. Such exploration inevitably requires an effective method to accurately control both the crystalline and electronic structures. This work applies high pressure to narrow the band gap of perovskite-like organometal halide, [NH₃-(CH₂)₄-NH₃]­CuCl₄ (DABCuCl₄), through the crystalline-structure tuning. The band gap keeps decreasing below ∼12 GPa, involving the shrinkage and distortion of CuCl₄^2–. Inorganic distortion determines both band-gap narrowing and phase transition between 6.4 and 10.5 GPa, and organic chains function as the spring cushion, evidenced by the structural transition at ∼0.8 GPa. The supporting function of organic chains protects DABCuCl₄ from phase transition and amorphization, which also contributes to the sustaining band-gap narrowing. This work combines crystal structure and macroscopic property together and offers new strategies for the further design and synthesis of hybrid perovskite-like alternatives

New Assembly of Acetamidinium Nitrate Modulated by High Pressure

High pressure is an essential thermodynamic parameter in exploring the performance of condensed energetic materials. Combination of high-pressure techniques and supramolecular chemistry opens a new avenue for synthesis of high energy density materials. Herein, we fabricate a new high-pressure-assisted assembly of energetic material acetamidinium nitrate (C₂N₂H₇⁺·NO₃^–, AN) with <i>P</i>-1 symmetry after a 0–12 GPa–0 treatment at room temperature, which exhibits a density that is 9.8% higher than that of the initial <i>P</i>2₁/<i>m</i> phase. Evolution of intermolecular lattice modes in Raman spectra and synchrotron X-ray diffraction (XRD) patterns provide strong evidence for this transition in the 1.3–3.4 GPa range. The mechanism involves relative motions between ionic pairs in the hydrogen-bonded array and distortions of building blocks

High-Pressure Effects on Hofmann-Type Clathrates: Promoted Release and Restricted Insertion of Guest Molecules

The search for effective methods to accurately control host–guest relationship is the central theme of host–guest chemistry. In this work, high pressure successfully promotes guest release in the Hofmann-type clathrate of [Ni­(NH₃)₂Ni­(CN)₄]·2C₆H₆ (Ni–Bz) and restricts guest insertion into Ni­(NH₃)₂Ni­(CN)₄ (Ni–Ni). Because of the weak host–guest interactions of Ni–Bz, external force gradually removes guest benzene from the host framework, leading to puckered layers. Further theoretical calculations reveal the positive pressure contribution to breaking the energy barrier between Ni–Bz and Ni–Ni, explaining guest release from an energy standpoint. Inversely, guest insertion is restricted in the synthesized host of Ni–Ni because of the steric hindrance effect at high pressure. This study not only reveals structural effects on host–guest behaviors but also proves the role of pressure in controlling host–guest interactions. This unique observation is also crucial for the further application of host–guest materials in sustained and intelligent drug release, molecular separation, and transportation

Pressure-Induced Phase Transition in Hydrogen-Bonded Supramolecular Structure: Ammonium Formate

High-pressure behaviors of hydrogen-bonded supramolecular structure, ammonium formate (NH₄⁺COOH^–, AF), have been investigated under pressure by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy up to 20 GPa. Under ambient conditions, AF exhibits three-dimensional hydrogen-bonded networks with two molecules crystallize in a monoclinic unit cell of space group <i>Pc</i>. A structural phase transition can be identified at around 1.8 GPa, as indicated by the abrupt changes in Raman spectra as well as the pressure dependence of major Raman modes. Furthermore, two new N–H stretching modes emerge, indicative of the construction of new hydrogen bonds. Rearrangement of the hydrogen-bonded networks is also deduced by the obvious changes of N–H stretching modes both in position and intensity. The reversible phase transition is confirmed by in situ synchrotron XRD experiments with the emergence of a new set of diffraction pattern. The high-pressure phase is found to have a structure with a monoclinic unit cell (space group <i>P</i>2₁) containing two molecules. The structural transformation is proposed to be a result of the rearrangement of the hydrogen-bonded networks. Detailed mechanism for the phase transition, high-pressure behaviors of hydrogen bonds, as well as the cooperativity of different noncovalent interactions are presented and discussed

High-Pressure Studies of Abnormal Guest-Dependent Expansion in {[Cu(CO₃)₂](CH₆N₃)₂}_<i>n</i>

High-pressure guest-dependent behaviors of porous coordination polymer {[Cu­(CO₃)₂]­(CH₆N₃)₂}_<i>n</i> (GCC) are investigated using synchrotron X-ray diffraction (XRD) and Raman techniques. In GCC, the host framework of 3D [Cu­(CO₃)₂]^2– coordination network presents a diamond-like topology, with guest guanidinium cations locating at the window of the pores through N–H···O hydrogen bonds. Above a critical pressure, the external force can squeeze the guanidinium ions into the pores, leading to the abnormal expansion of the structure. Meanwhile, the critical pressure for expansion can be effectively lowered when no pressure transmitting medium is employed. Moreover, nonhydrostatic effects can promote the insertion of guanidinium ions, along with the amorphization of the structure, and thus affect the reversibility of the structure after releasing the pressure. Our results show that pressure is an effective tool to tune the host–guest relationship and to prepare high-pressure phase host–guest materials. Meanwhile, this study broadens the understanding of host–guest chemistry and offers a new strategy for fabricating novel materials with applications of pressure switches and zero contraction material in porous coordination polymers

Structural Tuning and Piezoluminescence Phenomenon in Trithiocyanuric Acid

As an essential thermodynamic variable, pressure has a powerful ability to accurately control molecular structures and properties by modulating the noncovalent interactions therein. Based on this point, we utilized pressure to tune the structure and properties of trithiocyanuric acid (C₃H₃N₃S₃, TTCA), gaining deeper insight into its structural nature and structure–property relationships. During compression, layered TTCA undergoes molecular distortion and relative slippage between interlayers, as well as anisotropic and stepwise shrinkage of intralayer six-molecule rings. Importantly, these structural variations have a great influence on the luminescence properties of TTCA. Piezoluminescence is observed above ∼4 GPa, acompanied by the calculated shifting of valence-band top. In experiments, detailed stepwise compression of the intralayer structure was captured directly. The observations combine the microscopic structure and macroscopic properties together and are beneficial for the further design of luminescence materials, as well as pressure sensors and pressure switches

Reciprocating Compression of ZnO Probed by X‑ray Diffraction: The Size Effect on Structural Properties under High Pressure

Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds

Pressure-Induced Phase Transition in N–H···O Hydrogen-Bonded Molecular Crystal Oxamide

The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ∼14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ∼17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from <i>P</i>1̅ to <i>P</i>1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N–H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks

Pressure-Induced Irreversible Phase Transition in the Energetic Material Urea Nitrate: Combined Raman Scattering and X‑ray Diffraction Study

In situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction (XRD) have been employed to investigate the behavior of the energetic material urea nitrate ((NH₂)₂COH⁺·NO₃^–, UN) up to the pressure of ∼26 GPa. UN exhibits the typical supramolecular structure with the uronium cation and nitrate anion held together by multiple hydrogen bonds in the layer. The irreversible phase transition in the range ∼9–15 GPa has been corroborated by experimental results and is proposed to stem from rearrangements of hydrogen bonds. Further analysis of XRD patterns indicates the new phase (phase II) has <i>Pc</i> symmetry. The retrieved sample is ∼10.6% smaller than the ambient phase (phase I) in volume owing to the transformation from two-dimensional (2D) hydrogen-bonded networks to three-dimensional (3D) ones. The mechanism for the phase transition involves the cooperativity of noncovalent interactions under high pressure and distortions of the layered structure. This work suggests high pressure is an efficient technique to explore the performance of energetic materials, and to synthesize new phases with high density

Exploration of the Pyrazinamide Polymorphism at High Pressure

We report the high-pressure response of three forms (α, δ, and γ) of pyrazinamide (C₅H₅N₃O, PZA) by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 14 GPa. These different forms are characterized by various intermolecular bonding schemes. High-pressure experimental results show that the γ phase undergoes phase transition to the β phase at a pressure of about 4 GPa, whereas the other two forms retain their original structures at a high pressure. We propose that the stabilities of the α and δ forms upon compression are due to the special dimer connection that these forms possess. On the other hand, the γ form, which does not have this connection, prefers to transform to the closely related β form when pressure is applied. The detailed mechanism of the phase transition together with the stability of the three polymorphs is discussed by taking molecular stacking into account