Pressure-induced isosymmetric phase transition in sulfamic acid: A combined Raman and x-ray diffraction study

Acoustic Shock Wave-Induced Amorphous to Crystalline Phase Transitions of Li2SO4─Raman Spectroscopic and Thermal Calorimetric Approach

In this context, we have reexamined the acoustical shock wave-induced amorphous-glassy-crystalline-amorphous phase transitions in the Li2SO4 sample under 0, 1, 2, and 3 shocked conditions by implementing the detailed Raman spectroscopic approach. The recorded Raman spectroscopic data clearly reveal that the transition from the amorphous-glassy-crystalline state occurs because of a significant reduction of the translational disorder of lithium cations, particularly [Li (2)] ions wherein a slight reduction of the librational disorder of SO4 anions takes place, whereas the crystalline to amorphous transition occurs only at the third shocked condition because of the librational disorder of SO4 anions. The double degenerate υ2 and υ4 Raman modes provide a clear indication of the occurrence of the librational disorder of SO4 anions at the third shocked condition. Followed by the internal Raman modes, a detailed discussion is provided on the external Raman modes of the Li ions and SO4 ions with respect to the observed phase transitions, wherein it is found that the regions of lattice modes are significantly altered at each and every point of phase transition. Furthermore, the thermal and magnetic measurements have been performed for the above-mentioned state of Li2SO4 samples, whereby the obtained results of the magnetic loops and the thermal property resemble the observed structural transitions with respect to the number of shock pulses such that the inter-relationship of the structure-electrical-magnetic-thermal properties of Li2SO4 could be explored.

Can We Predict the Isosymmetric Phase Transition? Application of DFT Calculations to Study the Pressure Induced Transformation of Chlorothiazide

Isosymmetric structural phase transition (IPT, type 0), in which there are no changes in the occupation of Wyckoff positions, the number of atoms in the unit cell, and the space group symmetry, is relatively uncommon. Chlorothiazide, a diuretic agent with a secondary function as an antihypertensive, has been proven to undergo pressure-induced IPT of Form I to Form II at 4.2 GPa. For that reason, it has been chosen as a model compound in this study to determine if IPT can be predicted in silico using periodic DFT calculations. The transformation of Form II into Form I, occurring under decompression, was observed in geometry optimization calculations. However, the reverse transition was not detected, although the calculated differences in the DFT energies and thermodynamic parameters indicated that Form II should be more stable at increased pressure. Finally, the IPT was successfully simulated using ab initio molecular dynamics calculations.

Transformation between the Dark and Bright Self-Trapped Excitons in Lead-Free Double-Perovskite Cs2NaBiCl6 under Pressure

Understanding the relationships between the structure and the properties in lead-free double perovskites is significant for their applications in the optoelectronic field. Here the nonluminous Cs₂NaBiCl₆ crystal exhibits an unexpected broadband dual-color emission as the external pressure is increased to 6.77 GPa. The emission intensity is remarkably enhanced with further compression to 8.50 GPa. By analyzing the results of in situ high-pressure experiments and the density functional theory, we conclude that the dual-color emission is attributed to singlet self-trapped excitons (STEs) and triplet STEs, respectively. This phenomenon originates from the tilting and twisting of [BiCl₆]^3- caused by the transition of cubic Cs₂NaBiCl₆ to the tetragonal phase. Notably, the transformation between the dark and bright STEs in the Cs₂NaBiCl₆ crystal is demonstrated by ultrafast transient absorption experiments under different pressures. This work not only offers deep insight into the structure-property relationship in lead-free double perovskites but also opens the door for the design of new lead-free double perovskites.

High-Pressure Band-Gap Engineering in Lead-Free Cs2 AgBiBr6 Double Perovskite

Novel inorganic lead-free double perovskites with improved stability are regarded as alternatives to state-of-art hybrid lead halide perovskites in photovoltaic devices. The recently discovered Cs₂ AgBiBr₆ double perovskite exhibits attractive optical and electronic features, making it promising for various optoelectronic applications. However, its practical performance is hampered by the large band gap. In this work, remarkable band gap narrowing of Cs₂ AgBiBr₆ is, for the first time, achieved on inorganic photovoltaic double perovskites through high pressure treatments. Moreover, the narrowed band gap is partially retainable after releasing pressure, promoting its optoelectronic applications. This work not only provides novel insights into the structure-property relationship in lead-free double perovskites, but also offers new strategies for further development of advanced perovskite devices.

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.

High pressure studies of Ni3[(C2H5N5)6(H2O)6](NO3)6·1.5H2O by Raman scattering, IR absorption, and synchrotron X-ray diffraction

Molecular structure (a) and packing diagram (b) of 1. The green, grey, blue, red, and white spheres denote Ni, C, N, O, and H atoms, respectively.

Pressure-induced phase transition in hydrogen-bonded molecular crystal acetamide: combined Raman scattering and X-ray diffraction study

Two structural phase transitions are observed at ∼0.9 and ∼3.2 GPa in acetamide using in situ synchrotron X-ray diffraction (XRD) and Raman scattering techniques.

Luminescence Properties of Compressed Tetraphenylethene: The Role of Intermolecular Interactions

Mechanochromic materials with aggregation-induced enhanced emission (AIEE) characteristic have been intensively expanded in the past few years. In general, intermolecular interactions invariably alter photophysical processes, while their role in the luminescence properties of these AIEE-active molecules is difficult to fully recognize because the pressurized samples possess amorphous nature in many cases. We now report the high-pressure studies on a prototype AIEE-active molecule, tetraphenylethene, using diamond anvil cell technique with associated spectroscopic measurements. An unusual pressure-dependent color, intensity, and lifetime change in tetraphenylethene has been detected by steady-state photoluminescence and time-resolved emission decay measurements. The flexible role of the aromatic C-H···π and C-H···C contacts in structural recovery, conformational modification, and emission efficiency modulation upon compression is demonstrated through structure and infrared analysis.

Pressure-induced pseudoatom bonding collapse and isosymmetric phase transition in Zr2Cu: first-principles predictions

The structural evolution of tetragonal Zr2Cu has been investigated under high pressures up to 70 GPa by means of density functional theory. Our calculations predict a pressure-induced isosymmetric transition where the tetragonal symmetry (I4/mmm) is retained during the entire compression as well as decompression process while its axial ratio (c/a) undergoes a transition from ~3.5 to ~4.2 at around 35 GPa with a hysteresis width of about 4 GPa accompanied by an obvious volume collapse of 1.8% and anomalous elastic properties such as weak mechanical stability, dramatically high elastic anisotropy, and low Young's modulus. Crystallographically, the tetragonal axial ratio shift renders this transition analogous to a simple bcc-to-fcc structural transition, which implies it might be densification-driven. Electronically, the ambient Zr2Cu is uncovered with an intriguing pseudo BaFe2As2-type structure, which upon the phase transition undergoes an electron density topological change and collapses to an atomic-sandwich-like structure. The pseudo BaFe2As2-type structure is demonstrated to be shaped by hybridized dxz + yz electronic states below Fermi level, while the high pressure straight Zr-Zr bonding is accommodated by electronic states near Fermi level with dx(2) - y(2) dominant features.