Negative Linear Compressibility in Organic Mineral Ammonium Oxalate Monohydrate with Hydrogen Bonding Wine-Rack Motifs

Negative linear compressibility (NLC) is a relatively uncommon phenomenon and rarely studied in organic systems. Here we provide the direct evidence of the persistent NLC in organic mineral ammonium oxalate monohydrate under high pressure using synchrotron X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculation. Synchrotron X-ray powder diffraction measurement reveals that ammonium oxalate monohydrate shows both positive and negative linear compressibility along <i>b</i>-axis before 11.5 GPa. The red shift of the external Raman modes and abnormal changes of several selected internal modes in high-pressure Raman spectra further confirmed the NLC. DFT calculations demonstrate that the NH···O hydrogen bonding “wine-rack” motifs result in the NLC along <i>b</i>-axis in ammonium oxalate monohydrate. We anticipate the high-pressure study of ammonium oxalate monohydrate may represent a promising strategy for accelerating the pace of exploitation and improvement of NLC materials especially in organic systems

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 Phase Transformations of Zircon-Type LaVO₄ Nanorods

A combination of synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy has been used to study high-pressure behavior of the zircon-type LaVO₄ nanorods. In situ high-pressure XRD results identified an irreversible zircon-to-monazite phase transition at ∼5 GPa and a reversible transition to an undetermined second high-pressure phase (phase III) at ∼12.9 GPa. Through Le Bail refinements of the XRD patterns with zircon-type structure, we show that the zircon-type LaVO₄ nanorods possess the smallest bulk modulus among zircon-type rare-earth orthovanadates. Furthermore, negative pressure coefficients of external translational T­(E_g) and internal υ₂(B_2g) bending modes have been observed in Raman measurements. The Raman spectra of phase III with distinctive features have been fully recorded for the first time, and a related structure associated with a coordination increase for V is suggested in terms of the postmonazite phase in LaVO₄ nanorods. Finally, analysis of the transmission electron microscopy both before and after compression indicates that a large number of nanorods can be recovered in the quenched samples, allowing us to verify the orientation relationship for zircon-to-monazite phase transformation

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