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

A Protocol to Fabricate Nanostructured New Phase: B31-Type MnS Synthesized under High Pressure

Synthesis of nanomaterials with target crystal structures, especially those new structures that cannot be crystallized in their bulk counterparts, is of considerable interest owing to their strongly structure-dependent properties. Here, we have successfully synthesized and identified new-phase nanocrystals (NCs) associated with orthorhombic MnP-type (B31) MnS by utilizing an effective high-pressure technique. It is particularly worth noting that the generated new structured MnS NCs were captured as expected by quenching the high-pressure phase to the ambient conditions at room temperature. Likewise, the commercially available bulk rocksalt (RS) MnS material underwent unambiguously a reversible phase transition when the pressure was released completely. First-principles calculations further supported that the B31-MnS was more energetically preferable than the RS one under high pressure, which can be plausibly interpreted by the structural buckling with respect to zigzagged arrangements within B31 unit cell. Our findings represent a significant step forward in a deeper understanding of the high-pressure phase diagram of MnS and even provide a promising strategy to prepare desired nanomaterials with new structures that do not exist in their bulk counterparts, thus greatly increasing the choice of materials for a variety of applications

Discovery of High-Pressure Polymorphs for a Typical Polymorphic System: Oxalyl Dihydrazide

The variation of pressure is an effective experimental technique to explore new polymorphs of organic crystals. At ambient condition, oxalyl dihydrazide (C₂N₄O₂H₆, ODH) exhibits five polymorphs: α, β, δ, γ, and ε. Here we report the high-pressure response of the existed five forms of ODH by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 20 GPa. High-pressure experimental results show that all five polymorphs undergo phase transitions to new phases at different pressures, respectively. We propose that the special molecular conformation yields several geometric constructions for hydrogen-bonding arrangements. The detailed mechanisms of the phase transition and the high-pressure behaviors of the polymorphs are analyzed by considering molecular stacking

Size-Controlled Synthesis of Bifunctional Magnetic and Ultraviolet Optical Rock-Salt MnS Nanocube Superlattices

Wide-band-gap rock-salt (RS) MnS nanocubes were synthesized by the one-pot solvent thermal approach. The edge length of the nanocubes can be easily controlled by prolonging the reaction time (or aging time). We systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. Furthermore, these RS-MnS nanocubes could spontaneously assemble into ordered superlattices via the natural cooling process. The optical and magnetic properties were investigated using measured by UV–vis absorption, photoluminescence spectra, and a magnetometer. The obtained RS-MnS nanocubes exhibit good ultraviolet optical properties depending on the size of the samples. The magnetic measurements suggest that RS-MnS nanocubes consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperatures. Furthermore, the hysteresis measurements indicate these RS-MnS nanocubes have large coercive fields (e.g., 1265 Oe for 40 nm nanocubes), which is attributed to the size and self-assembly of the samples

Discovery of High-Pressure Polymorphs for a Typical Polymorphic System: Oxalyl Dihydrazide

The variation of pressure is an effective experimental technique to explore new polymorphs of organic crystals. At ambient condition, oxalyl dihydrazide (C₂N₄O₂H₆, ODH) exhibits five polymorphs: α, β, δ, γ, and ε. Here we report the high-pressure response of the existed five forms of ODH by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 20 GPa. High-pressure experimental results show that all five polymorphs undergo phase transitions to new phases at different pressures, respectively. We propose that the special molecular conformation yields several geometric constructions for hydrogen-bonding arrangements. The detailed mechanisms of the phase transition and the high-pressure behaviors of the polymorphs are analyzed by considering molecular stacking

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

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

Polymorphism and Formation Mechanism of Nanobipods in Manganese Sulfide Nanocrystals Induced by Temperature or Pressure

Manganese sulfide (MnS) nanocrystals (NCs) with three different phases were synthesized by one-pot solvent thermal approach. The crystal structures and morphologies were investigated using powder X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. We found that the crystal structure and morphology of MnS NCs could be controlled by simply varying the reaction temperature. The detailed growth process of MnS nanobipods, including the zinc blende (ZB)-core formation and wurtzite (WZ)-arms growth, provides direct experimental evidence for the polymorphism model. Furthermore, we have studied the stability of metastable ZB- and WZ-MnS NCs under high pressure and found that ZB-nanoparticles and ZB/WZ-nanobipods are stable below their critical pressure, 5.3 and 2.9 GPa, respectively. When pressures exceed the critical point, all these metastable MnS NCs directly convert to the stable rock salt MnS

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