At the 26 th AIRAPT conference in 2017, a task group was formed to work on an International Practical Pressure Scale (IPPS). This report summarizes the activities of the task group toward an IPPS ruby gauge. We have selected three different approaches to establishing the relation between pressure (P) and ruby R1-line shift (Δλ) with three groups of optimal reference materials for applying these approaches. Using a polynomial form of the second order, the recommended ruby gauge (referred as Ruby2020) is expressed by: P[GPa] = 1.87(+0.01) × 10 3 Dl l 0 1 + 5.63(+0.03) Dl l 0 ,where λ 0 is the wavelength of the R1-line near 694.25 nm at ambient condition. In June of 2020, the Executive Committee of AIRAPT endorsed the proposed Ruby2020. We encourage highpressure practitioners to utilize Ruby2020 within its applicable pressure range (up to 150 GPa), so that pressure data can be directly compared across laboratories and amended consistently as better scales emerge in the future.

In ethynylbenzene, elevated pressure favors cooperative ⋮CH···π(C⋮C) hydrogen bonds compared to ⋮CH···π(arene) and leads to the ordered polymorph nearly isostructural form with the crystals frozen by cooling. The high-pressure polymorph is more resistant to polymerization than the liquid and, in this respect, is a manifestation of Le Chatelier principle explained at the microscopic level for this compound. A new type of the polymorphic relation has been described.

The siliceous zeolite TON with a 1-D pore system was studied at high pressure by X-ray diffraction, infrared spectroscopy, and DFT calculations. The behavior of this material was investigated using nonpenetrating pressure-transmitting media. Under these conditions, a phase transition from the Cmc21 to a Pbn21 structure occurs at close to 0.6 GPa with doubling of the primitive unit cell based on Rietveld refinements. The pores begin to collapse with a strong increase in their ellipticity. Upon decreasing the pressure below this value the initial structure was not recovered. DFT calculations indicate that the initial empty pore Cmc21 phase is dynamically unstable. Irreversible, progressive pressure-induced amorphization occurs upon further increases in pressure up to 21 GPa. These changes are confirmed in the mid- and far-infrared spectra by peak splitting at the Cmc21 to Pbn21 phase transition and strong peak broadening at high pressure due to amorphization.

Black phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

Polar ordering has been induced by pressure in solid chloroform (trichloromethane), CHCl3, and bromoform (tribromomethane), CHBr3, obtained by isochoric and isothermal freezing in a diamond anvil cell. Structures of these new polymorphs have been determined by single-crystal X-ray diffraction, CHCl3 at 0.62 and 0.75 GPa and CHBr3 at 0.20 and 0.35 GPa. Despite different centrosymmetric structures of all low-temperature phases of CHCl3 (space group Pbcn) and CHBr3 (P6(3)/m, P1, and P3), the high-pressure phases are isostructural in space group P6(3). The polar phase of CHBr3 is formed at 295 K, already at the freezing pressure of approximately 0.1 GPa, while CHCl3 transforms from the Pbcn phase into the P6(3) phase between 0.62 and 0.75 GPa. It has been demonstrated that the electrostatic contribution to halogen...halogen and H...halogen interactions in the CHCl3 and CHBr3 molecular crystals is favorable for the polar aggregation and that this effect intensifies with increasing pressure.

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp 3 carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene co-crystals, naphthalene:octafluoronaphthalene (NOFN) and anthracene:octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study 2 shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22 % before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8 %) and 25 GPa in AOFN (8.7 %) indicates the pressure-induced breakdown of π−stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating a onedimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons, are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit cell parameters. We show that the polymerization takes place along the arene−perfluoroarene π−stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

Crystal structure of arsenolite, the cubic polymorph of arsenic(III) oxide, has been determined by single crystal X-ray diffraction up to 30 GPa. The bulk of the crystal is monotonically compressed with no detectable anomalies, to 60% of the initial volume at 30 GPa. In the structure the most compressed are As•••As contacts which contrasts with increased intramolecular As•••As distance in the deformed molecule. The ratio between As•••As inter-and intramolecular distances decreases from 1.47 at 0.1 MPa to 1.03 at 30 GPa. The As4O6 molecules are deformed to become more tetrahedron-like. Pressure above 3 GPa favours the formation of As4O6•2He inclusion compound in the surface layer increasingly deeper with pressure. The experimental As4O6 crystal compression has been compared with various theoretical models within the DFT framework. According to band-structure calculations the arsenolite band gap falls from 4.2 eV at ambient pressure to 2.7 eV at 27.8 GPa.

Transformations of NH•••N hydrogen bonds can be employed in new classes of electronic materials. Imidazole, a prototypic NH•••N bonded crystal, remains centrosymmetric when compressed to 2.7 GPa. However, its recrystallization at 1.2 GPa leads to a new polar phase which in turn can be decompressed to 0.5 GPa at least. This transformation and calculated potential-energy function of the H-atom correlate with the transforming dimensions of the hydrogen bond and reveal high polarity of nonionic NH•••N bonded compounds.