Fig. 1. Novel states of materials occur under a wide range of extreme pressures, which include methane hydrates at 0.1 GPa, diamond at 5 GPa, symmetric ice at 80 GPa, metallic hydrogen at 500 GPa, and Al electrides at 10 TPa. These pressures can be generated by modern high-pressure technologies in static and dynamic conditions (Fig. 2).

Fig. 2. (a) Modern high-pressure technologies, such as DAC, dynamic-DAC, gas guns, Z-machine, and NIF are capable of generating the extreme PT conditions in the deep interiors of the giant planets at different time scales. (b) Energy vs reaction coordination diagram, comparing kinetically constrained chemical processes between shock- and static-compressed materials, leading to decomposition and polymerization, respectively.

Fig. 3. A concept of pressure-induced chemistry in a 2D lattice, illustrating the evolution of chemical bonding and structure with increasing pressure, from weakly bound molecular solids to covalently bonded extended solids and ionic solids. These transformations are primarily driven by pressure-induced densification, promoting the delocalization and even ionization (or localization) of electrons from both valence and core states [Reproduced with permission from C.-S. Yoo, MRS Bull. 42, 724 (2017). Copyright 2017 Cambridge University Press].

Fig. 4. A thermodynamic representation of pressure-induced electron delocalization to high density, high energy extended solids. V_o signifies the specific volume of molecular solid under ambient conditions, whereas V_ES and V_MS represent the molecular solid and extended solid at the transition pressure. The pressure (or energy) offset of the transition (2 → 3) from the equilibrium value (the green line) signifies the presence of a large activation barrier. A similar kinetic barrier in the backward transition (3 → 4), on the other hand, makes it possible to recover the high-energy, high-density product under ambient conditions.

Fig. 5. Novel properties observed in low-Z extended solids: (a) Second harmonic generation of silica-like CO₂-V. (b) Superconductivity of dense CS₂. (c) High energy density of polymeric CO. (d) Colossal Raman scattering of layered polymeric nitrogen (LP-N) [Reproduced with permission from C.-S. Yoo, MRS Bull. 42, 724 (2017). Copyright 2017 Materials Research Society].

Fig. 6. Crystal structures of dense low-Z solids under extreme conditions, showing the pressure-induced ionization of 3D covalent bonds to 2D ionic solids in: (a) N₂ from the molecular ε(r-3m) phase to the singly bonded phase, 3D cg-N (I2₁3) at 110 GPa, and 2D layered polymeric LP-N (Pba2) at 150 GPa. (b) CO from 1D linear chain phase I (P2₁/m) to 3D ladder phase II (P2₁2₁2₁) and 2D layered phase III (Cmcm). (c) H₂O from 3D symmetric ice X (Pn-3m) at 80 GPa to distorted P2₁ phase at 1 TPa and 2D layered C2/m at 2 TPa. (d) NaCl from an oC8 phase at 320 GPa to oI8 at 650 GPa and oP14 above 700 GPa.

Fig. 7. A conceptual phase/chemical transformation diagram of molecular solids, signifying the pressure and temperature induced ionization leading to nonmolecular polymeric, metallic, and ionic solids. A delicate balance between the compression energy (PΔV) and the entropic energy (TΔS) under extreme pressure-temperature conditions gives rise to novel states and transitions of materials in both solids and liquids, which are strongly controlled by chemical kinetics. The compression energy at 100 GPa (∼10 eV) can strongly modify the valence electron configuration or chemical bonds of molecular solids; at 1 TPa (∼100 eV) it can even alter the core electron configuration and induce a new type of chemistry never experienced in the past.

Fig. 8. Various ice crystals, stable and metastable, formed along different thermal and kinetic path under rapidly modulating pressures using dynamic-DAC, including (a) single crystal ice VI (b) dendritic ice VI (c) metastable ice VII produced in the stability field of ice VI, and (d) high density amorphous (HDA) ice produced in no man’s land.

Fig. 9. Photographic images of polymeric carbon monoxide (pCO) products under ambient conditions, recovered after synthesis at high pressures: (a)–(c) pure pCO synthesized at 8, 9, and 10 GPa. The images show the presence of two polymeric products (I and II) (d) and (e) 10% H₂ doped pCO synthesized at 6 and 7 GPa, showing lower synthetic pressures than pure CO and strong luminescence [Reprinted with permission from Y. J. Ryu et al., J. Phys. Chem. C 121, 10078 (2017). Copyright (2017) American Chemical Society].

Fig. 10. Pressure-volume compression curves of extended (red circles) and molecular (green triangles and yellow squares) CON₂ phases in comparison with those of cg-N (blue line), LP-N (brown), δ-N₂ (yellow), and ε-N₂ (green). The inset shows the crystal structure of P4₃ solid, consisting of four-fold coordinated carbon atoms (brown), three-fold nitrogen (gray), and two-fold oxygen (red) in a three-dimensional framework structure [Reprinted with permission from C.-S. Yoo et al., J. Phys. Chem. C 122, 13054 (2018). Copyright (2017) American Chemical Society].

Fig. 11. Phase diagram of Fe(CO)₅, consisting of liquid, three molecular phases (noted as I, II, and III), and a polymeric product. Note that unreacted Fe(CO)₅ is stable within a limited pressure-temperature range, reflecting the weakness of Fe–CO back bonds with increasing temperatures. The inset shows the lustrous appearance of recovered Fe(CO)₅ polymer, coming from a graphite-like 2D carbon-oxygen polymer on the surface [Reprinted with permission from Y. J. Ryu et al., Sci. Rep. 5, 15139 (2015). Copyright 2015 Macmillan Publishers Ltd.].