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With recent advancements in high‐pressure technologies, combined with synchrotron X‐ray diffraction, Raman spectroscopy, and density functional theory calculations, the study of crystal structures under high‐pressure conditions has progressed rapidly. Among various chemical systems, nitrides have been extensively investigated due to their potential applications as superhard materials, high‐energy‐density materials, and superconductors. In this review, we summarized the crystal structures and nitrogen polymerization behavior in nitrides synthesized in high‐pressure experiments. This overview aims to facilitate the design of new nitrides and enhance understanding of the formation pathways and structural diversity of polynitride compounds. 

Abstract The insulator‐to‐metal transition in dense fluid hydrogen is an essential phenomenon in the study of gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using direct fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, an onset of metal‐like reflectance is observed in the visible spectral range atP>150 GPa andT≥ 3000 K. The reflectance increases rapidly with decreasing photon energy indicating free‐electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectance spectra also suggest much longer electronic collision time (≥1 fs) than previously inferred, implying that metallic hydrogen at the conditions studied is not in the regime of saturated conductivity (Mott–Ioffe–Regel limit). The results confirm the existence of a semiconducting intermediate fluid hydrogen state en route to metallization.