High-Pressure Synthesis of the β−Zn3N2β-Zn_{3}N_{2} Nitride and the α−ZnN4α-ZnN_{4} and β−ZnN4β-ZnN_{4} Polynitrogen Compounds

High-pressure nitrogen chemistry has expanded at a formidable rate over the past decade, unveiling the chemical richness of nitrogen. Here, the $Zn-N$ system is investigated in laser-heated diamond anvil cells by synchrotron powder and single-crystal X-ray diffraction, revealing three hitherto unobserved nitrogen compounds: $β-Zn_{3}N_{2}$, $α-ZnN_{4}$, and $β-ZnN_{4}$, formed at 35.0, 63.5, and 81.7 GPa, respectively. Whereas $β-Zn_3N_2$ contains the $N^{3–}$ nitride, both $ZnN_4$ solids are found to be composed of polyacetylene-like [$N_4$]$_∞$$^{2–}$ chains. Upon the decompression of $β-ZnN_4$ below 72.7 GPa, a first-order displacive phase transition is observed from $β-ZnN_4$ to $α-ZnN_4$. The $α-ZnN_4$ phase is detected down to 11.0 GPa, at lower pressures decomposing into the known $α-Zn_3N_2$ (space group Ia3̅) and $N_2$. The equations of states of $β-ZnN_4$ and $α-ZnN_4$ are also determined, and their bulk moduli are found to be $K_0$ = 126(9) GPa and $K_0$ = 76(12) GPa, respectively. Density functional theory calculations were also performed and provide further insight into the $Zn-N$ system. Moreover, comparing the $Mg-N$ and $Zn-N$ systems underlines the importance of minute chemical differences between metal cations in the resulting synthesized phases