${\mathrm{WN}}_6$ phase discovered at 126–165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group $R\overline{3}m$ which is consistent with the ${\mathrm{WN}}_6$ structure with armchairlike hexazine (${\mathrm{N}}_6$) rings, while strong $A_{1g}$ Raman mode confirms its $\mathrm{N}─\mathrm{N}$ single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent ${\mathrm{N}}_6$ rings, and repulsions between ${\mathrm{N}}_6$ rings that make ${\mathrm{WN}}_6$ ultrastiff and tough. The ${\mathrm{WN}}_6$ phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

Crystal structure prediction methods and first-principles calculations have been used to explore low-energy structures of carbon monoxide (CO). Contrary to the standard wisdom, the most stable structure of CO at ambient pressure was found to be a polymeric structure of $Pna{2}_1$ symmetry rather than a molecular solid. This phase is formed from six-membered (four carbon + two oxygen) rings connected by C=C double bonds with two double-bonded oxygen atoms attached to each ring. Interestingly, the polymeric $Pna{2}_1$ phase of CO has a much higher energy density than trinitrotoluene (TNT). On compression to about 7 GPa, $Pna{2}_1$ is found to transform into another chainlike phase of $Cc$ symmetry which has similar ring units to $Pna{2}_1$. On compression to 12 GPa, it is energetically favorable for CO to polymerize into a purely single bonded $Cmca$ phase, which is stable over a wide pressure range and transforms into the previously known $Cmcm$ phase at around 100 GPa. Thermodynamic stability of these structures was verified using calculations with different density functionals, including hybrid and van der Waals corrected functionals.