Anchoring effect of distorted octahedra on the stability and strength of platinum metal pernitrides

The successful preparation of platinum metal pernitrides ($\mathrm{Pm}{\mathrm{N}}_{2}$) in high-temperature and high-pressure experiments has aroused great scientific interest, since it has long been thought that these systems could not be prepared, and also because of their intriguing mechanical properties. Although it is now widely recognized that $\mathrm{Pm}{\mathrm{N}}_{2}$ phases can be stabilized under high pressure, the physical origin explaining their stability remains unknown. By means of high-throughput first-principles schemes, we reveal that the choice of specific space group of these pernitrides at high pressure can be quantified by the anchoring effect of distorted $\mathrm{Pm}{\mathrm{N}}_{6}$ octahedra. The competition between baddeleyite and marcasite is attributed uniquely to Pm dimerization, resulting in a profound enhancement of Pm-Pm bonding and N-N $\ensuremath{\pi}$ antibonding, while Pm-N bonding plays a secondary role. The observed mechanical strength and atomic deformation mechanism of $\mathrm{Pm}{\mathrm{N}}_{2}$ suggest that they are ultraincompressible yet soft. This is attributed to the breaking of elongated Pm-N bonds in the $\mathrm{Pm}{\mathrm{N}}_{6}$ octahedra, which is accompanied by a continuous semiconductor-semimetal-metal transition for the semiconducting $\mathrm{Pm}{\mathrm{N}}_{2}$ during straining. These findings shed light on the physical origin of high-pressure stabilization and highlight the importance of exploring deformation mechanisms in designing novel strong solids.