The structural stability of hexagonal tungsten mononitride (WN) has been studied combining scanning transmission electron microscopy and first-principles calculations. The results show that the WC-type WN with vacancies of 6∼8 at% is more stable than the previously proposed MnP-type and NiAs-type structures. Due to the larger vibrational entropy of the WC-type WN, the vacancy concentration required to stabilize the WC-type structure is lower at high temperatures. The results demonstrate the importance of vacancies and configurational and vibrational entropies in the structural stability of compounds synthesized at high temperatures.

The recent discovery of stable FeO2 under high pressures has aroused great interest in materials science, suggesting the existence of the 4+ oxidation state of late transition metals in their binary oxides. Compared with other cobalt and nickel compounds, reports on CoO2 and NiO2 are limited, and even the oxidation states of Co and Ni in CoO2 and NiO2 are the subject of debate in theoretical works. Herein, PbO2‐type CoO2 and pyrite‐type NiO2 are predicted using density functional theory calculations. The equations of state and relative enthalpies with known compounds are determined, showing that the PbO2‐type CoO2 and pyrite‐type NiO2 can be synthesized above 40 and 43 GPa, respectively. The calculations of elasticity and phonon spectra have confirmed their mechanical and dynamical stabilities at 0 GPa. The band structures and densities of states show that PbO2‐CoO2 is half metallic and pyrite‐NiO2 is semiconducting.