Defect-engineered room-temperature ferromagnetism in quasi-two-dimensional nitrided Co Ta 2 O 6

Thermal ammonolysis of quasi-two-dimensional (quasi-2D) $\mathrm{Co}{\mathrm{Ta}}_{2}{\mathrm{O}}_{6}$ yields the ${\mathrm{O}}^{2\ensuremath{-}}/{\mathrm{N}}^{3\ensuremath{-}}$ and anionic vacancy-ordered ${\mathrm{Co}}^{2+}{\mathrm{Ta}}_{2}^{5+}{\mathrm{O}}_{6\ensuremath{-}x}{\mathrm{N}}_{2x/3}{\ensuremath{\square}}_{x/3}$ ($x\ensuremath{\le}0.15$), which exhibits a transition from antiferromagnetism to defect-engineered above-room-temperature ferromagnetism. First-principles calculations reveal that the origin of ferromagnetism is a particular $\mathrm{Co}{\mathrm{O}}_{5}\mathrm{N}$ configuration with N located at Wyckoff position $8j$, which breaks mirror symmetry about the $ab$ plane. A pressure-induced electronic phase transition is also predicted at $\ensuremath{\sim}24.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, accompanied by insulator-to-metal transition and magnetic moment vanishing.