Fig. 1: Crystal structures and dynamic stability.

Top and side views of X₂NO₂ monolayer with (a) fcc, (b) FE-ZB′ and (c) FE−WZ′ phase (dashed lines denote a unit cell). The phonon spectra of Tl₂NO₂ monolayer with (d) fcc, (e) FE-ZB′ and (f) FE−WZ′ phase.