Polymorphism of two-dimensional antiferromagnets, AgF2 and CuF2

We present a theoretical study of relative stability, as well as of the magnetic and electronic properties, of AgF2 and CuF2, in two related structural forms: orthorhombic (ambient pressure form of AgF2) and monoclinic (ambient pressure form of CuF2), using density functional theory. We demonstrate that the P21/c -> Pbca structural transition is associated with a weakening of the intrasheet magnetic superexchange (|J2D|). This finding aligns with the flattening of two-dimensional (2D) layers, a smaller charge -transfer energy, and a stronger admixing of Agd/Cud-Fp states in the monoclinic structure, compared to the orthorhombic form. Consequently, monoclinic AgF2 should be targeted in experiments as it is likely to exhibit stronger magnetic coupling than its orthorhombic counterpart. The dynamically stable P21/c form of AgF2 could be achieved through two alternative paths: by applying negative strain or by rapidly quenching silver(II) difluoride from temperatures above 480 K to low temperatures.