Title: Polymorphism of two-dimensional antiferromagnets, AgF2 and CuF2

Abstract: We present 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 show, that at P21/c --> Pbca structural transition is associated with weakening of intra-sheet magnetic superexchange. This finding is consistent with the flattening of 2D layers, smaller charge-transfer energy and stronger admixing of Agd/Cud-Fp states in monoclinic structure, comparing to orthorhombic form. Consequently, monoclinic AgF2 should be targeted in experiment as it should show stronger magnetic coupling than its orthorhombic sibling. The dynamically stable P21/c form of AgF2 could be achieved via two alternative paths: by applied negative strain, or by rapid quenching silver(II) difluoride from temperatures higher than 480 K to low temperatures.