New Ideas for Understanding the Structure and Magnetism in AgF2: Prediction of Ferroelasticity.

In the search for new high-temperature superconductors it has been proposed that there are strong similarities between the fluoroargentate AgF2 and the cuprate La2CuO4. As a novelty we explore the origin of the possible layered structure of AgF2 by studying its parent high-symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF2 . Our findings first stress the large differences between CuF2 and AgF2 . Indeed, the parent structure of AgF2 is found to be cubic, naturally devoid of any layering, even though Ag2+ ions occupy trigonal sites which, nevertheless, allow the existence of a Jahn-Teller effect. The observed Pbca orthorhombic phase is found when the system is cooperatively distorted by a local Exe trigonal Jahn-Teller effect around the silver sites that creates both the geometrical and magnetic layering. While the distortion implies that two Ag2+ -F- bonds increase their distance by 15% and become softer, our simulations indicate that covalent bonding and interlayer electron hopping is strong, unlike the situation in cuprate superconductors and that, in fact, exfoliation of individual planes may be a harder task than previously suggested. As a salient feature, the present results prove that the actual magnetic structure in AgF2 is a direct consequence of vibronic contributions involved in the Jahn-Teller effect. Finally, our findings show that, due to the multiple minima intrinsic to the Jahn-Teller energy surface, the system is ferroelastic, a property that is strongly coupled to magnetism in this argentate.