Theoretical exploration of Be Ag(II) F phases and their magnetic properties using learning algorithms

Abstract

The search for novel silver(II) fluorides is driven by their potential as electronic and magnetic analogues to high temperature cuprate(II) superconductor precursors. Here, we explore the previously uncharted Be Ag(II) F chemical space using global structure prediction algorithms combined with first principles calculations. Focusing on the AgBeF4 stoichiometry, we identify the five lowest enthalpy polymorphs crystallizing in the C2, P minus 1, and P 21/c space groups. All polymorphs show an antiferromagnetic ground state, with AgBeF4_4 and AgBeF4_5 exhibiting unprecedented strong superexchange interactions of J equal circa to minus 460meV and J equal circa to minus 359meV respectively. Those high J values are due to the presence of either [Ag2F7] for AgBeF4_4, or related infinite [AgF2/2+2/1]2 minus chains for AgBeF4_5. Although the phases are found to be metastable with respect to binary difluorides, the thermodynamic analysis suggests that they could be targeted via synthetic routes employing fluorine radicals, with reaction enthalpies reaching minus 370 kJ/mol.