Rare case of magnetic Ag3+ ion: Double perovskite Cs2KAgF6

Normally 4d or 5d transition metals are in a low-spin state. Here using first-principles calculations, we report on a rare case of a high-spin S = 1 magnetic state for the Ag3+ ion in the double perovskite Cs2KAgF6. We also explored a possibility of a conventional low-spin S = 0 ground state and find an associated tetragonal distortion to be 0.29 angstrom. However, the lattice elastic energy cost and the Hund exchange loss exceed the e(g) crystal-field energy gain, thus making the low-spin tetragonal structure less favorable than the high-spin cubic structure. We conclude that the compact perovskite structure of Cs2KAgF6 is an important factor in stabilizing the unusual high-spin ground state of Ag3+.