Rare case of magnetic Ag$^{3+}$ ion: double perovskite Cs$_{2}$KAgF$_{6}$

TL;DR

This study predicts a rare high-spin S=1 magnetic state for Ag$^{3+}$ in the double perovskite Cs$_{2}$KAgF$_{6}$ using first-principles calculations, highlighting the role of lattice structure in stabilizing this state.

Contribution

First-principles calculations reveal a rare high-spin state of Ag$^{3+}$ in Cs$_{2}$KAgF$_{6}$, challenging typical low-spin expectations for such ions.

Findings

High-spin S=1 state for Ag$^{3+}$ in Cs$_{2}$KAgF$_{6}$

Low-spin tetragonal structure is less stable energetically

Lattice structure stabilizes the high-spin ground state

Abstract

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 Ag$^{3+}$ ion in the double perovskite Cs$_{2}$KAgF$_{6}$. We also explored a possibility of a conventional low-spin $S$=0 ground state and find an associated tetragonal distortion to be 0.29 {\AA}. 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 Cs$_{2}$KAgF$_{6}$ is an important factor in stabilizing the unusual high-spin ground state of Ag$^{3+}$.