Rare case of magnetic Ag$^{3+}$ ion: double perovskite   Cs$_{2}$KAgF$_{6}$
  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$^{3+}$. 
Ting Jia, Xiaoli Zhang, Ting Liu, Fengren Fan, Zhi Zeng, X. G. Li, D.   I. Khomskii, and Hua Wu
cond-mat.str-el