Cobalt in strontium titanate as a new off-center magnetic impurity

TL;DR

This study investigates cobalt impurities in SrTiO3, revealing their local structure, oxidation states, and off-center displacement, with implications for magnetic properties and synthesis control.

Contribution

It provides detailed analysis of cobalt's local structure, oxidation states, and off-center displacement in SrTiO3, highlighting how synthesis conditions influence impurity site occupancy.

Findings

Cobalt predominantly substitutes the A site under certain synthesis conditions.

Cobalt's oxidation state is +2 at the A site and +3 at the B site.

The Co impurity at the A site is off-center by approximately 1.0 Å.

Abstract

The local structure and oxidation state of the cobalt impurity in SrTiO$_3$ is studied by X-ray absorption fine structure (XAFS) spectroscopy. The synthesis conditions, under which cobalt predominantly (up to 76%) substitutes the atoms at the $A$ site of the perovskite structure, is found for SrTiO$_3$(Co) samples. By varying the synthesis conditions, it is possible to appreciably change the ratio between the concentrations of cobalt atoms incorporated into the $A$ and $B$ sites. It is established that the oxidation state of cobalt is +2 at the $A$ site and +3 at the $B$ site. It is revealed that the Co impurity at the $A$ site is off-center, and its displacement from the lattice site is $\sim$1.0 {\AA}. First-principles calculations show that an isolated Co$^{3+}$ ion at the $B$ site is diamagnetic, whereas the Co$^{2+}$ ion at the $A$ site is in a high-spin state ($S = 3/2$).