On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system

TL;DR

This study investigates how the oxidation state of nickel varies in BaTiO3-SrTiO3 solid solutions, revealing a transition from 4+ to 2.5+ with compositional changes, affecting electronic structure and optical properties.

Contribution

It provides new insights into the oxidation state variation of Ni in the BaTiO3-SrTiO3 system and links it to oxygen vacancy formation energies and electronic structure modifications.

Findings

Ni oxidation state changes from 4+ to 2.5+ across the system

Ni creates impurity bands affecting optical absorption

Different oxygen vacancy formation energies explain oxidation state variations

Abstract

XAFS studies of Ni-doped Ba$_{1-x}$Sr$_x$TiO$_3$ solid solution reveal that the Ni oxidation state changes from 4 in SrTiO$_3$ to 2.5 in BaTiO$_3$ when varying $x$. This change is accompanied by a noticeable change in the interatomic Ni-O distances in the first shell. The first-principles calculations show that nickel creates an impurity band in the forbidden band gap of BaTiO$_3$ and SrTiO$_3$, which explains the appearance of intense absorption of Ni-doped samples in the visible region. The analysis of the electronic structure of doped crystals and calculations of the oxygen vacancy formation energy in them show that different oxidation states of Ni in SrTiO$_3$ and BaTiO$_3$ can be explained by different formation energies of the oxygen vacancies in these compounds.