Local structural origins of the distinct electronic properties of Nb-substituted SrTiO$_3$ and BaTi$O_3$

TL;DR

This study reveals that subtle local structural differences in Nb-substituted SrTiO$_3$ and BaTiO$_3$ explain their contrasting electronic behaviors, with SrTiO$_3$ showing distorted non-cubic structure and BaTiO$_3$ exhibiting local cation off-centering.

Contribution

The paper uncovers the critical local structural distinctions responsible for the different electronic properties in Nb-doped SrTiO$_3$ and BaTiO$_3$, despite their similar structures.

Findings

SrTi$_{0.875}$Nb$_{0.125}$O$_3$ has a distorted non-cubic structure at 15 K.

BaTi$_{0.875}$Nb$_{0.125}$O$_3$ shows local cation off-centering.

The structural differences underpin the contrasting electronic behaviors.

Abstract

Near or less than 10% Nb substitution on the Ti site in perovskite SrTiO$_3$ results in metallic behavior, in contrast to what is seen in BaTiO$_3$. Given the nearly identical structure and electron counts of the two materials, the distinct ground states for low substitution have been a long-standing puzzle. Here we find from neutron studies of average and local structure, the subtle yet critical difference that we believe underpins the distinct electronic properties in these fascinating materials. While SrTi$_{0.875}$Nb_${0.125}$O$_3$ possesses a distorted non-cubic structure at 15 K, the BO$_6$ octahedra in the structure are regular. BaTi$_{0.875}$Nb$_{0.125}$O$_3$ on the other hand shows evidence for local cation off-centering whilst retaining a cubic structure.