Local Structural Origins of the Distinct Electronic Properties of Nb-Substituted SrTiO3 and BaTiO3

The perovskite SrTiO3 becomes metallic with 0.03% to 0.1% Nb substitution on the Ti site, while BaTiO3 remains insulating above 10% Nb substitution. 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 SrTi0.875Nb0.125O3 possesses a distorted noncubic structure at 15 K, (Nb/Ti)O-6 octahedra in the structure are regular. BaTi0.875Nb0.125O3, on the other hand, shows evidence for local cation off centering while retaining a cubic structure.