17O quadrupole coupling and the origin of ferroelectricity in isotopically enrichedBaTiO3 and SrTiO3

Abstract

The 17O electric field gradient (EFG) and chemical shift tensors have been determinedin the paraelectric and ferroelectric phases of isotopically enrichedBaTiO3 andSrTiO3 singlecrystals via 17O NMR. This is the first determination of the17O EFG and chemical shift tensors in any oxide perovskite crystal. The difference in the17O chemicalshifts between BaTiO3 and SrTiO3 is relatively small, whereas there is a large difference in the17O EFGtensors even in the cubic phases. Density functional theory calculations indicate that the difference inthe 17O EFGs is due to a crystal cell volume effect together with effects due to the difference in ionic radii ofBa and Sr that result in a larger distortion of the nearly spherical charge distribution around the oxygenin BaTiO3.