Extinction-free electron diffraction refinement of bonding in SrTiO ₃

Accurate low-order Fourier coefficients of the crystal potential of SrTiO₃ are measured by quantitative convergent-beam electron diffraction. The accuracy in the corresponding derived X-ray structure factors is about 0.1% for the strong low-order reflections (sin θ/λ < 0.3 Å^-1). This accuracy is better than for conventional X-ray diffraction and equivalent to the accuracy of the X-ray Pendellösung method. Combination of these structure factors with high-order X-ray diffraction measurements allows accurate bonding information to be obtained from a multipole model fitted to the experimental data. It is shown that Ti-O has a covalent component and that the Sr-O bond is mainly ionic. The role of Ti 3d electrons in Ti-O bonding is also discussed.