Local and electronic structure of Sr_1-xGd_xTiO₃ probed by X-ray absorption spectroscopy

Highlights: • Gd-doped SrTiO₃ powders were probed by X-ray absorption spectroscopy. • EXAFS at Gd L_III-edge shows regular GdO₁₂ without displacement of Gd atoms. • Disorder was observed beyond the first 12 O where Pm-3m symmetry does not fit well. • XANES at Ti L_III,II-edges show an asymmetric peak because of Ti displacement. • XANES at O K-edge depend on the hybridization between O 2p and Gd 5d4f6s states. -- Abstract: Gadolinium-doped strontium titanate (Sr_1-xGd_xTiO₃) is a typical perovskite structure material which has been studied due their thermomechanical, termoelectrical and electrochemical properties. In this study, local and electronic structure of Sr_1-xGd_xTiO₃ samples were analyzed through X-ray absorption spectroscopy measurements. The results obtained with the adjustment of extended X-ray absorption fine structure (EXAFS) spectra at Sr K-edge show that crystallographic model of Pm-3m space group is consistent with local structure around Sr atoms, as expected. This same analysis also reveals an increasing of the Debye-Waller as a function of the Gd content in some shells, which is associated with disorder induced by Sr vacancies due to the heterovalent Gd incorporation. EXAFS spectra at Gd L_III-edge for Sr_1-xGd_xTiO₃ samples indicates regular GdO₁₂ dodecahedra without displacement of Gd atoms from centrosymmetric position. A disorder was also identified in the shells beyond the first 12 O neighbors in which neither the crystallographic cubic structure of the SrTiO₃ nor the orthorhombic structure of the GdTiO₃ fits well. X-ray absorption near edge spectroscopy (XANES) spectrum at Ti L_III,II-edges shows an asymmetric peak because of the splitting between the e_g orbitals of 3d band for SrTiO₃ sample. The addition of Gd atoms to SrTiO₃ structure cause an enlargement of this peak and this split is associated with a small displacement of Ti atoms from their centrosymmetric position. Several features of the XANES spectra at O K-edge for Sr_1-xGd_xTiO₃ samples are affected by the increase of Gd concentration. According to our calculated projected density of states, these transitions are related to a reduction in the number of unoccupied O 2p - Ti 3d states caused by the split of Ti 3d band. Moreover, these XANES spectra also show a dependence of the increasing of the hybridization between O 2p and Gd 5d4f6s states.