X-ray photoelectron spectroscopy characterization of fluorite and perovskite phases in Sr1-xBi2+yTa2O9-z films

High-resolution x-ray photoelectron spectroscopy of strontium bismuth tantalate films revealed distinct chemical states of constituent atoms in fluorite and perovskite crystals. Reflecting the number of nearest-neighbor oxygen atoms coordinated with Bi3+ ions, the Bi 4f peak could be deconvoluted into Bi2O3 (sixfold coordination), Bi2O2 (fourfold coordination), and (Bi2O2)2+ (intermediate between sixfold and fourfold coordination) components. We found that amorphous and fluorite phases could be expressed as a mixture of Bi2O3 and Bi2O2, whereas the (Bi2O2)2+ component representing the bismuth oxide layer, being adjacent to the (SrTa2O7)2- block, predominated in the perovskite phase. Similarly, the Sr 3d peak could be deconvoluted into the components of Sr oxide (SrO) and Sr in (SrTa2O7)2-. The volume fraction of the (SrTa2O7)2- component increased at higher annealing temperatures. The Ta 4d peaks, in contrast, located at the same binding energy for all samples, indicated that an energetically stable TaO6 octahedron unit is preferentially created. The binding energies of the O 1s state corresponding to O-Bi, O-Ta, and O-Sr bonds were identified at 531, 530, and 528.9 eV, respectively.