The defect structure of SrTi1-xFexO3-y (x=0-0.8) investigated by electrical conductivity measurements and electron energy loss spectroscopy (EELS)

The electrical properties and defect structure of selected compositions in the SrTi1-xFexO3-y system (x = 0-0.8) have been studied using van der Pauw 4-point conductivity measurements and electron energy loss spectroscopy (EELS). Using X-ray powder diffraction and selected area electron diffraction (SAD), the basic crystal structure was determined to be cubic perovskite for all the investigated compositions. A superstructure cell 2.2.1 times the ordinary perovskite cell was found in materials with x = 0.6 and x = 0.8. The conductivity was measured on sintered tablets as a function of the partial pressure of oxygen (pO(2) = 10(-25) to 1 atm) at 600-1100 degrees C. The materials investigated are predominantly p-type electronic conductors at high, n-type conductors at low, and ionic conductors at intermediate oxygen partial pressures. All conductivity contributions increase with increasing iron content. This can be attributed to the acceptor role of the iron, decreased band gap and decreased activation energy for oxygen vacancy migration. The EELS spectra show a shoulder below the oxygen K-edge, increasing in magnitude with increasing iron content and oxygen partial pressure. This shoulder is assigned to empty electron energy states at some of the oxygen atoms, indicating that electron holes are associated with lattice oxygen in the structure. Spectra from the Fe L-edge showed small changes, suggesting that there are few or no empty states at the iron atoms. (C) 1997 Elsevier Science Ltd.