Strongly coupled thermal and chemical expansion in the perovskite oxide system Sr(Ti,Fe)O_3-α

The thermochemical expansion behavior of the mixed ionic and electronic conducting perovskite SrTi_1-xFe_xO_3-α (x = 0.05, 0.35) was characterized to evaluate its potential stability in operating electrochemical devices and to quantify the coupling between oxygen stoichiometry changes and lattice expansion in this material. Changes in oxygen stoichiometry and dimensions of dense specimens were measured using thermogravimetric analysis and dilatometry, respectively, over a range of temperatures up to 1000 °C and oxygen partial pressures down to 10^-4 atm, both under steady-state conditions and during thermal or oxygen activity excursions. The separate isothermal chemical expansion and iso-stoichiometric thermal expansion contributions to overall expansion were thereby deconvoluted. For x = 0.35, the isothermal coefficient of chemical expansion (CCE) was found to increase with increasing temperature (CCE = 3.0(3) × 10^-5 × T + 1.9(2) × 10^-2; T is in °C), while the iso-stoichiometric coefficient of thermal expansion (CTE) increased with increasing oxygen deficiency (CTE = -3.00 × 10^-5 × δ + 1.44 × 10^-5, per °C, where δ = x/2 - α). From these relationships, and the measured change in oxygen stoichiometry upon heating, the overall thermo-chemical expansion during heating in air was successfully modeled. The effective size of a vacant anion site was also estimated as a function of temperature from the measured CCEs and found to be comparable to that of an occupied anion site at high temperatures (≥900 °C). For x = 0.05, the changes in oxygen stoichiometry and expansion were smaller over a given temperature or oxygen partial pressure range, though the CCE appeared to be larger than for x = 0.35. Greater charge localization may be responsible for the higher CCE of the sample with the lower Fe content.