Perovskite SrCo0.6Fe0.2Nb0.2O3-z attracts attention as a promising material with high oxygen conductivity. The sample was investigated by means of high-temperature
X-ray powder diffraction and thermogravimetry. Phase transition was detected near 400 °C and accompanied with significant
mass loss. The phase transition affects oxygen mobility, important for the synthesis of oxygen permeable membranes. The unit
cell parameters are proved to change with temperature after two effects (1) reversible conventional thermal expansion and
(2) irreversible contraction-expansion due to the changes in the oxygen content. In situ high-temperature X-ray diffraction
experiments allowed us to separate the contributions and to measure them as a function of temperature.