Oxygen-deficient Sr2ScGaO5 single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO6 octahedra and GaO4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr2ScGaOs to be thermodynamically stable only above 1400 degrees C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr,ScGaO5 remains surprisingly kinetically stable upon heating from ambient temperature to 1300 degrees C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10(-4) S/cm at 600 degrees C, while oxygen 180/160 isotope exchange indicates a free oxygen mobility to set in at around 500 degrees C.