- Synthesis and XRD
High purity (= 99.9%) BaCO3, SrCO3, Co3O4, Fe2O3 and WO3 were used to prepare Ba0.5Sr0.5(CoxFey)1-zWzO3-d samples. The powders were ball-milled overnight in 2-propanol with ZrO2 balls (350 rpm , Fritsch Pulverisette 7 Planetary Mill), dried and heated initially to 700 °C for 6 hours followed by 900 °C for 8 hours before ball-milling again. The resulting powders were then pressed as pellets (1.3 cm diameter) and sintered at 1200 °C for 12 hours. 

Phase purity was determined using X-ray powder diffraction (Panalytical X-pert Pro Bragg-Brentano geometry laboratory X-ray diffractometer with Co Ka1 radiation = 1.78901 Å). For the detailed structural analysis of Ba0.5Sr0.5(Co0.7Fe0.3)0.6875W0.3125O3-d, synchrotron X-ray diffraction data of powder samples were recorded on Beamline I11 with a wide-angle position sensitive detector (? = 0.825623 Å) at Diamond Light Source, UK

Laboratory XRD patterns for BSCFW as-prepared, after annealing at 750 °C for 10 days in static air, annealing in flowing 1% CO2 in Ar at 650 °C for 5 days, annealing at 650 °C for 7 days while vaporised Cr from steel 316 fired at 850 °C was passed over the sample at 650 °C by flowing humid air, and fired to the fabrication condition of 950 °C for 3 hours with a 1:1 mass ratio mixture with SDC. Synchrotron X-ray diffraction patterns for as-prepared BSCFW and after long-term ASR test at 650 °C for 60 hours in air.

- Further structureal studies
For the detailed structural analysis of Ba0.5Sr0.5(Co0.7Fe0.3)0.6875W0.3125O3-?, synchrotron X-ray diffraction data of powder samples were recorded on Beamline I11 with a wide-angle position sensitive detector (? = 0.825623 ?) at Diamond Light Source, UK and time-of-flight neutron diffraction data of sintered pellets in a quartz tube under flowing air were recorded on the Polaris diffractometer at the ISIS facility, Rutherford Appleton Laboratory, UK. Structural parameters were refined by the Rietveld method with the Topas software.
Data at RT - pol74538__abscor_b1_TOF.dat and 390894_390901_reb_005.xye
Data at 600 - pol74539_42_abscor_b1_TOF.dat and 390904_390905_reb_005.xye

- AC impedance studies
The cathode materials were ball-milled in isopropanol overnight followed by further ball-milling with the pore-former V-006 (Heraeus Electronic Materials) for 3 hours, with a mass ratio between cathode materials and the pore-former of 1:0.7. The suspension was then screen-printed onto a sintered SDC electrolyte (Fuel cell materials) on both sides (area of ˜ 0.56 cm2 for each side), and dried in an oven at 100 °C: this was repeated 10 times (4 times for reference cathodes, see Supplementary Note 6 for full details). The cell was fired to 950 °C for 3 hours, and then coated with Au paste, followed by heating to 600 °C for 1 hour to ensure bonding to the cathode surface. ASR measurements were then performed in static air by AC impedance spectroscopy (Metrohm Autolab AUT84515) in the frequency range from 0.1 to 105 Hz with a perturbation of 10 mV. The impedance data were analysed using ZView software

- Electron microscopy
Thin lamella transmission electron microscopy (TEM) specimens were produced by the lift out method using a focussed (Ga) ion beam (FIB) instrument37. TEM was performed using a Schottky field emission gun equipped JEOL JEM 2100FCs microscope operating at 200 kV. Bright field images were collected using conventional TEM illumination and High-Angle Annular Dark Field (HAADF) images were recorded in scanning TEM (STEM) mode using a CEOS aberration corrected probe. Energy-dispersive spectrometry (EDS) measurements were performed in the same instrument using an EDAX windowless silicon drift X-ray detector and TEAM collection/analysis software.