- 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). Laboratory XRD patterns for samples as-prepared, after annealing at 750 °C for 5,10,15 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 and after long-term ASR test at 650 °C for 60 hours in air. The first column - 2 theta The second column - intensity - 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. Data at 400 - 283251-mythen_summed.dat and 283252-mythen_summed.dat Data at 450 - 283253-mythen_summed.dat and 283254-mythen_summed.dat Data at 500 - 283255-mythen_summed.dat and 283256-mythen_summed.dat Data at 550 - 283257-mythen_summed.dat and 283258-mythen_summed.dat Data at 600 - 283259-mythen_summed.dat and 283260-mythen_summed.dat Data at 650 - 283261-mythen_summed.dat and 283262-mythen_summed.dat Data at 700 - 283263-mythen_summed.dat and 283264-mythen_summed.dat Data at 750 - 283265-mythen_summed.dat and 283266-mythen_summed.dat Data at 800 - 283267-mythen_summed.dat and 283268-mythen_summed.dat - 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 The fabriacted symmetrical cells were cut into half and the cross section was examined by SEM. - Dilatometer The powder samples were uniaxially pressed into pellets and further pressed isostatically (Autoclave Engineers Cold Isostatic Press) under a pressure of 200 MPa, then sintered at 1200 °C for 12 hours. The sintered discs showed the relative density of = 90%. Thermal expansion profiles were collected using Netzsch DIL 402C under static air. - DC conductivitiy The powder samples were uniaxially pressed into pellets and further pressed isostatically (Autoclave Engineers Cold Isostatic Press) under a pressure of 200 MPa, then sintered at 1200 °C for 12 hours. The sintered discs showed the relative density of = 90% and were cut into bars (˜ 2 x 4 x 8 mm). Gold wires were attached to the bars (held in position with a small amount of gold paste) to make the I-V probes with four-in-a-line contact geometry and the electrical conductivity measurement was performed using the standard DC four-probe method (Keithley 2182 Nanovoltmeter and Keithley 220 Current Source).