A sol-gel method was used to synthesize the precursor followed by high-temperature calcination. NaNO3 (>99.0%, Sigma Aldrich), Me(CH3COO)2 (Me= Mg, Zn, Cu, CH3COOLi, >99%, Alfa Aesar), and Sc(NO3)3•9H20 (>99.99%, abcr Gute Chemie) were dissolved in 80 mL of deionized water in a stoichiometric ratio, mixed for 2 h at 50 ℃ before adding the corresponding amount of citric acid (>99.0%, Sigma Aldrich) (metal: acid=1:1) and mixing for a further 3 h. The temperature was then increased to 120 ℃ to evaporate the water, forming a gel after around 3 h. The gel is then transferred to a crucible and heated in a furnace at 500 ℃ for 5 h. After cooling down to room temperature, the materials were ground to powder, and a pellet was pressed under 15 MPa. The pellets were calcined at 900 ℃ for 10 h and at 950 ℃ for another 10 h in air. The pellets were then transferred to an argonfilled glovebox after the temperature cooled down to 200 ℃.