The mixed powders were ground by means of a planar ball milling machine (Retsch, PM100).After being placed into alumina crucibles, the ground powders were calcined at a constant temperature of 840 °C for 12 h for the Y358 and at a constant temperature of 915 °C for 24 h for the Y123, in air atmosphere (818 Eurotherm Controller/Programmer Muffle furnace). Calcined powders were finely reground and later again calcined for the purpose of homogeneity.