Of these phases, the cubic fluorite phase forms at ~9.3 𝑚𝑜𝑙% 𝑆𝑐 ! 𝑂 " and the tetragonal phase forms at ~6-9.3 𝑚𝑜𝑙% 𝑆𝑐 ! 𝑂 " exhibit the highest and second-highest ionic conductivity among other phases. The 𝑐 ↔ 𝑡 phase transition happens by oxygen ion displacement from fluorite ideal sites (8c sites). Increasing dopant levels has been shown to trigger the formation of an unwanted low conductive 𝛽 -rhombohedral phase transformed from a high conductive cubic phase which accompanies volume change . Study of ternary systems consisting of 𝑆𝑐 ! 𝑂 " -𝐶𝑒𝑂 ! -𝑍𝑟𝑂 ! demonstrated that the addition of a small amount (≤ 2%) of a third component such as ceria can stabilize the higher ionic conductivity phases over a wider dopant range of scandia . The reaction that creates oxygen vacancies in scandia-ceria-stabilized zirconia is expressed in the Kröger-Vink notation below.