The second and third insertion voltages are both about 4.7 V for both phases. The fourth Ca has a negative voltage and thus is not a stable phase, but it is important to note that for the forth Ca atom, the Na2 positions become more stable for further occupation than the Na1 positions. Going from the fourth to the fifth Ca the voltage is positive for both phases again, making this transition possible. The occupation with six Ca is most stable with three Ca in the Na1 positions and three in the Na2 positions. These voltages show nicely that the material might only be suitable as an electrode in the 1-3 occupation window, but is generally more suitable as a solid electrolyte.