(a) XPS spectra of the Ce 3d level in [(Pr, Nd)_1−x(La, Ce)_x]_2.14Fe₁₄B strips with x = 0.1–0.5. (b) The derived intensities of I f 0 (circles in red color) and ( I f 1 + I f 2 ) (squares in blue color) are plotted as a function of La–Ce content x.^[23]