Figure 1. Hall carrier concentration versus number of electrons per primitive cell Co₄Sb₁₂.

Data from refs 2, 8, 9, 18, 19, 32, 33, 34, 35. The number of electrons per primitive cell is calculated as the sum of q_i*x_i for partially filled (R₁)_x1(R₂)_x2…(R_n)_xnCo₄Sb₁₂, where q_i is the effective charge state and x_i is the filling fraction of the ith filler R in multiple-filled skutterudites. The solid line represents the theoretical curve calculated by using n=2x/a³, where x is the total number of electrons per primitive cell and a is the lattice constant of CoSb₃. The red double arrow represents the estimated high power factor region (see discussions below) and the red dashed line represents the number of electrons per primitive cell limited by the previously thought Ce filling fraction limit8.