来源：ACS Publications
Rare earths (RE) are crucial strategic resources and have demonstrated remarkable efficacy in material modification. However, there is a wide variety of RE elements, and the mechanism for regulating electronic structure is unclear, which limits their accurate selection and efficient utilization. The RE tetrad effect divides RE elements into four groups, with the boundaries set at the points where the 4f electrons are filled to 1/4, 1/2, and 3/4. Herein, we present a systematic strategy for regulating the electronic structure of Co-based spinel compounds via it. Research results on a series of RE-doped classic Co-based spinel compounds, Co3O4 and Co3S4, indicate that the spin state of Co can be regulated at the 1/4 and 3/4 positions of the RE tetrad effect, while the valence state of Co is significantly affected at the 1/2 position. The regulation of the spin states of Co3O4 and Co3S4 can be respectively attributed to the strains in the XY-plane and the Z-direction induced by RE. In addition, RE achieve dual-periodic regulation of the Co valence state through 4f–5d–6s–5p orbital hybridization to attain a state where the 4f orbitals are nearly half-filled. The relationship between the regulation mechanism of the RE tetrad effect on the electronic structure and the adsorption evolution of oxygen-containing species has been established. Finally, we have presented a design strategy for RE composite systems based on the tetrad effect, which plays a crucial role in achieving the accurate selection and efficient utilization of RE.