Role of 4f electrons and 3d-4f hybridization in metal-insulator transition in RE (La, Nd, Sm, Eu, Dy and Er)-doped vanadium dioxide for thermochromic applications

Vanadium dioxide is a promising thermochromic material for novel applications to smart optoelectronic devices due to its reversible insulator-to-metal transition which is accompanied by rapid variation in its optical properties. In this work, rare-earth (La, Nd, Sm, Eu, Dy and Er) dopants substituting vanadium site were investigated in detail to control the phase transition temperature (T_MIT) and optical properties of VO₂. It is found that T_MIT is mainly related to the nature of 3d-4f hybridization in RE-doped VO₂(M₁): the direct overlapping of 3d-4f orbitals contributes to V–V dimerization, but it is less pronounced to lower T_MIT; the indirect 3d-4f interaction via O-2p makes 4f electrons itinerant to e_g^π states to assist MIT from M₁ to R phase at lower temperatures. The dominating nature of 3d-4f hybridization varies with changing RE dopant's ionic radius and electronegativity. Er offered the lowest T_MIT with a decreasing rate of 12.8 °C/at. % due to effective 4f-O2p-3d hybridization. RE dopants were also found effective in changing optical properties by scaling E_g1 and E_g2 bandgaps of VO₂. Hence, the fashion of 3d-4f interaction in RE-doped VO₂ is found to play important role in modulating thermochromic properties of VO₂ for smart window applications and smart optoelectronics devices.