Tuning VO2 phase stability by Cr doping: insights from photoemission and X-ray absorption spectroscopic investigations

Abstract

This study investigates the electronic structure modifications induced by Cr doping in epitaxial VO₂ thin films coherently grown on m-Al₂O₃ (102) single-crystal substrates across various phase transitions. Utilizing advanced microscopic and spectroscopic techniques, the electronic properties of Cr_xV_1−xO₂ are systematically analysed. The insulator-to-metal transition temperature (T_IMT) is suppressed due to epitaxial strain, with subsequent Cr-doping elevating T_IMT toward room temperature. At 10 at% Cr doping, a metastable triclinic phase emerges from the monoclinic (M1) phase of pristine VO₂, while a first-order monoclinic (M2) phase is observed at 30 at% Cr doping. Photoemission spectroscopy reveals distinct spectral changes attributed to charge fluctuations, resulting in Cr³⁺ and V⁵⁺ polarons that suppress the insulating phase and enhance conductivity. These charge conversions drive site localization involving V⁴⁺ and V⁵⁺ at Cr³⁺ sites, consistent with the Mott–Peierls mechanism, indicating reduced V–V dimerization with increasing Cr concentration. X-ray absorption fine structure spectroscopy, combined with first-principles calculations, elucidates the atomic-scale structural evolution of Cr dopants and their role in modulating V–V dimerization. The local environment of Cr atoms exhibits intrinsic tetragonal-like symmetry, undergoing a concentration-dependent evolution from initial distortion to stabilization due to strong Cr–VO₂ lattice interactions across the IMT. These findings provide direct experimental evidences for the symmetric Cr core inducing de-twisting of the asymmetric monoclinic VO₂ lattice, forming more symmetric and less insulating VO₂.