Phase Evolution in Low Fe Concentration V1−xFexO2 Compounds: Phase diagram and Annealing Effects

Abstract

We present a comprehensive investigation into the synthesis, phase evolution and valence state of vanadium (V) in V_1−xFe_xO₂ (x = 0 %, 0.5 %, 0.75 %, 1.0 %) compounds. Polycrystalline samples have been synthesized with solid-state reaction method, followed by thermal annealing. X-Ray Powder Diffraction (XRPD) analyzed by Le Bail method revealed the transformation from monoclinic (M1) phase (space group: P2₁∕c) to triclinic (T) one with increasing Fe concentration. Additionally, a monoclinic (M2) phase (space group: C2∕m) emerged at 1.0 % Fe doping. Temperature-dependent XRPD and diffuse reflectance measurements elucidated the phase transitions during heating cycles, showing the impact of Fe doping on the system’s behavior. The construction of a complete phase diagram for the V_1−xFe_xO₂ system (x ≤ 1.0 %) was achieved, addressing ambiguities in the low-Fe concentration region. X-ray Photoelectron Spectroscopy (XPS) further confirmed the influence of Fe doping on the vanadium valence states, indicating an increase of V⁵⁺ sites and therefore a lattice distortion and stabilization of the triclinic phase. The metal-insulator transition temperature (T_MIT) appears to be almost constant. Post-annealing led to the reinstatement of the M1 phase in all samples, and a modified phase diagram was constructed. The accompanied decrease of V⁵⁺ ions contributed to the destabilization of the T and M2 phases, favoring the thermodynamically stable M1 phase. The findings provide valuable insights into the complex phase behavior of V_1−xFe_xO₂ compounds, showcasing a significant interplay between charge redistribution, the vanadium valence state, and the oxygen defects of the system.