Single-crystal synchrotron X-ray diffraction study reveals bulk intermediate M2 phase during the VO₂...

The structure of solid state grown VO₂ crystals was characterized by synchrotron single crystal X-ray diffraction in the temperature region 300-355 K. Upon heating, a phase transition from the M1(P2₁/c) phase to the R(P4₂/mnm) phase was observed at ∼341 K corresponding to the well-known insulator to metal (IMT) phase transition. Upon cooling however, the R phase proceeds first through the intermediate M2 phase between 345 and 340 K before converting to the M1 phase. This constitutes the first observation of the R → M2 → M1 phase progression in pristine free standing single crystals of VO₂, a result reported previously only in doped samples, nanocrystals, and thin films. The vanadium coordination in the M2(C2/m) phase is found to exhibit structural characteristics of both the M1(P2₁/c) and R(P4₂/mnm) phases containing both V-V dimers and 1-dimensional V chains along the c-axis. The presence of the insulating M2(C2/m) phase in undoped VO₂ is consistent with a Mott-Hubbard type IMT phase transition. Significant changes in the vanadium atomic displacement parameters observed at 340 K suggest thermal vibrations play a crucial role in the phase transition.