Single-crystal synchrotron X-ray diffraction study reveals bulk intermediate M2 phase during the VO ₂ insulator-to-metal transition

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(<i>P</i>2₁/<i>c</i>) phase to the R(<i>P</i>4₂/<i>mnm</i>) 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(<i>C</i>2/<i>m</i>) phase is found to exhibit structural characteristics of both the M1(<i>P</i>2₁/<i>c</i>) and R(<i>P</i>4₂/<i>mnm</i>) phases containing both V-V dimers and 1-dimensional V chains along the <i>c</i>-axis. The presence of the insulating M2(<i>C</i>2/<i>m</i>) 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.