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VO2 samples are grown with different oxygen concentrations leading todifferent monoclinic, M1 and triclinic, T insulating phases which undergo afirst order metal to insulator transition (MIT) followed by a structural phasetransition (SPT) to rutile tetragonal phase. The metal insulator transitiontemperature (Tc) was found to be increased with increasing native defects.Vanadium vacancy (VV) is envisaged to create local strains in the lattice whichprevents twisting of the V-V dimers promoting metastable monoclinic, M2 and Tphases at intermediate temperatures. It is argued that MIT is driven by strongelectronic correlation. The low temperature insulating phase can be consideredas a collection of one-dimensional (1-D) half-filled band, which undergoes Motttransition to 1-D infinitely long Heisenberg spin 1/2 chains leading tostructural distortion due to spin-phonon coupling. Presence of VV createslocalized holes (d0) in the nearest neighbor, thereby fragmenting the spin 1/2chains at nanoscale, which in turn increase the Tc value more than that of aninfinitely long one. The Tc value scales inversely with the average size offragmented Heisenberg spin 1/2 chains following a critical exponent of 2/3,which is exactly the same predicted theoretically for Heisenberg spin 1/2 chainat nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation ofMIT and SPT at the same time in VO2 can be explained from our phenomenologicalmodel of reduced 1-D Heisenberg spin 1/2 chains. The reported increase(decrease) in Tc value of VO2 by doping with metal having valency less (more)than four, can also be understood easily with our unified model, for the firsttime, considering finite size scaling of Heisenberg chains.

Role of 1-D finite size Heisenberg chain in increasing metal to insulator transition temperature in hole rich VO2