Ab Initio Calculations On The Phase Transition In VO₂

Hubbard energy,U,which represents the electronic correlation effect in the 3d shell of a V atom,has a large effect on the properties of vanadium dioxide.To explore the effects of U on the structure and electronic properties of VO₂,the LDA+U method was used in our density functional theory?DFT?calculations.The results revealed that monoclinic?M?VO₂ could be relaxed to rutile?R?one when U < 0.28 eV or a monoclinic metallic phase?Mm?when 0.28 eV < U < 0.44 eV,and keep steady when U > 0.44 eV.On the other hands,R-VO₂ can be relaxed to an M-VO₂ when U is larger than 4.75 eV.The large difference in critical U for R-M and M-R transitions derives from the influence of U on the energy barriers in these processes.When U is 0.3 eV,the energy barrier makes the Mm phase stable even if the total energy of R-VO₂ is smaller than that of Mm.When U is larger than 4.75 eV,the energy barrier in R-M transition is diminished and the M phase is stable.On the other hand,ab initio molecular dynamics method was used to simulate the phase transition of VO₂.The influence of lattice deformation on the phase transition temperature of VO₂ has been studied.It is found that the ratio of lattice parameters aM and bM?a_M/b_M?is related to the phase transition temperature in VO₂.Specifically,the growth of a_M/b_M will lead to an increase in the phase transition temperature and vice versa.This finding is in good agreement with the experimental results,and a proper explanation is given for this phenomenon.This result makes it possible to design the phase transition temperature in VO₂.