VO2: Orbital competition, magnetism, and phase stability

Abstract

The relative phase stability of VO2 is one of the most fundamental issues concerning the metalinsulator transition in this material but has so far largely unexplored theoretically. We investigate the relative stability of various phases of VO2 using different levels of energy functionals within density functional theory (DFT). It is found that straightforward applications of several popular energy functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, result in a wrong prediction for the ground state of VO2. In particular, although the HSE and DFT+U methods are able to produce a band gap in the M1 phase, they strongly favor the formation of local magnetic moments, a result that clearly disagrees with experiments. We also examine the effect of the occupation and the redistribution of the d derived t2g (i.e., dxz, dyz and d x 2 −y 2 ) orbitals of V atoms on the calculated relative phase stability of VO2. We find that a small change in d-occupation can result in a drastically different theoretical prediction. With the introduction of an orbital-dependent potential, a complete separation between the d x 2 −y 2 derived valence band and dxz and dyz derived conduction bands in the M1 phase is achieved, resulting in a slight redistribution of the d occupation and a more faithful account of the polarization of the t2g orbitals. This slight rearrangement of the d occupation also leads to a relative phase stability of VO2 (including structural and magnetic phases) that agrees well with experiment.

Hydrogen-doping induced reduction in the phase transition temperature of VO2: a first-principles study

“…30,31 The potentials were of the projector augmented wave (PAW) type, and the exchange-correlation part of the density functional was treated within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE). 36,37 Despite the fact that more sophisticated methods such as GW, local density approximation with dynamical mean field theory (LDA+DMFT) and particular hybrid functionals might yield better results, [38][39][40][41] the Dudarev DFT+U method has been shown to reasonably describe the electronic structure and strong correlation of VO 2 (M). 36,37 Despite the fact that more sophisticated methods such as GW, local density approximation with dynamical mean field theory (LDA+DMFT) and particular hybrid functionals might yield better results, [38][39][40][41] the Dudarev DFT+U method has been shown to reasonably describe the electronic structure and strong correlation of VO 2 (M).…”

Unraveling the Mott-Peierls intrigue in vanadium dioxide

“…Relatively recent calculations based on HSE hybrid functionals bring even worst results: both rutile and M1 phases are predicted to be magnetically ordered insulators, with the former lower in energy [65,66], even though earlier calculations were claimed to be more in accordance with experiments [25]. In turn, mBJ exchange potentials seem to predict the proper conducting behaviour of the R and M1 phases, as well as their lack of magnetism [67], which is erroneously predicted to occur also in the M2 phase [63]. This suggests that suppression of magnetic moments is somehow the rule of mBJ functionals applied to VO 2 , which only by chance is the correct result for R and M1 phases.…”

“…For instance, straight LDA or GGA methods do not find any gap opening in M1 and M2 phases [56,57]. Such gap is instead recovered by GW [58][59][60] or LDA+U [61][62][63], in all its variants. However, GW does not give easy access to the total energy, and therefore it does not explain why low temperatures should favour the M1 distorted phase against the rutile undistorted one.…”

“…However, GW does not give easy access to the total energy, and therefore it does not explain why low temperatures should favour the M1 distorted phase against the rutile undistorted one. In turns, LDA+U or GGA+U calculations, known to overemphasise the exchange splitting, predict the existence of local moments even in the rutile phase [61][62][63], not observed in experiments [64]. Relatively recent calculations based on HSE hybrid functionals bring even worst results: both rutile and M1 phases are predicted to be magnetically ordered insulators, with the former lower in energy [65,66], even though earlier calculations were claimed to be more in accordance with experiments [25].…”

Formation energies of intrinsic point defects in monoclinic VO2 studied by first-principles calculations

“…16 However, very limited number of theoretical investigations been reported on the intrinsic defects in VO 2 (M) have, which may be ascribe to the extreme sensitivity of its properties to theoretical approximations. [17][18][19][20][21] For instance, the calculations of the electronic structures based on density functional theory (DFT) within the local density approximation (LDA) or the generalized gradient approximation (GGA) are incapable of depicting the insulator properties of VO 2 (M) phase. 18 The methods such as hybrid functionals, although can reproduce the band gap for VO 2 (M) phase, they fail to reproduce a magnetic ground state.…”