Evidence of a Pressure-Induced Metallization Process in MonoclinicVO2

Abstract

Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO2 at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*∼10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P>P*. Differently from ambient pressure, where the VO2 metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO2. PACS numbers:Since the first observation of the metal to insulator transition (MIT) in several vanadium oxides, these materials attracted considerable interest because of the huge and abrupt change of the electrical properties at the MIT. As usual in transition metal oxides, electronic correlation strongly affects the conduction regime of vanadium oxides, although, in some compounds, lattice degrees of freedom seem to play an important role. This is the case of VO 2 , which undergoes a first order transition from a high temperature metallic rutile (R) phase to a low temperature insulating monoclinic (M1) one. At the MIT temperature, T c =340 K, the opening of an optical gap in the mid-infrared (MIR) conductivity and a jump of several order of magnitude in the resistivity are observed [1]. The interest on this compound is thus mainly focused on understanding the role and the relative importance of the electron-electron and the electron-lattice interaction in driving the MIT. Despite the great experimental and theoretical efforts [2], the understanding of this transition is still far from being complete [3,4,5,6,7]. In the R phase the V atoms, each surrounded by an oxygen octahedron, are equally spaced along linear chains in the c-axis direction and form a body-centered tetragonal lattice. On entering the M1 insulating phase the dimerization of the vanadium atoms and the tilting of the pairs with respect to the c axis lead to a doubling of the unit cell, with space group changing from C 5 2h (R) to D 14 4h (M1) [8,9]. As first proposed by Goodenough [10], the V-V pairing and the off-axis zig-zag displacement of the dimers lead to a band splitting with the formation of a Peierls-like gap at the Fermi level. First principle electronic structure calculations based on local density approximation (LDA) showed the band splitting on entering the monoclinic phase, but failed to yield the opening of the band gap [11,12]. In fact, as early pointed out [13], the electron-electron correlation has to be taken into account to obtain the insulating phase. A recent theoretical paper where the electronic Coulomb repulsion U is properly accounted for, shows that calculations carried out joining dynamical mean field theory with...

Optical properties for the Mott transition in VO2

“…17,20 In accordance with past results, the rutile tetragonal phase is observed near 70 • C after the MIT occurs. In our Raman measurements, although the rutile tetragonal phase indicating the SPT is not observed, a significant change of crystalline structure is shown above 75 • C. Thus, the SPT temperature may be regarded as 75 • C and it may be considered that a first-order MIT in VO 2 is not a structure-driven MIT.…”

Pressure–Temperature Phase Diagram of Vanadium Dioxide

“…Transitions between these phases can be driven by temperature, photo-excitation, hydrostatic pressure, uniaxial stress, or electrical gating. 1,[4][5][6][7][8][9][10][11][12] For example, the well-known, insulating M1 (monoclinic, space group P21/c) phase transforms to the metallic R (tetragonal, P42/mnm) phase at 68 o C under ambient pressure. 1 This M1-R transition can also be driven by hydrostatic pressure 6 or uniaxial compression.…”

Modification of electronic structure in compressively strained vanadium dioxide films

“…Thus, for experimental measurements of these systems to provide meaningful insight, the external parameters, the intrinsic interacting degrees of freedom, and the resulting properties must all be well characterized. Hence, measurements on samples subject to external perturbations, for example pressure and strain [5][6][7][8][9][10] , can provide additional insight into the underlying physics of these systems.…”