New studies explain insulator-to-metal transition of vanadium dioxide

New studies explain insulator-to-metal transition of vanadium dioxide
Changes in the crystal structure and electronic properties of vanadium dioxide occur during its insulator-to-metal phase transition (V blue; O red). Above 67°C (right), large-amplitude, nonlinear lattice vibrations (phonons) lead to a tetragonal crystal structure with mobile electrons (yellow) indicating that the vanadium dioxide is a metal. At lower temperatures (left), the electrons are localized in the atomic bonds in the distorted monoclinic crystal structure indicating that the vanadium dioxide is an insulator. Credit: Oak Ridge National Laboratory

When heated to just above room temperature, the electrical conductivity of vanadium dioxide (VO2) abruptly increases by a factor of 10,000. Experiments coupled with high-performance computation reveal how the unusually large lattice vibrations, which are the oscillations of atoms about their equilibrium positions, stabilize this highly conductive metallic phase.

Achievement of a quantitative description of phase competition and functionality in metal oxides could pay great dividends in the quest to predictively design materials with .

Vanadium dioxide (VO2), a "functional material" that could be used in applications such as smart windows and ultrafast field effect transistors, exhibits an insulator to metal transition upon heating to just above . At the transition temperature, its abruptly increases by a factor of 10,000 and the atomic lattice rearranges from a monoclinic to a tetragonal structure (see figure).

A fundamental description of the physical and electronic properties during the transition in VO2 has remained controversial for over 50 years. Researchers at Oak Ridge National Laboratory employed advanced neutron and X-ray scattering experiments at DOE user facilities, coupled with large-scale first-principles calculations with super computers, to determine the detailed mechanism for the transition.

Their studies, published in Nature, revealed that the thermodynamic force driving the insulator-to-metal transition is dominated by the lattice vibrations (phonons) rather than electronic contributions. In addition, a direct, quantitative determination of the phonon dispersions was achieved, as well as a description of how changing occupancies in the atomic orbitals participate in the phase transition. The low-energy phonons were found to change the bonds between atoms (i.e., electron orbitals), allowing some electrons to travel freely at higher temperatures leading to a metallic state.

This research demonstrates that anharmonic lattice dynamics play a critical role in controlling phase competition in , and provides the complete physical model vital for the predictive design of new materials with unique properties.

More information: J.D. Budai, J. Hong, M.E. Manley, E.D. Specht, C.W. Li, J.Z. Tischler, D.L. Abernathy, A.H. Said, B.M. Leu, L.A. Boatner, R.J. McQueeney, and O. Delaire, "Metallization of vanadium dioxide driven by large phonon entropy." Nature 515, 535-539 (2014). [DOI: 10.1038/nature13865]

Journal information: Nature

Citation: New studies explain insulator-to-metal transition of vanadium dioxide (2015, April 11) retrieved 23 May 2024 from
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