Vanadium dioxide: In transition

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  • Published: Nov 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Vanadium dioxide: In transition

Decades of debate

Vanadium atoms (blue) have unusually large thermal vibrations that stabilize the metallic state of a vanadium dioxide crystal. Red depicts oxygen atoms. Image credit: Oak Ridge National Laboratory

Combining X-ray and neutron scattering data for lattice dynamics measurements with theoretical models improves our understanding of the well-known insulator vanadium dioxide and its transition to a metallic state.

For half a century scientists have argued about what turns oxide insulators on. In an insulator, the electrons are not free to flow but if such a material can flip to a conducting metallic state, they flow freely. One point of view was held by Nobel Prize winning physicist Nevill Mott and others whose evidence suggested that direct interaction between the electrons was the key. Others, including Rudolf Peierls felt that the evidence pointed to atomic vibrations and distortions being the underlying mechanism.

Now, a team led by the Department of Energy’s Oak Ridge National Laboratory, in Tennessee has taken a critical step to help us understand how a classical transition metal oxide, vanadium dioxide, might make the insulator to conductor transition by quantifying the thermodynamic forces driving the change; they publish details in the journal Nature this month.

The proof is in the phonon

"We proved that phonons - the vibrations of the atoms - provide the driving force that stabilizes the metal phase when the material is heated," explains ORNL's John Budai, who co-led the study with colleague Jiawang Hong. "This insight into how lattice vibrations can control phase stability in transition-metal oxides is needed to improve the performance of many multifunctional materials, including colossal magnetoresistors, superconductors and ferroelectrics," adds Hong.

Vanadium dioxide has a wide range of technological applications particularly in improving recording and data storage media, strengthening structural alloys, and providing colour for synthetic jewels. It might one day be used in nanoscale actuators for switches, smart windows that control heat flow in and out, optical shutters for protecting delicate instrumentation on spacecraft and in ultrafast field-effect transistors for the next generation of microelectronics devices and spintronics technologies.

When heated to 340 Kelvin, vanadium dioxide turns from insulator to metal. Above 340 K, where entropy due to phonon vibrations dominates, its preferred state has all bond angles in the atomic unit cell at 90 degrees rather than skew. The phase change is fully reversible, so cooling a metal below the transition temperature reverts it to an insulator, and heating it past this point turns it metallic.

Property prediction

Before the current experiments, researchers knew how much total heat energy is absorbed during the transition of vanadium dioxide from insulator to conductor. What they did not know was how much of the material's entropy was due to disorder in the electrons and how much was due to atomic vibrations. "This is the first complete description of thermodynamic forces controlling this archetypical metal–insulator transition," explains Budai. The results were obtained by combining data from modern X-ray and incoherent neutron scattering experiments that provided information about the material's lattice dynamics and a calculation technique developed by Olle Hellman of Linköping University in Sweden that captures anharmonicity (a measure of non-linearity in the bond forces between atoms). With agreement between experiment and calculation, the team can now make new predictions about other materials that also undergo such transitions, or to predict what properties a novel material have.

Neutron measurements were obtained using the large neutron flux at the Spallation Neutron Source (SNS), while the X-ray measurements were carried out by the team using the Advanced Photon Source (APS) on crystals grown by Lynn Boatner. "The entropy of the lattice vibrations competes against and overcomes the electronic energy, and that's why the metallic phase is stabilized at high temperatures in vanadium dioxide," explains Budai. "Using comprehensive measurements and new calculations, we're the first [team] to close this gap and present convincing arguments for the dominant influence of low-energy, strongly anharmonic phonons."

Fundamentally, the experiments show that the lattice of vanadium dioxide is anharmonic in the metallic state, which complicates its vibration modes. Moreover, in metallic vanadium dioxide, each vanadium atom has one electron that is free to roam whereas in the insulating form that electron is trapped in a chemical bond that forms vanadium dimers.

"The theory provides a deep understanding of the experimental observations and reveals fundamental principles behind them," says Hong. "It also gives us predictive modelling, which will accelerate fundamental and technological innovation by giving efficient strategies to design new materials with remarkable properties."

"The next step is to test the generality of the findings," Budai told SpectroscopyNOW. "How broadly do these ideas apply to other materials, particularly those who exhibit a metal-insulator phase transition?  Ultimately, we hope to develop the ability to predictively design new materials with improved physical properties through calculations rather than by trial and error. We need fundamental materials science in order to create the reliability that would enable materials engineering."

Related Links

Nature 2014, online: "Metallization of vanadium dioxide driven by large phonon entropy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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