Scientists discover structural changes in metal oxides at low temperatures
When water boils, it converts into another phase, steam. Such transitions are commonplace in nature and frequently studied in scientific laboratories.
One of particular interest to scientists is the transition from a metal to an insulator (MIT), which can occur in some materials at various temperatures. Metals conduct electricity. Insulators do not. Scientists are seeking to exploit the switch from metal to insulator and back for many potential applications, including low-power electronics, specialized devices for monitoring temperature change in industrial settings, and smart windows/smart glass.
Scientists from the U.S. Department of Energyโs (DOE) Argonne National Laboratory, the University of Alabama and the University of California at Los Angeles have made a surprising discovery regarding structural changes that occur when one such material is cooled below itsย MITย temperature. That material is vanadium dioxide (VO2) to which the research team added varying amounts of another element, molybdenum.
โOur results demonstrated that tiny structural distortions form within the samples below theย MITย temperature,โ said Argonne senior physicist Ray Osborn of the Materials Science division.ย โโThese distortions are two dimensional shapes, that is, planes with length and width but essentially no thickness. And yet, on average, the overall three-dimensional structure of the sample remains intact.โ
Theย MITย in pure vanadium dioxide was first reported inย 1959. It is one of the few materials that undergoes this transition close to room temperature, which is highly desirable for practical applications. Sixty years after the discovery, the mechanism behind the transition remains a mystery. The team sought to better understand the physics of vanadium dioxide by incorporating molybdenum in the structure.
โAs a chemist, I am interested in understanding the effect on theย MITย from chemically modifying vanadium oxide by addition of elements like molybdenum,โ said Jared Allred, assistant professor at the University of Alabama.
As the team added more and more molybdenum to the vanadium dioxide, the signal in their data indicating theย MITย became less and less pronounced until it had almost vanished. That point occurred whenย 19% of the vanadium atoms had been replaced with molybdenum. Along with this drop in theย MITย signal was a decline in the temperature at which it happened. This temperature had been near room temperature for the pure vanadium dioxide and fell to minusย 190ย degrees Fahrenheit for the sample withย 19% molybdenum.
University of Alabama researcher Matthew Davenport prepared the samples for the study. The research team characterized the samplesโ atomic structure using X-ray scattering at beamlineย 6-ID-Dย at Argonneโs Advanced Photon Source (APS), aย DOEย Office of Science user facility. The team performed this analysis over a wide temperature range, from close to absolute zero to well above room temperature.
โThe methods we use at theย APSย enable us to gather large volumes of data and convert the findings to a detailedย 3Dย model of the atomic structure at the nanoscale,โ said Osborn. The team captured several hundred thousand images for each sample atย 10ย images per second. For the samples withย 19% molybdenum, unexpected rod-like shapes appeared in the images at about minusย 240ย degrees Fahrenheit, well below theย MITย temperature.
โThe rods indicated that novel two-dimensional structures emerge after the collapse of the three-dimensional order in micro-regions of this material,โ said Allred. Despite these changes in micro-regions, the overall three-dimensional structure of the material remains intact. On further study, the team also found that these two-dimensional structures are not perfectly flat.
Osborn described the eye-opening moment when team members first saw the X-ray scattering results:ย โโWe nearly fell out of our chairs,โ he said.ย โโWe saw in the X-ray scattering results a phenomenon that shouldnโt have been there: the rods were wavy โ something weโd never seen before. The waviness turned out to be a sign that these sheets in fact were not perfect, two-dimensional planes.โ
To better understand the mechanism behind these results, the team used a relatively new technique in X-ray scattering for data analysis, referred to asย 3D-difference pair distribution function analysis. This approach enabled the team to view the structure directly at an atomic scale, showing just how the atoms distort theย 2Dย planes in the material when below theย MITย temperature.
โWe didnโt solve that original problem โ the question about theย MITย mechanism in vanadium dioxide,โ admitted Allred. However, in probing possible explanations, this work should lead to a more complete physical model for theย MIT, and that understanding could help realize the commercial potential of this material in temperature-sensitive devices and energy-efficient systems.
An article on this research entitledย โโFragileย 3Dย Order in V1โxMoxO2โ appeared in Physical Review Letters.
In addition to Osborn and Allred, authors include Matthew A. Davenport (University of Alabama), Matthew J. Krogstad (Argonne), Logan M. Whitt (University of Alabama), Chaowei Hu (UCLA), Tyra C. Douglas (University of Alabama), Ni Ni (UCLA) and Stephan Rosenkranz (Argonne).ย This research was supported by theย DOEย Office of Basic Energy Sciences.
