Disorder is key in phase transitions of materials

Disorder is key in phase transitions of materials https://i0.wp.com/www.eresviral.com/wp-content/uploads/2018/11/El-desorden-es-clave-en-las-transiciones-de-fase-de-los-materiales.jpg?fit=237%2C146&ssl=1

Disorder is key in phase transitions of materials







Phase transitions are common events that radically change the properties of a material. One of the best known is the solid-liquid-gas transition of water. Each phase corresponds to a new positioning of the atoms within the material, which dictate the properties of the substance.



While this change in positioning can be easily studied in each phase, it is much more difficult to do so during a phase transition. This is because the atoms are incredibly small and the distances in which they move are, consequently, small and, as a result, they can occur very quickly. In addition, the materials consist of more than 1023 atoms, which makes it extremely difficult to track their individual movements.



A particularly interesting phase change is the metal-insulator transition in the vanadium dioxide (VO2) material. At room temperature, it is an insulator, and inside the crystal, the vanadium ions form periodic chains of vanadium pairs, known as dimers.



But when the material is heated, just above the ambient temperature, the atomic structure changes and the pairs break, although the material remains solid. At the same time, the conductivity of the material increases by more than 5 orders of magnitude and gives it a wide range of applications, including infrared detection.







ICFO researchers Simon Wall and Luciana Vidas work on the experimental setup in their laboratory. (Photo: ICFO)



One of the most intriguing properties of VO2 is that the phase transition can happen incredibly fast, taking into account that the only limit lies in how quickly the system can be heated. To explain this incredible speed of transition, the scientists suggested that there must be a cooperative movement between the vanadium ions, that is, each pair of vanadium ions is broken in the same way at the same time.



In turn, to understand the atomic structure of materials, scientists use a technique known as diffraction. In the last 30 years, this method has been extended to include the temporal resolution, in order to obtain the 'molecular film', that is, to directly film the movement of the atoms during the transition. When this technique was applied for the first time to vanadium dioxide in 2007, it seemed to confirm the image of the coordinated movement.



However, diffraction only measures the average atomic position and reveals little information about the actual trajectory taken by the individual atoms involved. For example, the diffraction method would see in the same way a band marching down an avenue, moving in a uniform and coordinated way, as a group of tourists covering the same distance, on average, but in a totally uncoordinated, wondering and stopping at random to look at the architecture of the city.



Now three researchers from the Institute of Photonic Sciences (ICFO) in Barcelona (Professor Simon Wall, PhD student Luciana Vidas and former postdoc Timothy Miller), in collaboration with scientists from Japan's Synchrotron Radiation Research Institute, American universities of Duke and Stanford and the Oak Ridge National Laboratory, have used a new technique capable of solving atomic routes.



It has also been crucial to carry out the study, published in the journal Science, the world's first X-ray laser located in the National SLAC Accelerator Laboratory (USA). This new light source allowed the researchers to examine the crystalline structure of the material with unprecedented detail, using a technique known as total X-ray scattering. In contrast to the widely held opinion, the authors found that the vanadium pairs break It was done in an extremely disorderly manner and more like tourists than the band.



As Simon Wall comments, the first author of the article: "This is the first time that we have actually been able to observe how atoms rearrange themselves in a phase transition without assuming that the movement is uniform. This suggests that the understanding of these transitions explained in textbooks should be rewritten. Now we plan to use this technique to explore more materials and understand where the role of clutter comes from. "



To date, VO2 has often been used as a guide to understand phases in more complex materials such as high temperature superconductors. The results obtained from this study suggest that these materials should also be reexamined. In addition, understanding the role of disorder in vibrating materials could imply a new perspective on how to achieve control of matter, especially in the field of superconductivity, which could have important implications for nanotechnology and optoelectronics. (Source: ICFO)

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