An international team with the participation of the Higher Council for Scientific Research (CSIC) has shown that a transition in the chemical bonding mechanism facilitates the storage of data in phase change materials, substances that are currently used, for example, in the manufacture of the latest generation mobiles. The results, published in the journal Science, could be used to optimize these materials and develop new faster and more effective information storage technologies.

Phase change materials (PCMs) are substances (compositions of antimony, tellurium and germanium) capable of storing large amounts of information efficiently. The data is recorded by applying heat pulses locally, thus changing from a vitreous to a crystalline state and vice versa.


Computer systems. / European XFEL / Jan Hosan

"These different states represent the 0 and the 1 of the binary code necessary to store information. However, until now it has not been possible to clarify exactly how these changes of state occur at the atomic level ", explains the researcher of the CSIC Jan Siegel, of the Optical Institute" Daza de Valdés ".

In the experiments carried out in the Linac Coherent Light Source in California (United States), scientists have used a technique called femtosecond X-ray diffraction to study the atomic changes that occur when materials change state. Using pulses of X-ray lasers lasting less than 10-13 seconds, it was possible to capture up to 10,000 images of the atomic structure at different times after initiating the phase change process with another laser pulse. The sequence of the images clarifies the different atomic changes produced during the process.

The crystallization process

To store information using phase change materials, they must be melted and then cooled quickly, thus becoming uncrystallized. They must also remain in this vitreous state while the data is required to be stored. This means that the crystallization process has to be very slow, to the point of almost not occurring, as in the case of normal glass used, for example in the manufacture of windows. However, at high temperatures, the same material has to be able to crystallize very quickly in order to erase the information. For decades, scientists have sought answers to this dilemma: that a material can form a stable glass but, at the same time, become unstable at high temperatures.

During the experiments, the scientists studied the rapid cooling process by which glass is formed. They found that, when the liquid cools sufficiently below the melting temperature, it goes through a structural change to form another liquid at a low temperature. This liquid can only be observed at very short time scales, before crystallization occurs. The two liquids do not only have different structures, but also different behaviors: the liquid at high temperature has a high atomic mobility that allows atoms to crystallize, that is, to group together to form an ordered structure. However, when the liquid passes below a certain temperature lower than the melting point, some chemical bonds become stronger and stiffer and can keep the atomic structure of the glass in place.

"The rigid nature of these chemical bonds prevents transformation and, in the case of memory devices manufactured with phase change materials, ensures that the information is not deleted", explains the CSIC researcher.

Peter Zalden, Europe X-FEL scientist and co-leader of the study, adds: "The current data storage technologies have reached the limit of scale, so new concepts are required to store the large amount of data that we will produce in the future. Our study explains how the phase-change process of this promising new technology can be fast and reliable at the same time. "
The published work has been led by Peter Zalden, of the European X-FEL, and Klaus Sokolowski-Tinten, of the University of Duisburg-Essen (Germany), with the participation of scientists from the Forschungszentrum Jülich, Institut Laue-Langevin, Lawrence Livermore National Laboratory, Lund University, Paul Scherrer Institute, SLAC National Accelerator Laboratory, Stanford University, University of Aachen, University of Potsdam and Jan Siegel of the "Daza de Valdés" Optical Institute of the CSIC.