Experiments

Amorphous materials under pressure

Glassy state, which is a universal property of supercooled liquids if they are cooled rapidly enough,is regarded as the fourth type of matter. The mysterious glass transition phenomenon, whichconnects the liquid and glassy states, is related widely to daily life, industry, materials preparation,organism preservation and a lot of natural phenomena. However, the exact and comprehensive physicalunderstanding of the glass transition and glass natures is considered to be one of the most challengingproblems in condensed matter physics and material science. Due to the random disorderedstructure, the characterization of the glasses is very difficult, and this leads to problems for understandingthe formation, deformation, fracture, nature, and the structure–properties relationship ofthe glasses.Metallic glass, which is a newcomer in glassy family (discovered in 1959) and at the cutting edge ofcurrent metallic materials research, is of current interest and significance in condensed matter physics,materials science and engineering because of its unique structural features and outstandingmechanical, many novel, applicable physical and chemical properties (ref 1-4). Metallic glasses havealso been the focus of research advancing our fundamental understanding of liquids and glassesand provide model systems for studying some long-standing fundamental issues and have potential
engineering and functional applications. The recently developed bulk metallic glass (BMG) forming
systems are complicated multicomponent alloys that vitrify with remarkable ease during conventional
solidification. The BMGs represent a novel and exciting group of metallic materials with many
favorable and applicable properties as compared to their crystalline counterparts. Their unique combinations
of mechanical, physical, tribological and chemical properties are of current interest and
importance. Nevertheless due to their complexity and to the difficulty of characterizing a structurally disordered material,the birth of the
novel metallic glasses also presents a lot of unresolved issues, outstanding questions and challenges.
We have recently started the study of the phase diagram of amorphous Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> (GST). It is one of the most studied chalcogenide phase change materials, because of its use as optical memory in DVD re-writable optical discs and in phase change non volatile random access memories (PCRAM). The memory concept is based on the possibility to switch between the amorphous and the crystalline phase by applying electric or laser pulses. The large difference in the electrical and optical properties between the two phases assures the existence of two well defined logic states. The data retention of a phase change memory is determined by the thermal stability of the amorphous phase. The crystallization to the metastable face centred cubic (fcc) structure occurs at about 140°C in blanket films. At higher temperatures (>250°C) the material converts into the stable hexagonal structure (hcp).
We have performed a series of X ray diffraction measurements along isothermal pressure scans at room and at high temperature, observing the phase transitions occurring along the P-T path followed. A careful work of analysis is now in progress in order to determine the new cristalline phases discovered.

1 Greer AL. Confusion by design. Nature 1993;366:303.
2 Cahn RW, Greer AL. Metastable states of alloys. In: Cahn RW, Haasen P, editors. Physical metallurgy. Elsevier Science BV; 1996 [chapter 19].
3 Schroers J, Paton N. Amorphous metal alloys form like plastics. Adv Mater Process 2006;61:164.
4 Löffler JF. Bulk metallic glasses. Intermetallics 2003;11:529


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