Structures of Optical Recording Materials Ge 2 Sb 2+x Te 5 Studied by Pulsed Neutron Diffraction
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Structures of Optical Recording Materials Ge2Sb2+xTe5 Studied by Pulsed Neutron Diffraction Shin-ichi Shamoto, Tomitsugu Taguchi, Toshiyuki Matsunaga1, Noboru Yamada2, Kenji Ishii3, Toshiya Inami3 Neutron Science Research Center, Japan Atomic Energy Research Institute (JAERI), Tokai, Ibaraki, 319-1195, JP 1 Characterization Technology Group, Matsushita Technoresearch, Inc., Moriguchi, Osaka 5708501, JP 2 Optical Memory Group, Matsushita Electric Industrial Co., Ltd. Moriguchi, Osaka 570-8501, JP 3 Synchrotron Radiation Research Center, JAERI, Mikazuki, Hyogo, 679-5148, JP ABSTRACT Metastable cubic Ge2Sb2+xTe5 compounds, which can be prepared only as thin films, have been studied by both of neutron and x-ray powder diffraction measurements. Large displacement of germanium atoms in a crystalline cubic Ge2Sb2Te5 phase is found by PDF analysis. By adding Sb, the crystalline cubic phases coexist with two types of amorphous phases, depending on the quenching temperature. One has a first sharp diffraction peak similar to the amorphous Sb at Q ~1.1 Å-1. Another has a new peak at Q ~0.74 Å-1. Based on our crystal structure analyses, the composition of the new amorphous phase was estimated to be about Ge7Sb8Te10. The composition of the crystalline solid was also found to be different from the matrix, resulting in the nucleation dominated crystal growth in this system. INTRODUCTION A Ge2Sb2+xTe5 compound exhibits four different phases. The first stable crystalline phase has a trigonal structure. The second metastable crystalline phase has a cubic structure. The third one is a nanocrystalline cubic phase. The last one is an amorphous phase. The latter three phases have been prepared only as thin films. These phases have different optical reflectivity depending on their electronic conductivity, which can be used as a memory. Their rapid structural phase transition (
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