Structures of optical recording materials Ge₂Sb _2+xTe₅ studied by pulsed neutron diffraction

Metastable cubic Ge₂Sb_2+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 Ge₇Sb ₈Te₁₀. 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.