Local crystal structures of Ge₂Sb₂Te₅ revealed by the atomic pair distribution function analysis

Two types of optical recording crystalline material Ge₂Sb ₂Te₅ have been studied by neutron and X-ray powder diffraction measurements. One is the crystalline cubic-phase Ge ₂Sb₂Te₅. The other is nanocrystalline-phase Ge₂Sb₂Te₅. In the former, a large atomic displacement of germanium atoms was found by the atomic pair distribution function (PDF) analysis of neutron powder diffraction data. According to the electronic band structure calculation of isoelectronic GeTe, the electrical conductivity at the valence band with holes is not severely affected by the randomness at the cation site, whereas the thermal conductivity is significantly reduced by the randomness. For the nanocrystalline phase, the local crystal structure and the particle size distribution were obtained simultaneously by PDF analysis using a spherical-particle form factor. The lattice parameter is longer than that of the crystalline phase, suggesting an antimony-richer composition than the matrix. This compositional deviation would form the nanoparticle, resulting in nucleation-dominated crystal growth.