Crystal Structure determination of Ge₂Sb₂Te₅ and Se-doped Ge₂Sb₂Te₅ material

The [GeTe]_n[Sb2Te3]_m layered materials have been widely used as component of data storage devices and electronic memories. Ge₂Sb₂Te₅ is a well-known phase change material that exhibits three phases: an amorphous phase, a rock-salt NaCl-type metastable phase and a hexagonal stable phase. To obtain the amorphous phase in bulk samples, we synthesized Ge₂Sb₂Te_5-xSe_x where we replaced Te with Se. At about 90 % of Se, the system becomes amorphous by quenching. The Se atom function as a stabilizer of the amorphous phase and helps to trigger the local displacements of the Sb and Ge atoms within the unit cell, thus breaking the long-range order and render the system into an amorphous one. The rich physics within this transition will be discussed. Moreover, Ge₂Sb₂Te₅ is associated with naturally occurring stacking faults and a topological phase transition depends sensitively on the distribution of these faults. It has been proposed theoretically that depending on the stacking sequence, GST can undergo a topological phase transition from the low temperature topological insulating phase with the so-called Kooi structure to the high temperature Weyl semi-metal phase with the Ferro structure. We synthesized the Kooi structure and the results from the DFT calculations will be discussed.