Jxa 8200 superprobe electron probe microanalyzer

Thin films of GSST were prepared using thermal evaporation from a single Ge₂Sb₂Se₄Te₁ source. Bulk starting material of Ge₂Sb₂Se₄Te₁ was synthesized using a standard melt quench technique from high-purity (99.999%) raw elements^{58 (link)}. The film deposition was performed using a custom-designed system (PVD Products, Inc.) following previously established protocols^{59 (link),60 (link)}. The substrate was not actively cooled, although the substrate temperature was measured by a thermocouple and maintained at below 40 °C throughout the deposition process. Stoichiometries of the films were confirmed using wavelength-dispersive spectroscopy (WDS) on a JEOL JXA-8200 SuperProbe Electron Probe Microanalyzer (EPMA) to be within 2% (atomic fraction) deviation from target compositions.

Major element compositions of the quenched melts and coexisting silicate minerals from all experimental run products were analyzed using JEOL JXA-8200 Superprobe electron probe micro-analyzer at Massachusetts Institute of Technology. Analyses were performed using a 15 kV accelerating potential and a 10 nA beam current. The beam diameter varied depending on the target point. For quenched melt pools, beam diameters varied between 3 μm to 10 μm (mostly 5 μm) depending on the size of the melt pools. For silicate minerals, a focused beam (1 µm) was used. Data reduction was done using CITZAF package⁷⁸ . Counting times for most elements were 40 s on peak, and 20 s on background. In order to prevent alkali diffusion in glass, Na was analyzed first for 10 s on peak and 5 s on background. All phases (melt and coexisting minerals) were quantified using silicate and oxide standards. The compositional maps for different major elements were performed at similar instrumental setup using a focused beam. Major element compositions of melts and minerals are reported in Supplementary Data 2 and 5, respectively.