1：Experimental Design and Method Selection

Polycrystalline powder samples of the Cu2ZnGe(S1-xSex)4 solid solution were synthesized by solid state reaction from the pure (5N) elements Cu, Zn, Ge, S, and Se. The weighed elements were placed in a graphite boat and sealed in evacuated silica tubes which, in turn, were placed in a tubular one-zone furnace.

2：Sample Selection and Data Sources

The chemical homogeneity and chemical composition of the synthesized polycrystalline powders were proven by quantitative wavelength dispersive X-ray spectroscopy (WDX) using an electron microprobe of the type JEOL JXA 8200 Superprobe. The quantitative element analysis includes an error of 2 %. 35 grains per sample were measured with 10 measurement points each.

3：List of Experimental Equipment and Materials

JEOL JXA 8200 Superprobe for WDX spectroscopy, LEO Gemini 1530 SEM for EDX analysis, graphite boat, silica tubes, tubular one-zone furnace.

4：Experimental Procedures and Operational Workflow

The samples were heated up to temperatures between 700 °C and 750 °C depending on the S:Se anion ratio, held for 300 h, and cooled down to room temperature with a cooling rate of 10 K/h. Intermediate steps at 250 °C, 450 °C, and 650 °C were introduced for balancing between reaction velocity and partial pressure of S and Se.

5：Data Analysis Methods

Chemical analysis was performed by WDX spectroscopy and EDX analysis. The elemental distribution within specific grains was analyzed by EDX using SEM.