**3. Crystal Growth via Chemical Vapor Transport**

All procedures for the preparation were performed in a glove box under argon atmosphere. The elemental educts silver, chromium, red phosphorus and sulfur were weighed out in a molar ratio of Ag : Cr : P : S = 1 : 1 : 2 : 6 and homogenized in an agate mortar. 0.5 g of reaction mixture were loaded in a quartz ampule (6 mm inner diameter, 2 mm wall thickness) together with approx. 50 mg of the transport agent iodine. Immediately prior to use, the ampule was cleaned by washing with distilled water, rinsing with isopropanol and, subsequently, baking out at 800 ◦C for at least 12 h in an electric tube furnace. This is done to avoid contamination of the reaction volume with (adsorbed) water. The filled ampule was then transferred to a vacuum pump and evacuated to a residual pressure of 10−<sup>8</sup> bar. To suppress the unintended sublimation of the transport agent during evacuation, the end of the ampule containing the material was cooled with a small Dewar flask filled with liquid nitrogen. After reaching the desired internal pressure, the valve to the vacuum pump was closed, the cooling was stopped and the ampule was sealed under static pressure at a length of approximately 12 cm.

**Figure 2.** (**a**) Graphical illustrations of the temperature profile for the CVT growth of AgCrP2S<sup>6</sup> and (**b**) schematic drawing of an ampule during CVT. Arrows indicate the mass flow of the volatile transport species (top) and the flow of the released transport agent back to the charge (bottom). (**c**,**d**): As-grown crystals of AgCrP2S6. A orange square in the background corresponds to 1 mm × 1 mm for scale.

The ampule was carefully placed in a two-zone tube furnace in such a way that the reaction mixture was only at one side of the ampule which is referred to as the charge region. As illustrated in Figure 2a, the furnace was initially heated homogeneously to 750 ◦C at 100 ◦C/h. The charge region was kept at this temperature for 274.5 h while the other side of the ampule, which is the sink region (see Figure 2b), was initially heated up to 800 ◦C at 100 ◦C/h, dwelled at this temperature for 24 h and then cooled back to 750 ◦C at 1 ◦C/h. An inverse transport gradient is formed, i.e., transport from sink to charge, to clean the sink region of particles which stuck to the walls of the quartz ampule during the previous preparation steps. This ensures improved nucleation conditions in the following step. Then the sink region was cooled to 690 ◦C at 0.5 ◦C/h to gradually form the thermal transport gradient resulting in a controlled nucleation. With a final gradient of 750 ◦C (charge) to 690 ◦C (sink), the ampule was dwelled for 100 h. After this period of time, the charge region was cooled to the sink temperature in 1 h before both regions were furnace cooled (i.e., the heating elements were turned off) to room temperature.

Shiny plate-like crystals of AgCrP2S<sup>6</sup> in the size of approximately 2 mm × 2 mm × 100 µm were obtained. As example, as-grown single crystals are shown in Figure 2c,d. These crystals exhibit a layered morphology and are easily exfoliated, which is typical for bulk crystals of (quasi-)2D materials.

As shown in Figure 3a, some as-grown crystals exhibit small spherical particles (likely solidified droplets) of a secondary phase (bright) and pieces of a second secondary phase (dark). As the secondary phases are found only on the surface, exfoliating the crystals is sufficient to remove the secondary phases and results in crystals with clean surfaces, as illustrated in Figure 3b.

**Figure 3.** SEM image with chemical contrast (BSE detector) of (**a**) an as-grown crystal of AgCrP2S<sup>6</sup> with superficial impurities and (**b**) a piece of the same crystal after exfoliation with a clean surface.
