**6. Summary and Conclusions**

We report optimized crystal growth conditions for the quarternary compound AgCrP2S<sup>6</sup> via Chemical Vapor Transport (CVT). A temperature profile adapted from the CVT growth of ternary *M*2P2S<sup>6</sup> compounds is sufficient to yield crystals of the target AgCrP2S<sup>6</sup> phase in the mm-size. On some crystals, traces of a superficial impurity phase is found which could be readily removed by exfoliation.

The as-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus and have the expected composition of AgCrP2S<sup>6</sup> based on EDX spectroscopy. The pXRD pattern is indexed in the space group *P*2/*a* in agreement with the literature [19]. The *P*2/*a* space group, on which the zig-zag type arrangement of *M* and *M*0 is based on, can be well distinguished from, e.g., the *C*2/*m* and *C*2/*c* space groups due to reflections that are systematically absent for *C* centering. Starting from the model of Colombet et al. [19], a refined structural model is obtained using the Rietveld method. This model contains a

notable distortion of the AgS<sup>6</sup> and P2S<sup>6</sup> coordination environments, while the CrS<sup>6</sup> units remain antiprismatic with a slight trigonal distortion.

The zig-zag stripe-like arrangement in AgCrP2S<sup>6</sup> and the alternating arrangement of *M* and *M*0 , which is reported, e.g., for CuCrP2S6, are promising to yield interesting magnetic and electronic structures. While only few such quarternary phosphorus sulfide compounds have been synthesized until now, many more combinations of a 1+-ion and a 3+-ion can be expected to form analogous compounds. Furthermore, the fundamental idea of replacing *MX*<sup>+</sup> by *M* (*X*−1)+ 0.5 *M* 0(*X*+1)+ 0.5 may be adoptable to the closely related structures such as, *M*3<sup>+</sup> 2 (Si,Ge)2Te<sup>6</sup> compounds.

The single crystals of AgCrP2S<sup>6</sup> that were obtained using the presented growth conditions allow for studies of the low dimensional magnetic interactions including the magnetic anisotropy of this compound in the future, which may lead to a better fundamental understanding of low dimensional magnetism. Furthermore, the van der Waals layered structure makes exfoliation easily possible and, thus, our successful growth of single crystals paves the way for further manufacturing of few-layer or even monolayer samples of AgCrP2S6.

**Author Contributions:** Investigation, S.S., Y.S., S.A.; data curation, S.S., Y.S.; writing—original draft preparation, S.S., S.A.; writing—review and editing, S.S., S.A.; supervision, B.B., S.A.; funding acquisition, B.B., S.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by the Deutsche Forschungsgemeinschaft (DFG) via Grant No. DFG A.S 523\4-1. S.S. acknowledges financial support from GRK-1621 graduate academy of the DFG. B.B. acknowledges financial support from the DFG through SFB 1143 (project-id 247310070). Y.S acknowledge the support of BMBF through UKRATOP (BMBF). S.A., B.B. and S.S. thank DFG for financial support in the frame of the joint DFG-RSF project-id 405940956.

**Data Availability Statement:** The refined crystal structure model and the powder X-ray diffraction dataset of AgCrP2S<sup>6</sup> presented in this study are openly available in the Crystallography Open Database (COD), COD ID: 3000295 under https://www.crystallography.net/cod/3000295.html.

**Acknowledgments:** The publication of this article was funded by the Open Access Fund of the Leibniz Association.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Single crystals of AgCrP2S<sup>6</sup> are available from the corresponding author.
