**1. Introduction**

Among the magnetic quasi-two-dimensional materials that have recently moved in the focus of (quasi-)two-dimensional (2D) materials research [1–3], the *M*2P2S<sup>6</sup> class of layered materials offers a plenitude of isostructural compounds with different magnetic properties depending on *M* [4,5]. Thus, *M*2P2S<sup>6</sup> allows to investigate fundamental aspects of low dimensional magnetism and several members may be promising for future applications, e.g., complementing non-magnetic (quasi-)2D materials in heterostructures or in spintronic devices [6,7]. Furthermore, future applications in the field of catalysis are conceivable due to the structural similarity to the non-magnetic 2D materials such as graphene or the transition metal dichalcogenide compounds for which such applications are already discussed [8–10].

Regarding the crystal structure, the *M*2P2S<sup>6</sup> family consists of van der Waals layered compounds which share a honeycomb network of *M*2<sup>+</sup> and, most prominently, a dominantly covalent [P2S6] <sup>4</sup><sup>−</sup> anion located in the voids of the honeycomb [4,5]. In the bulk, such layers are stacked on top of each other only interacting via weak van der Waals forces. Consequently, these compounds can be easily exfoliated potentially down to a single layer [11,12].

Several isovalent substitution series of *<sup>M</sup>*2<sup>+</sup> by another *<sup>M</sup>*02<sup>+</sup> (e.g., (Mn1−*x*Fe*x*)2P2S<sup>6</sup> [13], (Mn1−*x*Ni*x*)2P2S<sup>6</sup> [14], (Fe1−*x*Ni*x*)2P2S<sup>6</sup> [15,16] and (Zn1−*x*Ni*x*)2P2S<sup>6</sup> [4]) are reported to exhibit solid solution behavior and, thus, imply a random distribution of the substituents on the honeycomb network, as illustrated in Figure 1a. Beyond isovalent substitution, Colombet et al. [17–20] demonstrated that a substitution of *M*2<sup>+</sup> 2 by *M*1+*M*03<sup>+</sup> also yields several stable compounds. In contrast to the isovalent substitution series however, *M*1<sup>+</sup> and *M*03<sup>+</sup> do not randomly occupy the *M* positions in the lattice but order either in an alternating

**Citation:** Selter, S.; Shemerliuk, Y.; Büchner, B.; Aswartham, S. Crystal Growth of the Quasi-2D Quarternary Compound AgCrP2S<sup>6</sup> by Chemical Vapor Transport. *Crystals* **2021**, *11*, 500. https://doi.org/10.3390/ cryst11050500

Academic Editor: Raghvendra Singh Yadav

Received: 12 April 2021 Accepted: 26 April 2021 Published: 2 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

or in a zig-zag stripe-like arrangement on the honeycomb, as illustrated in Figure 1b,c, respectively. The former arrangement is attributed to a minimization of repulsive Coloumb interactions (i.e., charge ordering). The latter is observed for compounds for which *M*1<sup>+</sup> and *M*03<sup>+</sup> have notably different sizes and, thus, is dominantly driven by a minimization of lattice distortion and steric effects [4,17].

**Figure 1.** Schematic illustration of the different arrangements of *M* and *M*0 on the honeycomb lattice of *M*2P2S6. (**a**) Random distribution for *M*2+*M*02+P2S6. (**b**) Alternating/triangular arrangement and (**c**) zig-zag stripe like arrangement for *M*1+*M*03+P2S6.

With *M*03<sup>+</sup> being a magnetic ion (e.g., V3<sup>+</sup> or Cr3+) and *M*1<sup>+</sup> being non-magnetic (e.g., Cu1<sup>+</sup> or Ag1+), the magnetic sublattices formed in *M*1+*M*03+P2S<sup>6</sup> extend the magnetic structures of the usually magnetically hexagonal *M*2P2S<sup>6</sup> compounds by an alternating/triangular and a zig-zag stripe-like magnetic arrangement [4,17,19]. The stripe-like magnetic structure is especially notable, as each stripe of magnetic ions is well isolated from the adjacent magnetic stripes by a stripe of non-magnetic ions. Although the corresponding compound still has a (quasi-)2D layered crystal structure, the magnetic structure can be expected to exhibit 1D magnetic characteristics. Indeed, several indications for such low dimensional magnetism are reported for *M*1+*M*03+P2S<sup>6</sup> with *M* = Ag and *M*<sup>0</sup> = Cr [19,21], making it an interesting compound for further studies.

However, until now only details on the synthesis of AgCrP2S<sup>6</sup> via solid state synthesis are reported (although Mutka et al. [21] mention CVT grown crystals, they do not report any further details or conditions regarding the crystal growth) [19]. Although small crystals in the µm scale could be obtained by solid state synthesis, which allowed for a structural solution based on single crystal X-ray diffraction, significantly larger crystals are needed for detailed investigations of the physical properties including anisotropies.

As a crystal growth method of choice, for macroscopic AgCrP2S<sup>6</sup> single crystals, the chemical vapor transport (CVT) technique is suitable due to the contained volatile elements such as S and P. Phosphorus and sulfur are both volatile and readily evaporate at elevated temperatures. The generation of volatile intermediate transition metal species for the vapor transport using so-called transport agents is well established [22]. CVT is the crystal growth technique of choice for virtually all ternary *M*2P2S<sup>6</sup> compounds [4,5]. For example, Taylor et al. [23] and Nitsche et al. [24] report the successful crystal growth of *M*2P2S<sup>6</sup> with *M* = Mn, Fe, Ni, Cd and Sn by CVT using either chlorine or iodine as transport agent and we present the crystal growth of mixed transition metal phosphorus sulfides of the substitution series (Fe1−*x*Ni*x*)2P2S<sup>6</sup> [16] and (Mn1−*x*Ni*x*)2P2S<sup>6</sup> [14] by the same technique with iodine as agent. To determine a suitable temperature gradient for the CVT growth of the quarternary compound AgCrP2S6, several growth experiments with different temperature profiles were conducted. The temperature profile, which is reported hereafter, resulted in the best crystal size and quality as well as regarding impurity contributions and opens up access to macroscopic AgCrP2S<sup>6</sup> single crystals. In addition to the crystal growth of AgCrP2S6, we also present a comprehensive compositional and structural characterization of the obtained crystals.
