Crystal growth, exfoliation, and magnetic properties of quaternary quasi-two-dimensional ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$

We report optimized crystal growth conditions for the quaternary compound ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive x-ray spectroscopy and powder x-ray diffraction. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ is structurally closely related to the $M_2{\mathrm{P}}_2{\mathrm{S}}_6$ family ($M$: transition metal), which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a platelike, layered morphology as well as a hexagonal habitus. We present successful exfoliation of such as-grown crystals down to thicknesses of 2.6 nm corresponding to four layers. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ crystallizes in the monoclinic space group $C2/c$. Magnetization measurements reveal an antiferromagnetic ground state with $T_{\text{N}}\approx 30$ K and a positive Curie-Weiss temperature in agreement with dominant ferromagnetic intralayer coupling. Specific heat measurements confirm this magnetic phase transition and the magnetic order is suppressed in an external magnetic field of about 6 T (8 T) applied parallel (perpendicular) to the $ab$ plane. At higher temperatures between 140–200 K, additional broad anomalies associated with structural changes accompanying antiferroelectric ordering are detected in our specific heat studies.

Figure 1

Comparison of the structures of (a) ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ [27] (as exemplary member of the ternary $M_2{\mathrm{P}}_2{\mathrm{S}}_6$ compounds) and (b) ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$. Top: view on the $ab$ plane; bottom: view along $b$ illustrating the monoclinic stacking of layers in the $ac$ plane. The dashed lines show the contour of the unit cell. The ${\mathrm{CuS}}_6$ structural unit is shown separately in (c) to illustrate the different Cu positions in ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ according to the structural model proposed by Colombet et al. [17], with the partial filling of the Cu spheres corresponding to their site occupancies.

Figure 2

(a) Graphical illustration of the temperature profile for the CVT crystal growth of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ 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) [19].

Figure 3

(a) As-grown and (b) freshly exfoliated crystals of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$. An orange square in the background is $1\times 1 {\mathrm{mm}}^2$ for scale.

Figure 4

SEM images of a ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ crystal with (a) topographical contrast (using an SE detector) and (b) with chemical contrast (using a BSE detector).

Figure 5

Exfoliated flakes of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ on ${\mathrm{SiO}}_2$ substrates. (a): optical image and (b): atomic force microscope image of an exfoliated ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ flake (scale-bar: $5\mu \text{m}$). (c): Height profile extracted from the AFM image along the white dashed line in (b). The red dashed lines indicates 2,6 nm thickness.

Figure 6

Oxidation study of exfoliated ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ flakes. a,b,c: optical images of a ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ flake and c,d,e: AFM images respectively 1, 14 and 30 days after exfoliation. g,h,i: Height profiles corresponding to the black, red and blue dashed lines, respectively in a,b,c. No changes are visible, neither with optical nor with AFM microscopy. j,k,l: Optical images of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ flakes one year after exfoliation. Strong signs of degradation are visible on all flakes.

Figure 7

PXRD pattern of pulverized ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ crystals acquired using the Cu-${\mathrm{K}}_{{\alpha }_1}$ radiation ($\lambda =1.54059$ Å) compared to the calculated pattern based on the optimized structural model from Rietveld refinement. Inset shows a zoom in of the high angle fit.

Figure 8

(a) Single crystal x-ray diffraction precession image for the $\mathit{hk}0$ layer. The reciprocal plane is normal to the $c^{*}$-axis. (b) The precession image for the $0\mathit{kl}$ layer with index guidance in $k$ and $l$ direction.

Figure 9

(a) Thermal evolution of the normalized magnetization $M{H}^{-1}\left(T\right)$ and its first derivative for $H=1$ kOe applied parallel and perpendicular to the crystallographic $ab$ plane. $T_{\text{N}}$ is indicated by the black dotted line. (b) Inverse of the normalized magnetization as a function of temperature ${\left(M{H}^{-1}\right)}^{-1}\left(T\right)$. The black dashed lines are linear regressions of the temperature regime 100–300 K.

Figure 10

(a) Field dependent magnetization $M\left(H\right)$ at 5 K with $H\le 55$ kOe applied parallel (red) and perpendicular (violet) to the crystallographic $ab$ plane. The green curve shows $M\left(H\right)$ for $H\perp ab$ after demagnetizing field correction as explained in the text. The inset shows a zoomed-in view of the low-field regime up to 10 kOe. (b) $M\left(H\right)$ at 1.8 K with $H\le 70$ kOe applied parallel to the $ab$ plane. The spin-only saturation moment of ${\mathrm{Cr}}^{3+}$ (assuming $S=3/2$ and $g=2$) is marked by the grey dotted line.

Figure 11

Temperature-dependent zero-field specific heat capacity of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$. The red line corresponds to the Debye model for the phononic contribution, while the blue line corresponds to the extrapolation of the obtained phonon function (Eq. (1)) for the whole temperature range. The inset shows a zoom into the low-temperature region with antiferromagnetic long-range ordering at $T_{\text{N}}$ = 31 K.

Figure 12

Zero-field magnetic specific heat of ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ plotted as $C_{}\text{mag}/T$ vs $T$ (left scale) and the corresponding magnetic entropy $S_{}\text{mag}\left(T\right)$ (right scale).

Figure 13

Temperature and field dependence of the specific heat for ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ (a) for fields applied perpendicular to the $ab$ plane. The inset shows specific heat for 70 kOe. (b) For fields applied parallel to the $ab$ plane. The inset shows specific heat for 50 kOe.