  We report an optimized chemical vapor transport method to grow single
crystals of (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ where x = 0, 0.3, 0.5, 0.7 & 1.
Single crystals up to 4\,mm\,$\times$\,3\,mm\,$\times$\,200\,$\mu$m were
obtained by this method. As-grown crystals characterized by means of scanning
electron microscopy, and powder x-ray diffraction measurements. The structural
characterization shows that all crystals crystallize in monoclinic symmetry
with the space group $C2/m$ (No. 12). We have further investigated the magnetic
properties of this series of single crystals. The magnetic measurements of the
all as-grown single crystals show long-range antiferromagnetic order along all
crystallographic principal axes. Overall, the N\'eel temperature TN is
non-monotonous, with increasing $Ni^{2+}$ doping the temperature of the
antiferromagnetic phase transition first decreases from 80 K for pristine
Mn$_2$P$_2$S$_6$ (x = 0) up to x = 0.5, and then increases again to 155 K for
pure Ni$_2$P$_2$S$_6$ (x = 1). The magnetic anisotropy switches from
out-of-plane to in-plane as a function of composition in
(Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ series. Transport studies under hydrostatic
pressure on the parent compound Mn$_2$P$_2$S$_6$ evidence an insulator-metal
transition at an applied critical pressure of ~22 GPa
