  By varying the parameters controlling the growth of crystals, including the
thermodynamic variables, such as temperature, pressure, and reagent composition
and the kinetic factors, namely reaction time and cooling rate, we found the
most appropriate conditions for the reproducible growth of the
nonstoichiometric antiperovskite superconductors MgC1-xNi3-y. Bulk single
crystals of MgC1-xNi3-y were grown by a self-flux method at 3 GPa and 1700 C
using a mixture of Mg, C, and Ni powders in a molar ratio 1:1.25:3. The
as-grown black colored crystals, mechanically extracted from solidified lump,
exhibit various irregular three dimensional shapes, with flat surfaces and
maximum dimensions up to ~ (1-1.2) x (0.8-1.0) x (0.4-0.6) mm3. Single-crystal
x-ray diffraction refinement confirmed the high structural perfection of the
grown crystals (Space group Pm-3m, No 221, Z = 1, a = 3.7913(1) {\AA}, and V =
54.5(1) {\AA}3), but also the presence of deficiencies on the C and Ni sites.
Temperature-dependent magnetization measurements showed a single-phase
behaviour with a critical temperature (Tc) ranging between 6.3 and 6.8 K due to
the slightly different C and Ni stoichiometries of MgC1-xNi3-y crystals. The
growth of relatively large crystals reported here could provide a helpful
guidance for further syntheses of various 3d-based antiperovskite
intermetallics under high pressure.
