  Halide perovskites are promising semiconductor materials for optoelectronics.
One of their unique features is the synthesis, that could be performed by the
crystallization from the solution. This method is fast, but practically
couldn't be optimized due to the strongly non-equilibrium conditions of crystal
growth. Another quality limitation is the typical incorporation of organic
solvent molecules into crystals when perovskite crystallization is performed
from solution with organic solvent. Here we report an alternative method of
perovskite single crystal growth based on the counterdiffusion of reagents in
the gel medium. This counterdiffusion-in-gel crystallization (CGC) method is
based on the difference of solubility between perovskites and lead halide in
the corresponding halide acid, HX. The difference of solubility between
perovskites and lead halide leads to the formation of perovskite single
crystals of sufficiently large size with an excellent optical quality at
constant room temperature. Particularly, here we report the growth of MAPbI3
and MAPbBr3 halide perovskite single crystals (MA = CH3NH3+) by the
counterdiffusion of the organic-solvent-free reagents in the U-tube filled with
the silica gel. Their photoluminescence spectra recorded at 4 K demonstrate the
excitonic resonances with the full width < 2.5 meV, which proves their
excellent optical quality. The proposed method can be used in fabrication of
high-quality halide perovskite single crystals for both fundamental research,
and for applications where the absence of defects is a critical requirement.
