The results of studying crystallization in space are reviewed with focus on the growth of semiconductor and protein crystals. The history of the problem is considered, the influence of microgravity on the crystal growth is investigated, and the main experimental data on crystal growth in zero gravity are analyzed. The studies performed in this field at the Institute of Crystallography, Russian Academy of Sciences (IC RAS), are reviewed in detail.

Optically homogeneous mixed K₂Ni _x Co_{(1 − x)}(SO₄)₂ · 6H₂O crystals are grown from solutions of different compositions by the temperature-reduction technique in static and dynamic regimes. The optical characteristics of the grown crystals are measured: transmittance reaches 80% in the wavelength range of 240–290 nm and no more than 9% in the visible spectral range. The thermal stability of the crystals is studied. It is established that the thermal stability of mixed K₂Ni _x Co_{1 − x} (SO₄)₂ · 6H₂O crystals is higher than that of K₂Co(SO₄)₂ · 6H₂O crystal. The defects of the mixed crystals grown in static and dynamic regimes are investigated by X-ray topography.

This review, for the first time, summarizes the results of studies of the defect formation mechanisms in mixed crystals grown from aqueous solutions. The general mechanism of interaction of a crystal with a foreign solution is described (reaction of isomorphous replacement). As a result of this reaction, the crystal surface turns into a mosaic of local areas where multidirectional processes (dissolution and growth) occur simultaneously. Data on mosaic microinhomogeneity, which is a new type of composition inhomogeneity inherent solely to multicomponent crystals, is presented. A new mechanism for the mismatch stress relaxation in heterocompositions of brittle crystals grown from low-temperature solutions is described; in this case, the formation of misfit dislocations is impossible and stress relaxation occurs due to the formation of numerous inclusions at the interface. The general concept of growing high-quality mixed crystals from solutions is described, using the example of K₂(Co, Ni)(SO)₂ · 6H₂O (KCNSH) mixed crystals.

Studies of the growth of large-area free-standing single-crystalline films of p-quinquephenyl are presented. High-quality crystals were grown by slow cooling of a hot chlorobenzene solution. Worse quality large-area free-standing single crystals of p-quinquephenyl were also grown by using physical vapor transport and used for comparison. The crystal structure of p-quinquephenyl at 293 K and 85 K was refined by single-crystal X-ray diffraction. The optical absorption and photoluminescence spectra of solutions and crystalline films were obtained and analyzed; a positive solvatochromic effect was detected.

The mosaic microinhomogeneity in mixed crystals of the isomorphic series (Co,Ni)K₂(SO₄)₂ · 6H₂O has been studied. The growth processes have been observed in situ in optical and confocal microscopes and the distribution of composition components has been studied by the energy-dispersive method. The results make it possible to develop model concepts about the formation of composition microinhomogeneity in solid solutions.