Growth of high-perfect mixed K2NixCo1-x(SO4)2·6H2O crystals for fabrication of high-efficiency UV optical filters

•The regimes of the solution feeding that increase the mixed crystal homogeneity have been realized.•The regimes of the growth solution feeding can be calculated to obtain crystals of the required composition.•The use of the shaper allows one to eliminate the sectoral inhomogeneity of the crystal.•Growth at a constant temperature with feeding eliminates zonal inhomogeneity.•The optical filters made from mixed crystals K2NixCo(1-x)(SO4)2·6H2O demonstrate high optical quality.