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[en] An isotopic effect in the distribution of H and D is found out in the solid solution TiN_0.40H_0.19D_0.19 with combined isotopic composition, consisting in that the isotopes H and D are arranged in one type of tetrahedral interstices (space group P3^-ml) with different coordinates z, i.e. the splitting of the tetrahedral positions 2(d) to hydrogen (H) and deuterium (D) positions is observed. It follows from this that in the ordered 'layered' structure of the solid solution TiN_0.40H_0.19D_0.19 two different isotopes of the same element can not be on one plane. The analysis of coordination polyhedrons (environment) of H and D atoms in the solid solution TiN_0.40H_0.19D_0.19 is carried out; the differences of polyhedrons surrounding H and D isotopes are revealed. (author)