[en] The intensity of the second harmonic generated in powdered samples of the compositions Sr_0.75Ba_0.25Nb₂O₆, Sr_0.61Ba_0.39Nb₂O₆, and Sr_0.50Ba_0.50Nb₂O₆ under irradiation by a pulsed-periodic YAG: Nd laser is measured. The effective values of the quadratic nonlinear susceptibility of these materials with respect to the reference sample of potassium dihydrophosphate crystals are estimated. The results obtained are compared with the precise parameters of the atomic structure of the single crystals with the corresponding compositions. It is established that the dominant contribution to the nonlinear optical susceptibility of the single crystals under investigation comes from the chains aligned parallel to the c axis of the crystal and composed of Nb octahedra with regularly alternating increased and decreased Nb-O interatomic distances. The occurrence of such a sequence in the crystal structure suggests an alternation of stronger and weaker chemical bonds in the chains -O=Nb-O=Nb-O=. These bonds are characterized by different charge electron densities, a phenomenon confirmed by the X-ray diffraction analysis, which indicates not only a difference in the Nb-O bond lengths but also an anharmonicity in the thermal vibrations of the niobium atoms. The revealed anharmonicity is associated with the difference between the chemical bonds of the niobium atoms with the oxygen atoms located in trans positions in the octahedra