Classification and relationship «structure - property» for binary noncentrosymmetric tungstates and molybdates

An empirical way to classify the known tungstates and molybdates M_mE_nW_pO_t is presented. As a basic criterion the shortest chemical bond length L(E, M-O) between cation and oxygen in crystal lattice is taken. Here the cations E and M are defined by the relation L(E-O) les 200 pm <L(M-O). On the plane L(M-O) - L(E-O) the acentric tungstate and molybdate crystals are positioned inside three partly intersected ellipses of LtacentricityGt. The tungstate and molybdate crystals with high non-linear and others physical parameters are concentrated near the axes of the ellipses or positioned near the boundary of the rosette revealing nonmonotonic dependency of the properties on the shortest chemical bond lengths. The distribution of tungstate crystals on point symmetry is sharply nonmonotonic with maximum for C_2v, D₂ and T_d. This approach allows to classify acentric tungstates and molybdates and predict NLO properties.