Distribution, structures and nonlinear properties of noncentrosymmetric niobates and tantalates

The distribution of 117 noncentrosymmetric niobates and tantalates over different crystal systems and types of space formation of Nb, Ta-O polyhedrons have been revealed. The dependence of polyhedron space formation in the crystal lattice of the compound on stoichiometric concentration (SC) of niobium and tantalum is established. Individual Nb, Ta-O octahedrons are found for SC=19-7.5, and chains and layers of the octahedrons appear in the range SC=11.0-5.2. Only frame formations of Nb, Ta-O octahedrons are possible under SC<5.2.