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.