KNiB₄ O₆ F₃ : A Layered Fluorooxoborate with Charge-Oriented Ordering

A layered fluorooxoborate, KNiB₄ O₆ F₃ , contains a new (B₄ O₆ F₄ )^4- group built of one planar (BO₃ )^3- triangle and three tetrahedral (BO₃ F)^4- units. Those units are joined together by sharing the oxygen atoms to form a 2-dimensinal (BO_3/2 F)₃ BO_3/2 )_∞ layer. K⁺ and Ni²⁺ occupy on the sides with F^- and O^2- of the (BO_3/2 F)₃ BO_3/2 )_∞ layer with a high (positive charge) to high (negative charge) and low to low coordination. Such kind of charge-oriented ordering is found to be governed by the stabilization energy of Coulomb interaction of the cations in certain sites. It is hoped that this mechanism of ordering may provide an additional tool for designing new structures with favourable properties, such as ferroelectrics or nonlinear optical materials.