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Published online by Cambridge University Press: 01 February 2011
The rational design of crystal structures, in particular noncentrosymmetric materials, and how to differentiate polar, polar-chiral, and chiral structures, is an ongoing theme in crystal engineering. In KNaNbOF5, the combination of a second-order Jahn Teller active d0 transition metal oxyfluoride anionic unit and mixed K/Na cation coordination environments are shown to result in a polar structure (space group Pna21). The crystal structure analysis of the Na/K-O/F interactions reveals that the potassium cations form one of the two contacts to the under-bonded oxide ions. These interactions satisfy the expected bond valence sums and Pauling's second crystal rule (PSCR), leading to O/F ordering and acentric packing of the [NbOF5]2− anionic unit.