Cation−Anion Interactions and Polar Structures in the Solid State

Complicated structures where oxygen and fluorine are found together in one framework, where
deviations from Pauling's second crystal rule (PSCR) are expected, often result in structures with important
physical properties. The [NbOF5]2- anion and therefore all the individual Nb−O and Nb−F bonds are ordered
in noncentrosymmetric KNaNbOF5 and centrosymmetric CsNaNbOF5. The Na/K− and Na/Cs−O/F
interactions in these phases, in particular the expected deviations from PSCR and the bond valence model,
reveal the essential role of the small potassium cations in the acentric packing of the [NbOF5]2- anion.
KNaNbOF5 crystallizes in the orthorhombic and polar space group Pna21 (No. 33) with lattice constants a
= 11.8653(11) Å, b = 5.8826(6) Å, c = 8.1258(8) Å, and Z = 4, while CsNaNbOF5 crystallizes in the
orthorhombic space group Pbcn (No. 60) with lattice constants a = 8.3155(7), b = 13.3176(11), c = 11.1314(9), and Z = 8.