Michael R. Marvel

How Lewis Acidity of the Cationic Framework Affects KNaNbOF5 Polymorphism

The valence matching principle is used to explain the loss of inversion symmetry in the noncentrosymmetric (NCS) polymorph of KNaNbOF5 in comparison to its centrosymmetric (CS) polymorph. The [NbOF5](2-) anion has five contacts to both potassium and sodium in the NCS polymorph, whereas in the CS polymorph there are only four contacts to potassium and six contacts to sodium. The lower average Lewis acidity of the cationic framework in the NCS polymorph relative to the CS polymorph reflects the loss of inversion symmetry. This lower average Lewis acidity is achieved during hydrothermal synthesis with a potassium-rich solution when the K:Na ratio in the reaction is greater than 1:1, as the Lewis acidity of potassium is lower than that of sodium. The contrasting coordination environments are manifested in secondary distortions that weaken the primary Nb═O interaction and lengthen the Nb═O bond in the NCS polymorph. An unusual heat-induced phase transition from the CS to the NCS polymorph was studied with in situ powder X-ray diffraction. The transition to the NCS polymorph upon cooling occurs through an intermediate phase(s).

Noncentrosymmetry in Mixed Metal Oxide-Fluorides, Can We Control It?

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 KNaNbOF 5 , the combination of a second-order Jahn Teller active d 0 transition metal oxyfluoride anionic unit and mixed K/Na cation coordination environments are shown to result in a polar structure (space group Pna2 1 ). 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 Paulings second crystal rule (PSCR), leading to O/F ordering and acentric packing of the [NbOF 5 ] 2− anionic unit.