Structures with deviations from Pauling’s second crystallographic rule (PSCR) can result in lower symmetry, including the loss of an inversion center. According to PSCR, cations with the largest positive potential occupy sites with the largest negative potential, and vice versa. One strategy to deviate from PSCR is to deviate from the valence matching principle, which states that more stable structures form when the cation’s Lewis acidity closely matches the anion’s Lewis basicity. Analysis of two polymorphs of KNaNbOF5 [1,2] demonstrates how violation of the valence matching principle can result in a NCS compound. The NCS and polar polymorph contains a cationic network whose average Lewis acidity is lower than that of the centrosymmetric polymorph. The slightly less acidic cationic network that corresponds to the polar polymorph can be achieved synthetically if the K:Na ratio in the reaction solution is greater than 1:1 because the Lewis acidity of K is less than that of Na [3]. The Lewis acidity of the reaction solution therefore matches the Lewis acidity of the cationic framework. This synthetic principle has led to a procedure to grow large single crystals suitable for single crystal property measurements. [1] M. Marvel, J. Lesage, J. Baek, P. Halasyamani, C. Stern, K. Poeppelmeier, J. Am. Chem. Soc. 129, 13963 (2007). [2] R. Pinlac, C. Stern, K. Poeppelmeier, Crystals, 1, 3 (2011). [3] K. Chang, A. Vinokur, R. Pinlac, M. Suchomel, M. Marvel, K. Poeppelmeier, Inorg. Chem. 53, 6979 (2014).

Low temperature synthesis of (noncentrosymmetric) oxide-fluoride materials

249th National Meeting (2015)

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