For a material to exhibit second-order nonlinear optically-activity, such as second-harmonic generation (SHG), a crystal must belong to a noncentrosymmetric crystal class. The hydrothermal syntheses and analyses of noncentrosymmetric materials often examine solid-state structures on a 'molecular' basis with emphasis on modular basic-building units. The use of polar basic building units within an extended structure can endow polarity upon the bulk. This polarity must be conserved from the individual units to the bulk, eg the polar units should not be aligned antiparallel as such an arrangement would result in a zero net dipole. In order to target such materials, an understanding of the reaction conditions, equilibria, and crystallization processes is necessary. As a demonstration, we present the structures of NaVOF4(H2O) and NaVO2-x-F2+x (x = 1/3,  = 1/6)  in which different NCS structures can be created from the same reagent ratios but varied temperatures. The temperature acts to create vanadium-oxide fluorides that are of (i) increased dimensionality and (ii) reduced oxidation states. The structure of the mixed valence compound has the 4+/5+ vanadium cations disordered; interestingly this disorder actually increases polar alignment of the basic building units. This is counter intuitive as disorder typically generates nonpolar materials. This concept will be explored in the context of generating polar materials and the possibilities of creating mixed-valence compounds by finding appropriate chemical equilibria for their synthesis.

In situ reduction to generate a second-harmonic generation active, mixed-valence sodium vanadium oxide-fluoride

247th National Meeting (2014)

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