Solid State Oxyfluorides

Northwestern University, Evanston IL

Investigators

Abstract

Technical Summary Technologies that require high-performance nonlinear optical (NLO) materials with enhanced optical properties at the microscopic and macroscopic level depend on inorganic materials that exhibit large optical nonlinearities (possessing a dipole). 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. These materials are essential for the modification of the amplitude, phase, or frequency of an optical signal. Polar distortions in metal centered octahedra are the origin of the nonlinear optical response in metal oxides. Octahedrally coordinated transition metal cations in groups 4,5,6 are unstable in mixed metal oxides with respect to intraoctahedral distortions, which can be understood through the second order Jahn-Teller theorem. This program explores research on noncentrosymmetric structures based on acentric transition metal oxyfluorides. These materials will provide a large and new class of solids with properties associated with piezoelectricity, pyroelectricity, ferroelectricity and second harmonic generation (SHG), all properties associated with noncentrosymmetric space groups. This work is supported by a grant from the Solid State and Materials Chemistry Program in the Division of Materials Research. Non-technical Summary The rational design of structures based on the chemical nature of molecular components, a longstanding and exciting research topic in organic solid-state chemistry, is an emerging theme of crystal engineering. Technologies that require high-performance nonlinear optical (NLO) materials with enhanced optical properties, however, depend on inorganic materials that exhibit large optical nonlinearities. Structures lacking inversion symmetry are a requirement for important current and future technologies that have been created by numerous inventions during the past two decades. These materials provide a large and new class of solids for studies in basic science associated with the noncentrosymmetric space groups. We propose an exploratory research program on these materials, which were highlighted recently in an article entitled "China's Crystal Cache" (Nature 2009, 457, 953-955). In this Nature article the critical national need for synthesis-based efforts to discover and to grow suitable crystals of these new materials was emphasized. This work is supported by a grant from the Solid State and Materials Chemistry Program in the Division of Materials Research at the National Science Foundation (DMR-1005827).