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).
Our research creates functional materials that can enable the use of lasers and piezoelectric devices. These materials have these particular properties (known as ‘nonlinear’ properties) as the crystalline materials belong to a class that does not have a symmetry condition known as a center of inversion. For this reason, they are called "noncentrosymmetric" (NCS) materials. One familiar example of a nonlinear property, called second harmonic generation, is used to upconvert infrared laser light into the visible spectrum making green laser pointers possible. Piezoelectricity is used in products ranging from high quality vibration sensors, speakers, cigarette lighters, and quartz watches. In the course of our research we produced several new crystal compounds possessing noncentrosymmetric (NCS) character. More significantly, we have discovered structural factors that can cause compounds to adopt NCS structures, and have developed new and useful strategies to help us in synthesizing more compounds with these structures as we continue our research. Using x-ray radiation and single crystal x-ray diffraction we are able to very accurately determine the positions and arrangement of atoms in our crystal compounds, this in turn made in-depth analysis of these structural features possible. The crystal symmetries of the oxyfluorides are determined by the network of contacts the surrounding network provides within the crystals. We found that a very useful strategy for synthesizing new NCS compounds involves thinking about these compounds as containing what we call Basic Building Units (BBUs). BBUs coordinate with spherical cations such as sodium and potassium ions to make up the crystal structures we observe. BBUs are multi-atom, molecular anions made of oxide anions and fluoride anions surrounding transition metal cation centers. With some transition metal centers, the resulting BBUs become distorted and adopt a dipole moment. Compounds with BBUs that have a dipole moment are more likely to crystallize lacking inversion symmetry. We also demonstrated that BBUs that have a bent, or ?-shapes, are more likely to assemble in NCS structures than linear BBUs. Notably, within the figure provided, we found that we could make a molecule of CuVOF4(H2O)6 that has a ?-shape. This molecule is believed to crystallize from solution when the two ions [Cu(H2O)6]2+ and [VOF4(H2O)]2- combine. Progress toward the understanding of how to optimize the assembly of oxyfluoride anions into ordered NCS structures is helping to advance the field of inorganic synthesis. Our research has helped to advance the field of inorganic chemistry closer to the goal of being able to perform targeted synthesis of inorganic compounds based on desired structural features.
