Emerging technologies require high-performance nonlinear optical (NLO) materials that exhibit enhanced optical properties at the microscopic and macroscopic level. The rational design of crystal structures, in particular noncentrosymmetric (NCS) materials, and how to target polar, polar-chiral, and chiral structures, is an ongoing theme in crystal engineering. A new class of potentially high performance NLO inorganic materials, solid-state oxide-fluorides, has been identified with the synthesis of new compounds containing [MOxF6-x]2- units in inorganic, solid-state environments. In the process it has been discovered that i) the use of fluoride ligands with early-transition metals (ETMs) enhances the second-order Jahn-Teller distortion in comparison to pure oxide ETM compounds, ii) the synthesis of NCS materials can be achieved with the use of polar basic-building units (BBUs), iii) design of NCS BBUs can predictably lead to NCS structures, iv) the synthesis of compounds that contain two, separate anionic BBUs is likely to result in an NCS compound, and v) the synthesis of ETM oxide-fluoride compounds can produce energy-storage materials such as high-potential primary batteries. Drawing from these learned principles, a program of discovery-based research on NCS structures based on acentric ETM oxide-fluorides (ETMOFs) is planned. These materials comprise a large and new class of solids with properties associated with piezoelectricity, pyroelectricity, ferroelectricity and second harmonic generation (SHG). The structural-property relationships that give rise to high, low, or null nonlinearities of the NLO material are examined. Finally, the growth of large crystals (on the order of 1 cm3) allows detailed analysis with measurements of the NLO tensor and phase-matchability properties, and other properties of interest such as piezoelectricity. The materials synthesized will have potential use in relaxor ferroelectrics and non-linear optics. These materials will have a high damage threshold and the potential for bulk growth to create large, single crystals suitable for optical use and examination. This work is supported by the Solid State and Materials Chemistry Program.
NON TECHNICAL SUMMARY As recently described in a Nature News Feature, the highly-efficient non-linear optical (NLO) oxide-fluoride material KBe2BO3F4 and other NLO materials are essential to act as wave guides for the modification of the amplitude, phase, and/or frequency of an optical signal. Such materials are used in femtosecond laser spectroscopy and-more recently-to generate UV laser light for nanolithography. Such NLO materials are increasingly needed to upconvert laser light to higher energies by second-harmonic generation (SHG). Current research and engineering activities in the field of NLO materials typically concern synthesis of NLO crystals; however, research is lacking in understanding how to design NLO materials and why materials exhibit a particular NLO response. While exploring the fundamental driving factors that create NLO materials, research will be performed to grow large (on the order of cm3) single crystals with a floating zone image furnace to allow a quantitative understanding of the interaction of light with the NLO material. This project concerns such analyses and progress made towards rational synthesis of solid-state inorganic materials and analysis of their structure-property relationships. These analyses of basic principles of the chemistry by graduate students and postdoctoral associates - who are engaged in local education, teaching, mentoring, and leadership - will facilitate broad understanding of stable solid-state compounds.
