Research in the Halasyamani group involves the synthesis, characterization, and development of structure-property relationships in new functional inorganic materials. Functionalities we are interested in include second-order non-linear optical behavior, i.e. second-harmonic generation, piezoelectricity, ferroelectricity, pyroelectricity, and multi-ferroic behavior. In addition to bulk phase synthesis, we also have research programs in large single crystal growth. We utilize a range of diffraction (powder and single crystal X-ray) and non-diffraction (TG/DSC, powder SHG, pyroelectric, etc.) measurements to characterize the materials.
Specific research areas include -
A broad area that we are investigating involves the synthesis, characterization, and structure-property relationships in new polar oxides. Polar materials, those that exhibit a macroscopic dipole moment, exhibit a host of technologically important properties including piezoelectricity, ferroelectricity, and pyroelectricity. We have demonstrated that the incidence of polarity in any new oxide material can be increased by incorporating second-order Jahn-Teller (SOJT) disorted cations. With respect to crystal chemistry, SOJT distortions are observed in oxide environments of octahedrally coordinated d0 transition metals, e.g. Ti4+, Nb5+, and W6+ and cations with non-bonded electron pairs, e.g. Sn2+, Sb3+, and Te4+. We have exploited this distortion and have successfully synthesized a variety of polar materials. We characterize the materials through second-order non-linear optical, i.e. SHG, measurements as well as piezoelectric and polarization measurements. Our recent work has incorporated theoretical calculations to better understand the energetic of polarization reversal.
Another area of research we are investigating involves large single crystal growth – crystals centimeter size or larger. Large single crystals are crucial for detailed physical property measurements that can provide a far better understanding of structure-property relationships. We use a top-seeded solution growth (tssg) technique to grow centimeter size crystals of our functional materials. Currently, we have eight (8) tssg crystal growth furnaces with a maximum temperature of 1200C. This technique has enabled us to grow large crystals of a variety of materials including Na2TeW2O9 and LiFeP2O7 (see Figures above – left and right respectively).
An additional area of research we are involved with concerns multi-ferroic fluorides. For our purposes, multi-ferroic materials exhibit ferroelectricity, i.e. reversible polarization, and some sort of magnetic ordering. We are investigating magnetic ordering in materials that undergo a ‘non-classic’ ferroelectric mechanism, e.g., BaNiF4 (see Figure). As seen in the Figure, the green octahedra and yellow spheres represent the structure of the ‘up’ polarization of BaNiF4, whereas the gray octahedral and spheres represent the structure of the ‘down’ polarization. The structural re-orientation through the NiF6 octahedral rotations and Ba2+ displacements result in the polarization reversal, switching between ‘up’ and ‘down’, i.e. the ferroelectric behavior. We are investigating a host of mixed-metal fluorides that could undergo this non-classic ferroelectric mechanism and exhibit magnetic ordering.