Jun Yang – Projekte
Quantum Chemical Investigation of the Structure and Stability of Various Borate Containing Crystalline Solids
Jun Yang, Johannes Weber, Michael Dolg
Institute for Theoretical Chemistry
University of Cologne
During the last several decades, oxoborates polyanion compounds increasingly attracted considerable interest from the chemical community and beyond. This is largely due to their potential nonlinear optical properties in the numerous practical applications. A lot of efforts have been put into the generation of new oxoborates. Among the binary rare earth oxoborates of general composition REB3O6, only a few compounds (RE=La, Nd, Sm, Tb, Gd and Pr) have been fully structurally determined. They exhibit isostructural series in the monoclinic symmetry, with the exception of TbB3O6 which has structural flexibility and undergoes at about 143K a phase transition from monoclinic to orthorhombic symmetry. However, for the other compounds REB3O6 only incomplete structural information exists in some literatures.
It is clearly recognized that both the polyanion triangle planar/non-planar [BO3]3- and tetrahedral [BO4]5- are linked via corner oxygen atoms to form these solids. But it is still magic behind these structures and their flexibilities what favors the final crystal stability and thus decides its phase transition behavior under a certain external pressure/temperature condition. We are trying to answer whether or not TbB3O6 really acts, as suggested in literature, to “play the transient role which is allowed to be the terminal member of the isostructural monoclinic series of REB3O6 with RE=La~Tb, but also the starting point of an assumed orthorhombic series for the smaller lanthanides”
For the above purpose, we aim at the investigation of the structures and stabilities of lanthanide-containing oxoborates using first-principles and ab initio calculations. Within the quantum scheme, not only the lattice equilibrium but also the dynamical performance should be taken into account, since the P/T-dependence of phonon normal vibrational modes actually contributes to crystal stabilities. These modes could be obtained from ab initio calculations for harmonical vibrations combined with the phenomenological analysis by the thermal-expansion of anharmonical corrections where first-principle accurate methods are nearly not applicable to solids with so many atoms. At the second point, the lanthanide cations with different sizes, relativistic effects, polarizabilities and so on maybe delicately affect both the equilibrium and dynamical properties of REB3O6 solids, and therefore are responsible for the structural series of REB3O6. Finally, the structural variability due to the phase transition of TbB3O6 (or other REB3O6) is driven by proper pressures or temperatures and related to the variational content ratio of the Fundamental Basic Block (FBB) triangle-BO3 and tetrahedral-BO4, which would impose alterations upon the electronic structures and vibrational modes of REB3O6 solids. We expect hopefully to calculate the phase-diagram for offering theoretical hints for experiments.
This project is part of GK 549 (Azentrische Kristalle) and financially supported by Deutsche Forschungsgemeinschaft (DFG). Our investigations are performed in collaboration with Professor Dr. Ladislav Bohaty (Institute of Crystallography, University of Koeln).