Crystal structure and single-crystal spectra of Gd(Eu)Al3(BO3)4. Anisotropic ligand polarization contributions to the ƒ–ƒ transition probabilities in EuIII

Abstract

The polarized single-crystal spectra of GdAl₃(BO₃)₄ containing 3 % Eu³⁺ have been measured, with particular reference to the absolute ƒ–ƒ absorption intensities, and the crystal structure of the host lattice has been determined from single-crystal X-ray data with a refinement by least-squares methods to R= 0.058. The trigonal crystal has unit-cell dimensions a= 9.3014 (7), c= 7.2571 (7)Å, space group R32 and Z= 3. The lanthanide ion is coordinated to an oxygen atom of six borate ions and has a D₃ site symmetry. The intensities of the ƒ–ƒ transitions in the [EuO₆] cluster of the crystal are compared with the corresponding intensities measured for a D₃ and a C_3h[EuO₉] cluster in a tris(oxy-diacetate) and a nonahydrate Eu^III single crystal, respectively, and with the theoretical intensities based upon the dynamic-polarization mechanism. In terms of the mechanism, the comparisons show that the polarizability anisotropy of the ligand bonds is generally significant, and dominant for particular transition-types, and that, for the [EuO₉] coordination clusters, the intensity contributions of the individual trigonal ligand sets, [EuO₃] and [EuO₆], constructively or destructively interfere, dependent upon the structural details of the ligand coordination polyhedron and the particular leading multipole components of the ƒ–ƒ transition moment.