Effect of symmetry reduction on the electronic transitions in polytypic GdAl3(BO3)4:Eu:Tb crystals

Abstract The existence of a recently described monoclinic phase ( C 2/ c , Z = 8) (Beregi et al., 2012) in addition to the well-known Huntite type rhombohedral ( R 32) polytypic modification of the GdAl 3 (BO 3 ) 4 (GAB) crystal at room temperature provides a unique possibility to investigate the incorporation of rare earth dopants into slightly modified crystal lattice by spectroscopic methods. In these characteristic GAB structures the dopant ions, e.g. Tb 3+ or Eu 3+ , possess slightly different neighbor geometries and local symmetries. The Tb 3+ : 7 F 6 → 5 D 4 and Eu 3+ : 7 F 0,1,2 → 5 D 0,1,2 electronic transitions were successfully identified in the absorption spectra using polarization, concentration and temperature dependent measurements in both polytypic modifications. The positions of the investigated Tb lines are shifted by up to 10 cm −1 due to symmetry changes. In addition, some of the Eu lines show splittings of about 4–30 cm −1 as a consequence of the change of the local environment. From the room temperature absorption measurements some of the low energy crystal field levels of 7 F and 5 D states of the Eu 3+ ions were successfully determined for both modifications.