Spectroscopic and Crystal Field Investigation of Kramers-Ions: Nd:YAB—a Case Study of the Crystal Field Structure of the I Ground State

Single crystals of yttrium aluminium borate, YAl₃(BO₃)₄, (referred to as YAB) doped with 20 and 40 mol% of Nd³⁺ were grown using a flux growth method. Inconsistencies in regard to the reported ground state splitting of the doped material are pointed out. A consistent splitting and assignment of the ⁴I_9/2 ground state levels of the Kramers-ion Nd³⁺ was obtained by a combination of both, temperature-dependent ⁴I_9/2→²P_9/2 polarized absorption spectroscopy and room temperature ⁴F_9/2→⁴I_9/2 luminescence spectroscopy. The group theoretical implications of the crystal field analysis are considered and discussed.