Trends in Hamiltonian parameters determined by systematic analysis of f-d absorption spectra of divalent lanthanides in alkali-halides hosts and supported by first calculations of the Nd2+ electronic structure: I. SrCl2:Ln2+

Synthesis and characterization of systems containing divalent lanthanide elements (Ln2+) is still largely an unexplored area. Until recently the absorption spectra were obtained nearly exclusively for Ln2+:CaF2 crystals, whereas most studied ions are Sm2+, Eu2+, Tm2+ and Yb2+. The scarcity of data on spectral properties of Ln2+ ions and usage of different methodologies for their analysis hinders uncovering systematic changes in Hamiltonian parameters and thus inherent trends across the Ln series. Here we report the first calculations of the crystal-field (CF) energy levels for the 4f35d1 configuration of Nd2+ ion. The novel absorption spectra for Nd2+ and Dy2+ ions in SrCl2 as well as new spectra for Sm2+, Eu2+, Tm2+, and Yb2+ ions in SrCl2 obtained independently by us are analyzed using uniform methodology proposed here. Our data are supplemented by comprehensive reanalysis of available spectral data for Sm2+, Eu2+, Tm2+, and Yb2+ ions in SrCl2. This approach yields refined and more consistent sets of Hamiltonian parameters. Systematic analysis of free-ion parameters and CF parameters for the six Ln2+ ions in SrCl2 studied by us provides sufficiently large amount of data that allows to reveal inherent trends across the Ln series. Our results enable for the first time to check if identified trends would be sufficiently systematic to allow for predicting the spectrum for any lanthanide ion based on the sets of parameters derived from the spectra obtained for another ion in the same matrix.