Spectroscopic determination of site symmetry and space group in lanthanide-doped crystals: Resolving intricate symmetry aspects for β-NaLnF4

There exists a controversy in the hitherto reported studies concerning the actual space group that accounts for spectroscopic results observed for the trivalent lanthanide (Ln(3+)) ions doped into beta-NaLnF(4) hosts. The two competing assignments of the space group: P (6) over bar or P6(3)/m, bear on the associated point symmetry groups, which describe the local site symmetry of the dopant Ln ions. This paper aims at resolving this controversy. For this purpose two approaches are utilized. First, several new samples were synthesized and characterized by photoluminescence (PL) spectroscopy. Both the emission and excitation PL spectra have been measured. The analysis of the present experimental data indicates unequivocally that the dopant Ln(3+) ions enter the crystal lattice beta-NaGF(4) and beta-NaYF4 at two crystallographically distinct sites. Hence, the correct space group for these hosts is P (6) over bar and not P6(3)/m. The intricacies arising from the sample preparation methods, which bear on the interpretation of the results, are also clarified. Second, the theoretical modeling of the crystal-field parameters (CFPs) is carried out based on superposition model to correlate the available crystallographic and spectroscopic data. The crystal-field analysis enables to ascribe unambiguously the spectroscopically determined sites to the crystallographic sites la and if. In this way an interpretation of photoluminescence spectra is achieved that fully corroborates the available crystallographic data. The influence of structural disorder occurring in beta-NaLnF(4) family of compounds on the D-5(0) -> F-7(1) photoluminescence spectra of dopant Eu3+ ion and its role in interpretation of experimental data is also discussed. (C) 2015 Elsevier Ltd. All rights reserved.