Eu ions as a crystal-field probe for low-symmetry sites in doped phosphors - a case study: Eu at triclinic sites in LiRE(BO) (RE = Y, Gd), YBO and ZnO and at trigonal sites in YAl(BO).

We present crystal-field (CF) calculations of energy levels () of Eu ions doped in various hosts aimed at exploring the low-symmetry properties of CF parameters (CFPs) and reliability of CFP modelling with decreasing site symmetry. The hosts studied are: LiY(BO), LiGd(BO), YBO, and ZnO with Eu at triclinic sites; YAl(BO) with Eu ions at trigonal symmetry. Two independent CFP modelling approaches utilizing the hosts' structural data are employed: the exchange charge model (ECM) and the superposition model (SPM). We adopt the Eu actual site symmetry and not the approximated one. The values calculated using CFPs modelled by the ECM and SPM mutually agree with the observed ones. For triclinic symmetry, the ECM/CFPs and SPM/CFPs were numerically distinct, yet turned out to be physically equivalent yielding identical rotational invariants, ( = 2, 4, 6) and . For trigonal symmetry, both CFP sets agree numerically, thus and are identical. This disparity poses a dilemma, since the modified crystallographic axis system was used in both approaches. The standardization of the triclinic CFPs using the 3DD package was performed to solve this dilemma. It has enabled discussing standardization aspects in experimental and computed CFP sets and elucidating intricate low-symmetry aspects inherent in CFP sets. Understanding of low-symmetry aspects in CF studies may bring about a better interpretation of the spectroscopic and magnetic properties of rare-earth ion doped host crystals. Thus, our study could provide more deep insights into the importance of clear definitions of axis systems and adequate treatment of actual site symmetry in the modelling of CFPs for low-symmetry cases which is essential for technological applications and engineering of rare-earth activated phosphor materials.