Analysis and simulation of optical and magnetic properties of lanthanide aluminates LnMgAl₁₁O₁₉ (Ln=La/Nd,La/Eu,Pr) with magnetoplumbite‐like structure

Single crystals of LnMgAl₁₁O₁₉ aluminates with a magnetoplumbite‐like structure, (Ln=La_1−xNd_x, La_1−x Eu_x, Pr), are grown by the flame fusion (Verneuil) or floating zone methods. Optical absorption or fluorescence spectra of these crystals reveal that Ln³⁺ ions occupy at least two or three different sites, instead of only one as in the ideal magnetoplumbite structure. Eu³⁺ fluorescence under dye‐laser excitation leads to the identification of two low symmetry (probably c_2v) sites labeled A and B. Ab initio crystal field parameters for Eu³⁺ are calculated from the atomic coordinates deduced from the crystal stucture of LaMgAl₁₁O₁₉. These parameters and the experimental energy levels are used to derive an acceptable set of B^k_q parameters for the A site. These B^k_q corrected for the Nd³⁺/Eu³⁺ radial integrals ratio are used in turn to derive the Nd³⁺ Racah and spin‐orbit coupling parameters which fit the Nd³⁺ absorption spectrum at 4 K. At least two sets of parameters ‘‘high ²P_1/2’’ and ‘‘low ²P_1/2’’ are required, confirming the multisite character of La_1−xNd_xMgAl₁₁O₁₉. For this compound, the ab initio crystal field parameters are used to calculate the magnetic susceptibility and Nd³⁺ ESR g values. The fairly good agreement between calculated and experimental magnetic parameters confirms that the real sites of the Lanthanide ion arise from minor perturbations of the normal magnetoplumbite one. These perturbations could arise from Al–Mg site disorder, and/or the presence of oxygen vacancies in the lattice.