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

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Abstract
Single crystals of LnMgAl11O19 aluminates with a magnetoplumbite‐like structure, (Ln=La1−xNdx, La1−x Eux, 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 c2v) 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 Bkq parameters for the A site. These Bkq 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 2P1/2’’ and ‘‘low 2P1/2’’ are required, confirming the multisite character of La1−xNdxMgAl11O19. 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.