Optical study of LaAlO3:Eu at temperatures approaching the rhombohedric → cubic transition

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Abstract
The LaAlO3:Eu fluorescence spectrum was investigated at 77, 300, and 500 °K, that is, approaching the rhombohedric→cubic transition. A progressive modification of spectra is noted, owing to a shift in point symmetry at the europium site (D3 → Oh). Assignment of irreducible representations of the levels is aided by the observation of degeneracies. A calculation of the crystal‐field parameters Bkq was tried, based on the model of point charges and coordinated with the structural data for LaAlO3 (R 3̄ c). The calculated values are very different from the experimentally measured parameters; however, these latter can be approached fairly well if the calculated parameters B2q, B4q, and B6q are multiplied by factors of 0.26, 1.93, and 7.5, respectively. Diagonalizing the interaction matrix of the 4f6 configuration thus accounts for the experimental splitting of the 7F levels at 77, 300, and 500 °K. A priori calculation of the Bkq parameters thus permits an approximation of the x parameter which characterizes the deformation of perovskite with respect to the ideal cubic form.