Quadrupole effects in the NMR spectra of 27Al in disordered mixed compounds CaREAlO4 (RE=La, Pr, Eu, Y)

The NMR spectra of 27Al are measured in several disordered rare-earth aluminates CaREAlO4. A change in the shape of the resonance line of the transition 1/2↔−1/2 in the transverse orientation in the rare-earth series RE=La, Pr, Eu, Y is observed, from a narrow (∼16 kHz), structureless line in the La compound to a wide (∼130 kHz), three-component line in the Y compound. This is attributed to a nonuniform distribution of the electric field gradient tensor at the 27Al site. The electric field gradient tensor at the 27Al site in a cluster (AlO6)−9 is calculated by a nonempirical method (MO LCAO SCF) in the “deformable oxygen octahedron” model. The results are found to be in satisfactory agreement with the experimental values of 〈Vzz〉 and can be used to obtain the experimental splitting of the resonance line in the Y compounds for reasonable values of the parameters of the local deformations of the AlO6 octahedron. The nature of the strong axial correlations in the distribution of the Ca and Y atoms in CaYAlO4 is discussed.