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Production of alkaline earth or rare earth precursors crystallized in the form of transition alumina and comprising homogeneous spherical particles of nanometric diameter. The precursor compounds are defined as having the following characteristics : They conform to the formula (1) or (2) : a(M 1>O).b(MgO).c(Al 2O 3) (1) a(M 2>O 1.5).b(MgO).c(Al 2O 3) (2) M 1>an alkaline earth; M 2>yttrium or cerium and terbium combined; a, b and c : conform to the relation : 0.25 +- a +- 4 ; 0 +- b +- 2 ; 0.5 +- c +- 9. They crystallize in the form of a transition alumina and comprise spherical and homogeneous particles with pore diameter at least 10 nm. Independent claims are also included for the preparation and use of the compounds.