NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC AND ORBITAL ORDERING IN (SMNIO3)-SM-154 AND (EUNIO3)-EU-153

Neutron-powder-diffraction experiments on (SmNiO3)-Sm-154 and (EuNiO3) -Eu-153 at different temperatures have been performed. At variance wit h the case of Pr and Nd nickelates, the metal-insulator transition tem perature in these compounds are different from the Neel temperature (T (M-I)approximate to 400 K, T-N approximate to 230 K for Sm and T(M-I)a pproximate to 480 K, T-N approximate to 220 K for Eu) so that the magn etic ordering develops upon cooling in an insulating matrix. With decr easing temperature the crystal structure undergoes a 0.15% lattice exp ansion at TM-I. The crystal symmetry Phnm does not change within the e xperimental error, but the Ni-O average distance increases and the sup erexchange angle Ni-O-Ni decreases slightly in the insulating regime. The magnetic structure of both compounds has the same propagation vect or k=(1/2,0,1/2) as observed in Pr and Nd nickelates. This feature lea ds us to speculate that the orbital ordering responsible for the magne tic structure is a characteristic of the insulating state of the RNiO3 series (at least for light rare earths R). A search for small nuclear superstructure peaks characterizing the orbital ordering has been car ried out without success, The consequences of our experimental results are discussed in the context of current ideas about superexchange in oxides.