Neutron-diffraction study of the magnetic and orbital ordering in154SmNiO3and153EuNiO3

Neutron-powder-diffraction experiments on ${}^{154}{\mathrm{SmNiO}}_3$ and ${}^{153}{\mathrm{EuNiO}}_3$ at different temperatures have been performed. At variance with the case of Pr and Nd nickelates, the metal-insulator transition temperature in these compounds are different from the Néel temperature ($T_{M-I}\approx 400\mathrm{K},$ $T_N\approx 230\mathrm{K}$ for Sm and $T_{M-I}\approx 480\mathrm{K},$ $T_N\approx 220\mathrm{K}$ for Eu) so that the magnetic ordering develops upon cooling in an insulating matrix. With decreasing temperature the crystal structure undergoes a 0.15% lattice expansion at $T_{M-I}.$ The crystal symmetry $\mathrm{Pbnm}$ does not change within the experimental error, but the Ni-O average distance increases and the superexchange angle Ni-O-Ni decreases slightly in the insulating regime. The magnetic structure of both compounds has the same propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$ as observed in Pr and Nd nickelates. This feature leads us to speculate that the orbital ordering responsible for the magnetic structure is a characteristic of the insulating state of the $R{\mathrm{NiO}}_3$ series (at least for light rare earths $R$). A search for small nuclear superstructure peaks characterizing the orbital ordering has been carried out without success. The consequences of our experimental results are discussed in the context of current ideas about superexchange in oxides.