Neutron-powder-diffraction experiments on 154 SmNiO 3 and 153 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 Ϸ400 K, T N Ϸ230 K for Sm and T M -I Ϸ480 K, T N Ϸ220 K for Eu͒ 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 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 kϭ( 1 2 ,0, 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 RNiO 3 series ͑at least for light rare earths R͒. 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.

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The crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa2Cu3O7 the lattice parameters and most interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series can be explained by the increase of Tc from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 series compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa2Cu3O7 approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for the Pr ions. This indicates for PrBa2Cu3O7 a highly anisotropic 4f-CuO2 valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO2 planes: the superconductivity is lost when the puckering angle exceeds a critical value of about 167.3.

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