Metal-Insulator Transitions, Structural and Microstructural Evolution of RNiO 3 (R ) Sm, Eu, Gd, Dy, Ho, Y) Perovskites: Evidence for Room-Temperature Charge Disproportionation in Monoclinic HoNiO 3 and YNiO 3 Jose´ A. Alonso,* Marı ´a J. Martı ´nez-Lope, Marı ´a T. Casais, Miguel A. G. Aranda, and Marı ´a T. Ferna´ ndez-Dı ´az Contribution from the Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, E-28049 Madrid, Spain, Departamento de Quı ´mica Inorga´ nica, Cristalografı ´a y Mineralogı ´a, Facultad de Ciencias, UniVersidad de Ma´ laga, E-29071 Ma´ laga, Spain, and Institut Laue-LangeVin, B.P. 156, 38042 Grenoble Cedex 9, France ReceiVed NoVember 20, 1998 Abstract: RNiO 3 nickelates have been prepared under high oxygen pressure (R ) Sm, Eu, Gd) or high hydrostatic pressure (R ) Dy, Ho, Y) in the presence of KClO 4 . The samples have been investigated at room temperature (RT) by synchrotron X-ray powder diffraction to follow the evolution of the crystal structures and microstructures along the series. The distortion of the orthorhombic (space group Pbnm) perovskite progressively increases along the series, leading for the smallest Ho 3+ and Y 3+ cations to a subtle monoclinic distortion (space group P2 1 /n) which implies the splitting of the Ni positions in the crystal. This symmetry was confirmed by neutron powder diffraction; the crystal structures for RdHo and Y were refined simultaneously from RT synchrotron and neutron powder diffraction data. In both perovskites the oxygen octahedra around Ni1 and Ni2 positions are significantly distorted, suggesting the manifestation of Jahn-Teller effect, which is almost absent in the nickelates of lighter rare earths. The very distinct mean Ni-O bond distances observed for Ni1 and Ni2 atoms at RT, in the insulating regime, suggest the presence of a charge disproportionation effect, considered as driving force for the splitting of the Ni positions. The metal-insulator (MI) transitions for RNiO 3 (R ) Gd, Dy, Ho, Y), above room temperature, have been characterized by DSC. The transition temperatures for Gd, Dy, Ho, and Y oxides in the heating runs are 510.7, 563.9, 572.7, and 581.9 K, respectively. The increasing rate of T MI for Dy, Ho, and Y materials is lower than that expected from the variation of T MI for the larger rare earth perovskites. This is probably related to the subtle monoclinic distortion found for Ho and Y nickelates. The high-resolution synchrotron X-ray powder patterns have revealed changes in the microstructure along the series. Powder patterns for orthorhombic RNiO 3 (R ) Sm, Eu, Gd, Dy) display asymmetric tails for some reflections which are due to structural mistakes such as stacking faults or regular intergrowths. These mistakes are not present in monoclinic RNiO 3 (RdHo, Y) nickelates. Introduction The discovery of thermally driven metal-insulator (MI) transitions in the RNiO 3 perovskites (R ) rare earth) 1 has stimulated intense research efforts. These paradigmatic oxides are the best opportunity known so far to investigate the closing of the charge-transfer gap in structurally simple, undoped, narrow-band oxides. The transition temperature, T MI , between the low-temperature insulating state and the high-temperature metallic state, rises systematically as the rare-earth size becomes smaller, i.e., as the distortion of the perovskite with respect to the ideal structure (aristotype) increases. 2-5 The existence of electronically induced subtle structural changes associated with the transition was reported earlier. 3 The interest in rare-eath nickelates has been reinforced after the discovery of an unexpected magnetic ordering in PrNiO 3 and NdNiO 3 associated with the electronic localization. 6 In these compounds the magnetic ground state consists of alternating ferromagnetic and antiferromagnetic Ni-O-Ni couplings that violate the inversion center at the Ni site. This suggests the existence of a nonuniform orbital distribution of the single e g electron, 6 similar (but not equal) to that recently found in some insulating phases of manganese perovskites. However, thus far orbital ordering has not been observed from the anisotropy of the Ni-O bond distances in NiO 6 octahedra, even though low-spin Ni 3+ (t 6 2g e 1 g ) is as susceptible to undergoing the Jahn-Teller (JT) effect as Mn 3+ (t 3 2g e 1 g ) does in stoichiometric LaMnO 3.00 . This is probably a manifestation of the higher covalent character of the Ni-mixed oxides because the larger crystal field in the nickelates makes the ionic picture less adequate than for the manganates. A recent neutron diffraction search for small nuclear superstructure peaks due to a possible orbital ordering * Corresponding author. E-mail: jalonso@fresno.csic.es. (1) Lacorre, P.; Torrance, J. B.; Pannetier, J.; Nazzal, A. I.; Wang, P. W.; Huang, T. C. J. Solid State Chem. 1991, 91, 225 (2) Torrance, J. B.; Lacorre, P.; Nazzal, A. I.; Ansaldo, E. J.; Nieder- mayer, Ch. Phys. ReV.B 1992,45, 8209. (3) Garcı ´a-Mun˜oz, J. L.; Rodrı ´guez-Carvajal, J.; Lacorre, P.; Torrance, J. B. Phys. ReV.B 1992, 46, 4414. (4) Alonso, J. A.; Martı ´nez-Lope, M. J.; Rasines I. J. Solid State Chem. 1995, 120, 170. (5) Medarde, M. J. Phys.: Condens. Matter 1997, 9, 1679. (6) Garcı ´a-Mun˜oz, J. L.; Rodrı ´guez-Carvajal, J.; Lacorre, P. Phys. ReV. B 1994, 50, 978. 4754 J. Am. Chem. Soc. 1999, 121, 4754-4762 10.1021/ja984015x CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999