Charge disproportionation and search for orbital ordering in NdNiO 3 by use of resonant x-ray diffraction V. Scagnoli,* U. Staub, M. Janousch, A. M. Mulders, and M. Shi Swiss Light Source, Paul Scherrer Institut, CH-5232, Villigen PSI, Switzerland G. I. Meijer IBM Research, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland S. Rosenkranz Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA S. B. Wilkins European Commission, Joint Research Center, Institute for Transuranium Elements, Hermann von Helmholtz-Platz 1, 76344 Eggenstein- Leopoldshafen, Germany and European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France L. Paolasini European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France J. Karpinski and S. M. Kazakov Laboratorium für Festkörperphysik, ETH-Zürich, CH-8093 Zürich, Switzerland S. W. Lovesey Diamond Light Source Ltd. and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, United Kingdom Received 26 April 2005; revised manuscript received 11 July 2005; published 14 October 2005 Detailed resonant x-ray diffraction experiments including azimuthal angle scans were performed on NdNiO 3 around the Ni K absorption edge, allowing us to investigate the electronic changes associated with the metal- to-insulator transition. The influence on the observed reflections of charge disproportionation and the asphe- ricity of the Ni electron density is evaluated. The asphericity, due to the distortion of the oxygen octahedra, persists also in the metallic phase, but its influence on the unrotated - 033 diffraction signal is found to be negligible, in contrast to the case for the superlattice reflections found in manganites and magnetite. We conclude that the - scattering intensity is consistent with charge disproportionation at the Ni site. No 1/201/2 type reflections were found, making unlikely the simultaneous occurrence of orbital order with this previously proposed wave vector. Moreover, the asphericity observed in the 4p shell in the metallic state indicates that there is no orbital degeneracy. Therefore, no orbital ordering is expected to occur at the metal- insulator transition. DOI: 10.1103/PhysRevB.72.155111 PACS numbers: 78.70.Ck, 71.30.+h I. INTRODUCTION The 3d transition-metal oxides exhibit a fascinating vari- ety of physical properties, ranging from high-T c supercon- ductivity to colossal magnetoresistance. These strongly cor- related electron systems often show metal-to-insulator transitions, whose microscopic origin is of on-going interest. Many are Mott insulators with a half-filled 3d band, for which the electron cannot hop to neighboring atoms. Spin and orbital degrees of freedom may survive and can play a significant role in the metal-insulator transitions. Particular interesting cases are those temperature driven metal-insulator transitions at which the itinerant charge carriers become lo- calized. This occurs in various transition-metal oxides such as manganites, 1 cobaltides, 2–4 or nickelates, 5,6 but also in other oxides such as magnetite. 7,8 The interplay and the com- petition between charge, orbital, and spin degrees of freedom can lead to charge and orbital ordered ground states for which the electrons are localized. Recent resonant x-ray scattering RXS experiments have challenged the interpretation of charge ordering CO of the Fe 2+ and Fe 3+ ions in magnetite below the Verwey transition. 9,10 The azimuthal and energy dependence of cer- tain superlattice reflections have been explained with a model without ionic charge order, which contradicts the CO model of Verwey. 7,8 Doubts about charge ordering have also been raised for manganites. 11–13 The observation of forbid- den reflections by means of RXS was first interpreted as “direct” evidence for CO and orbital ordering OO, 14,15 and a model was proposed based on CO at two different Mn sites, i.e., Mn I =3+ and Mn II =4+, and OO at the Mn I site. 16 Subsequent studies showed that the anisotropy of the scatter- ing tensor must be taken into account to describe the azi- muthal angle dependence of the CO type reflections. 12,13 PHYSICAL REVIEW B 72, 155111 2005 1098-0121/2005/7215/1551117/$23.00 ©2005 The American Physical Society 155111-1