With the exception of metallic LaNiO₃, all the members of the RNiO₃ series (R = rare earth and Y) undergo a sharp metal to insulator (M-I) transition at temperatures T_MI ranging between 130K (Pr) and 600K (Lu). The mechanism of the gap opening is not yet fully understood and has been a matter of controversy since its discovery in 1991 [1]. Recently, the existence of a symmetry decrease from orthorhombic Pbnm to monoclinic P2₁/n below T_MI was reported for the last members of the series (Ho to Lu) and interpreted as signature of an incomplete 2Ni³⁺ ==> Ni^3+δ + Ni^3-δ charge disproportionation (CD) with δ = 0.3 [2,3]. This mechanism provided a new framework for the M-I transition and, due to the diamagnetic nature of Ni⁴⁺, it could also account for the unusual, non-centrosymmetric magnetic structure displayed by these compounds in the particular case of δ = 1 . However, due to the smallness of the associated structural changes, no diffraction studies -even on single crystals- could provide full structure refinements supporting the existence of 2 distinct Ni sites for the first members of the series (Pr to Dy).
In this study we report new, combined ultra-high resolution neutron powder diffraction and x-ray absorption measurements on the full RNiO₃ series. In contrast with previous studies, we observe a nearly complete  charge disproportionation for the nickelates with small rare earths, as well as a progressive evolution towards a non-disproportionated state moving from Lu (δ = 0.8) to Pr (δ = 0.2) [4,5]. The implications of these results for the stability of the magnetic structure and the eventual existence of ferroelectricity are discussed.
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[1]  P. Lacorre, J.B. Torrance, J. Pannetier, A.I. Nazzal, P.W. Wang, T. Huang and   R. Siemens, J. Solid State Chem. 91, 225 (1991).
[2]  J.A. Alonso, J.L. García-Muñoz, M.T. Fernández-Díaz, M.A.G. Aranda, M.J. Martínez-Lope and M.T. Casais, PRL 82, 3871 (1999).           
[3]  J.A. Alonso, M.J. Martínez-Lope, M.T. Casais, J.L. García-Muñoz, M.T. Fernández-Díaz, M.A.G. Aranda, PRB 64, 94102 (2001).
[4]  M. Medarde, M.T. Fernández-Díaz and P. Lacorre, submitted to PRL.
[5] M. Medarde, C. Dallera, M. Grioni,  J. Mesot, M. Sikora, P. Glatzel, J.A. Alonso and M.J. Martínez-Lope, in preparation.

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