Charge disproportionation in RNiO3 (R = Tm, Yb) perovskites observed in situ by neutron diffraction and 57Fe probe Mossbauer spectroscopy

An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO3 and YbNiO3 perovskites across the metal-insulator transition, with T-MI = 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below T-MI to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group P2(1)/n, containing two inequivalent Ni1((3+sigma)+) and Ni2((3-sigma)+) cations; above T-MI, the samples become orthorhombic, space group Pbnm, with a single site for Ni3+. The Fe-57 Mossbauer spectra of iron-doped (at 1.5%) RNiO3 (R = Tm, Yb) samples recorded below T-MI exhibit for Fe1 and Fe2 (replacing Ni1 and Ni2 sites) hyperfine parameters corresponding to large (Ni1O(6)) and small (Ni2O(6)) octahedra. The remarkable difference between the quadrupole splittings (Delta(1) approximate to 0.3 mm/s and Delta(2) approximate to 0.07 mm/s) of Fe1 and Fe2 sites in RNi0.985Fe0.015O3 is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at Fe-57 ions, and estimate, by using the experimental Delta(1) and Delta(2) values, the contributions of the 3d, 3p, and 2p electrons (overlap distortion and covalence effects). Above T-MI, a unique state for iron atoms is observed, upon metallization of the sample.