Orbital ordering in NdNiO 3 and SmNiO 3 investigated by Mössbauer spectroscopy

A M\"ossbauer study of ${}^{57}\mathrm{Fe}$ doped ${\mathrm{Ni}}^{\mathrm{III}}$ perovskites, ${\mathrm{NdNi}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{O}}_{3}$ and ${\mathrm{SmNi}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{O}}_{3},$ in the magnetic ordering domain $(T\ensuremath{\ll}{T}_{N})$ has revealed that the Fe probes occupy two types of sites, i.e., Fe(1) and Fe(2), showing very different values of the magnetic hyperfine fields ${(H}_{1}\ensuremath{\approx}430\char21{}450\mathrm{kOe},{\mathrm{H}}_{2}\ensuremath{\approx}15\char21{}22\mathrm{kOe}).$ The close values of the isomer shift (\ensuremath{\delta}) and the second-order quadrupole perturbation parameter (\ensuremath{\varepsilon}) in the subspectra, for both Fe sites, have underlined that all the Fe sites are crystallographically equivalent. In the light of orbital ordering model suggested by the neutron diffraction studies, the partial substitution of ${\mathrm{Ni}}^{\mathrm{III}}$ ${(t}_{2g}^{6}{e}_{g}^{1})$ by ${\mathrm{Fe}}^{3+}$ ${(t}_{2g}^{3}{e}_{g}^{2})$ may induce significant topological frustration of ${\mathrm{Fe}}^{3+}$ spins in surroundings of ${\mathrm{Ni}}^{\mathrm{III}}$ orbitals, leading to two different magnetic environments around Fe below ${T}_{N}.$ Additionally, the M\"ossbauer spectra at 300 K $(g{T}_{N})$ indicate that all the available sites for probe atoms are crystallographically equivalent in the insulating ${\mathrm{SmNi}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{O}}_{3}$ as well as in the metallic ${\mathrm{NdNi}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{O}}_{3}.$ The present result may be an evidence of the important role of the orbital ordering in determining the electronic properties of the ${\mathrm{Ni}}^{\mathrm{III}}$ perovskites containing large rare earth.