Charge disproportionation inRNiO3(R=Tm, Yb) perovskites observedin situby neutron diffraction and57Fe probe Mössbauer spectroscopy

Abstract

An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO${}_{3}$ and YbNiO${}_{3}$ perovskites across the metal-insulator transition, with ${T}_{\mathrm{MI}}$ $=$ 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below ${T}_{\mathrm{MI}}$ to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group $P$2${}_{1}$/$n$, containing two inequivalent Ni1${}^{(3+\ensuremath{\sigma})+}$ and Ni2${}^{(3\ensuremath{-}\ensuremath{\sigma})+}$ cations; above T${}_{\mathrm{MI}}$, the samples become orthorhombic, space group Pbnm, with a single site for Ni${}^{3+}$. The ${}^{57}$Fe M\ossbauer spectra of iron-doped (at 1.5$%$) $R$NiO${}_{3}$ ($R$ $=$ Tm, Yb) samples recorded below ${T}_{\mathrm{MI}}$ 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 (${\ensuremath{\Delta}}_{1}$ \ensuremath{\approx} 0.3 mm/s and ${\ensuremath{\Delta}}_{2}$ \ensuremath{\approx} 0.07 mm/s) of Fe1 and Fe2 sites in $R$Ni${}_{0.985}$Fe${}_{0.015}$O${}_{3}$ is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at ${}^{57}$Fe ions, and estimate, by using the experimental ${\ensuremath{\Delta}}_{1}$ and ${\ensuremath{\Delta}}_{2}$ values, the contributions of the 3$d$, 3$p$, and 2$p$ electrons (overlap distortion and covalence effects). Above ${T}_{\mathrm{MI}}$, a unique state for iron atoms is observed, upon metallization of the sample.