On the evolution of the DyNiO3 perovskite across the metal–insulator transition though neutron diffraction and Mössbauer spectroscopy studies

Abstract

The structural changes of polycrystalline DyNiO₃ perovskite across the metal–insulator transition (T_MI = 564 K) have been studied by high resolution neutron diffraction techniques together with Mössbauer spectroscopy, in a sample doped with 1.5 at.% ⁵⁷Fe. In the insulating (semi-conducting) regime, below T_MI, the perovskite is monoclinic, space group (SG) P2₁/n, and the crystal structure contains two chemically different Ni1 and Ni2 cations, as a result of the charge disproportionation of Ni³⁺ cations. The β parameter, characterizing the low-temperature monoclinic distortion, is smaller than 90.04° for T < T_MI, indicating a strongly pseudo-orthorhombic symmetry, although the internal monoclinic symmetry, implying the splitting and shifts of oxygen positions around the two Ni sites is perfectly detected by neutrons. Above T_MI, DyNiO₃ becomes orthorhombic, SG Pbnm. Upon heating across T_MI, there is an abrupt convergence of the two sets (Ni1 and Ni2) of three Ni–O bond lengths, in the monoclinic-insulating phase, to three unique Ni–O distances in the orthorhombic-metallic phase upon entering the metallic region. The ⁵⁷Fe Mössbauer spectra of an iron-doped (1.5 at.%) DyNiO₃ sample recorded in the insulating, paramagnetic temperature range (T_N < T < T_MI) are discussed by supposing that the Fe³⁺ probe cations replace nickel in the two octahedral Ni1 and Ni2 sites. Electric field gradient calculations have shown that the ⁵⁷Fe hyperfine parameters of Fe1 and Fe2 subspectra reflect a specificity of local structure corresponding to large (Ni1O₆) and small (Ni2O₆) octahedra. At T > T_MI, the ⁵⁷Fe spectrum gives clear evidence for the formation of an unique state for iron probe atoms and could, therefore, imply that the charge disproportionation in the (NiO₆) subarray completely vanishes at the insulator→metal transition.