On the lack of monoclinic distortion in the insulating regime of EuNiO3 and GdNiO3 perovskites by high-angular resolution synchrotron X-ray diffraction: a comparison with YNiO3

Abstract

Rare-earth nickelates RNiO₃ (R = Y, La⋯Lu) are electron-correlated perovskite materials where the interplay between charge and spin order results in a rich phase diagram, evolving from antiferromagnetic insulators to paramagnetic metals. They are well-known to undergo metal–insulator (MI) transitions as a function of temperature and the size of the rare-earth ion. For intermediate-size Eu³⁺ and Gd³⁺ ions, the MI transitions are described to happen at T_MI = 463 K and 511 K, respectively. We have investigated their structural evolution across T_MI with the excellent angular resolution of synchrotron X-ray diffraction, using high-crystalline quality samples prepared at elevated hydrostatic pressures. Unlike YNiO₃, synthesized and measured under the same conditions, exhibiting a characteristic monoclinic phase (space group P2₁/n) in the insulating regime (below T_MI), the present EuNiO₃ and GdNiO₃ samples do not exhibit such a symmetry, but their crystal structures can be defined in an orthorhombic superstructure of perovskite (space group Pbnm) in all the temperature interval, between 100 and 623 K for Eu and 298 K and 650 K for Gd. Nevertheless, an abrupt evolution of the unit-cell parameters is observed upon metallization above T_MI. A prior report of a charge disproportionation effect by Mössbauer spectroscopy on Fe-doped perovskite samples seems to suggest that the distribution of two distinct Ni sites must not exhibit the required long-range ordering to be effectively detected by diffraction methods. An abrupt contraction of the b parameter of EuNiO₃ in the 175–200 K range, coincident with the onset of antiferromagnetic ordering, suggests a magnetoelastic coupling, not described so far in rare-earth nickelates. The magnetic susceptibility is dominated by the paramagnetic signal of the rare-earth ions; however, the AC susceptibility curves yield a Néel temperature corresponding to the antiferromagnetic ordering of the Ni moments of T_N = 197 K for EuNiO₃, corroborated by specific heat measurements. For GdNiO₃, a χT vs. T plot presents a clear change in the slope at T_N = 187 K, also consistent with specific heat data. Magnetization measurements at 2 K under large fields up to 14 T show a complete saturation of the magnetic moments with a rather high ordered moment of 7.5μB per f.u.