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

Rare-earth nickelates RNiO3 (R = Y, LaMIDLINE HORIZONTAL ELLIPSISLu) 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 P2(1)/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 Mossbauer 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 Neel 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 chi T 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 7.5 mu B per f.u.

Automated summary

Rare-earth nickelates are electron-correlated perovskite materials with complex phase diagrams, undergoing metal-insulator transitions as a function of temperature and rare-earth ion size. EuNiO3 and GdNiO3 exhibit orthorhombic superstructures of perovskite across a wide temperature range, with abrupt evolution of unit-cell parameters upon metallization. Additionally, EuNiO3 shows magnetoelastic coupling behavior and GdNiO3 demonstrates clear antiferromagnetic ordering characteristics.