High-Pressure Preparation, Crystal Structure, Magnetic Properties, and Phase Transitions in GdNiO₃ and DyNiO₃ Perovskites

Strongly distorted RNiO 3 (R = Gd, Dy) perovskites, containing Ni 3+, have been prepared under high-pressure conditions: 90 MPa of O 2 pressure (R = Gd) or 2 GPa of hydrostatic pressure in the presence of KClO 4 (R = Dy). These materials have been characterized by X-ray diffraction, neutron powder diffraction (NPD) (for DyNiO 3 ), DSC, magnetic measurements, and specific heat measurements. In contrast with the next member of the series, HoNiO 3, which shows a subtle monoclinic distortion at room temperature, DyNiO 3 exhibits orthorhombic symmetry, as shown by NPD data. A noticeable distortion is observed in NiO 6 octahedra, at variance with the almost regular octahedra exhibited by the first members (R = La, Pr, Nd) of the series: it is interpreted as a manifestation of the Jahn−Teller character of the Ni 3+ cation, enhanced in the RNiO 3 perovskites with heavier rare earths, showing weaker, less covalent Ni−O bonds. DSC measurements show sharp endothermic peaks at 510.9 K (Gd) and 564.1 K (Dy) in the heating run, which have been assigned to the corresponding metal−insulator transitions of both charge-transfer perovskites, based on the analogous behavior observed for the precedent members of the RNiO 3 series. Subtle slope variations in the susceptibility vs T curves, highly dominated by the strong paramagnetic signal of Gd 3+ and Dy 3+, indicate the onset of antiferromagnetic ordering of the Ni 3+ sublattice, confirmed by specific heat measurements, below T N values of 185 and 154 K, respectively. Additionally, the Dy 3+ sublattice becomes ordered below 8 K.