High-pressure preparation, crystal structure, magnetic properties, and phase transitions in GdNiO3 and DyNiO3 perovskites

Strongly distorted RNiO3 (R = Gd, Dy) perovskites, containing Ni3+, have be en prepared under high-pressure conditions: 90 MPa of O-2 pressure (R = Gd) or 2 GPa of hydrostatic pressure in the presence of KClO4 (R = Dy). These materials have been characterized by X-ray diffraction, neutron powder diff raction (NPD) (for DyNiO3), DSC, magnetic measurements, and specific heat m easurements. In contrast with the next member of the series, HoNiO3, which shows a subtle monoclinic distortion at room temperature, DyNiO3 exhibits o rthorhombic symmetry, as shown by NPD data. A noticeable distortion is obse rved in NiO6 octahedra, at variance with the almost regular octahedra exhib ited by the first members (R La, Pr, Nd) of the series: it is interpreted a s a manifestation of the Jahn-Teller character of the Ni3+ cation, enhanced in the RNiO3 perovskites with heavier rare earths, showing weaker, less co valent 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 perovskit es, based on the analogous behavior observed for the precedent members of t he RNiO3 series. Subtle slope variations in the susceptibility vs T curves, highly dominated by the strong paramagnetic signal of Gd3+ and Dy3+, indic ate the onset of antiferromagnetic ordering of the Ni3+ sublattice, confirm ed by specific heat measurements, below T-N values of 185 and 154 K, respec tively. Additionally, the Dy3+ sublattice becomes ordered below 8 K.