High-temperature crystal structures of orthorhombic and rhombohedral PrNiO₃

Crystal structures of orthorhombic and rhombohedral PrNiO3 have been studied by high-temperature X-ray powder diffraction. In the orthorhombic PrNiO3 (space group Pbnm) Ni-O octahedra are slightly distorted, and the O-Ni-O bond angles are close to ±3.4° of the 90° angle of an undistorted octahedron. The octahedral tilt is large and decreases from 22.2° to 20.5° when the temperature increases from room temperature to 400°C. The coordination number of Pr is less than 12 and may effectively be only 8. The atoms move from orthorhombic positions toward the so-called 'ideal cubic perovskite positions' with increasing temperature. However, they move in such a way that the structure assumes a rhombohedral symmetry when the temperature reaches 500°C. In the rhombohedral PrNiO3 (space group R3c) the Ni-O octahedra are trigonally distorted, and the Pr atoms have a coordination number 12. The rotation of octahedra necessary to produce the rhombohedral structure decreases slightly from 11.38° to 11.00° when the temperature increases from 500° to 600°C. © 1990.