On the Distortion of the NiO6 Octahedron in BaNiGd2O5

The NiO6 octahedron in BaNiGd2O5 is tetragonally distorted with two short and four long bonds, and has a further 10° orthorhombic distortion of the angles in the equatorial plane. These distortions are shown to arise from the strain required to maintain commensurability between the Ni-O and Gd-O bonds along the a axis. The high strain energy in the structure is reduced by the relaxation of the d -electrons of the Ni atoms as shown by Burdett and Mitchell (J. Am. Chem. Soc.112, 6571 (1990)).