On the distortion of the NiO[sub 6] octahedron in BaNiGd[sub 2]O[sub 5]

The NiO[sub 6] octahedron in BaNiGd[sub 2]O[sub 5] is tetragonally distorted with two short and four long bonds, and has a further 10[degrees] 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)). 8 refs., 2 figs., 1 tab.