Structure and Magnetization of (ND₄)₂Ni(SO₄)₂.6D₂O And (NH₄)₂Ni(SO₄)₂.6H₂O At 2-K by Neutron-Diffraction

Abstract

The structure of (ND₄)₂Ni(SO₄)₂.6D₂O has been determined at 2 K by means of neutron diffraction using two crystals, one mounted about the b axis and one about the c axis, in normal-beam Weissenberg geometry. Totals of 642 and 689 averaged unique reflections were obtained and refined together, allowing differing scale and extinction parameters, to give R(F²) = 0.053. The weighted average cell dimensions and atomic positional parameters are reported. The crystals are monoclinic, P2₁/a; a 9.073, b 12.305, c 6.255 Ǻ, β 106.9°. The structure is compared to that obtained by X-ray diffraction at c. 295 K. The similarity of the structure, including the hydrogen bonding, to that of the manganese compound, is very great, apart from the metal-oxygen bond lengths.

The structure of (NH₄)₂Ni(SO₄)₂.6D₂O was obtained by the study of one crystal mounted down the c axis, collecting only the hk0 and hk1 layers, also at 2 K. A total of 441 averaged unique reflections were obtained, and the structure refined to R_w(F²) = 0.043. Given the poor resolution in the c direction, the resultant structure is almost indistinguishable from that of the deuterated compound, except that the NiO₆ octahedron seems more regular.

The magnetization at 2.00 K and 4.60 T is shown to be in reasonable agreement with the value expected from the 90 K E.S.R . z.f.s . parameters, and the experimentally observed 8% difference between the deuterated and hydrogenous salts could well arise from a difference in z.f.s .parameters consequent upon the structural alterations.