Covalency effects in neutron diffraction from ferromagnetic and antiferromagnetic salts

Abstract

It is shown that covalent bonding affects the absolute intensities and form factors for neutron diffraction from antiferromagnetic and ferromagnetic salts. The general effect is to reduce the absolute intensity below the theoretical value for a purely ionic salt and to make the form-factor curve somewhat flatter (except in directions very close to the forward direction). In the case of Ni2+ and Cr3+ salts the reductions in absolute intensities observed by Nathans, Alperin and others are roughly consistent with the degree of covalency indicated by the nuclear magnetic resonance measurements of Shulman and Knox. Furthermore, Alperin's observed form-factor curve for NiO can be largely explained by allowing for the same degree of covalency. On the other hand, we have not been able to explain the observed absolute intensities or the form factors for Mn2+ salts in these terms. It is also shown that Bragg scattering experiments on crystals with large unit cells or measurements of diffuse elastic scattering should reveal a sharp forward peak in the form factor.