Structure of Nickel Chromite

Abstract

Nickel chromite is a normal spinel whose structure has a tetragonal distortion below about 310°K. The structure parameters have been determined by neutron diffraction at 77°K, at which temperature the c/a ratio of a face-centered unit cell is 1.04. The space group is I41/amd with four Ni ions in positions 4(a), eight Cr ions in positions 8(d), and 16 O ions in positions 16(h) with y = 0.239 and z = 0.392. The two independent Cr-O distances are 1.98 A and 1.99 A, indicating that the octahedron is not greatly distorted. The tetrahedron around the nickel is elongated by nearly 14% parallel to the c axis. Below a Curie temperature of about 65°K, nickel chromite has a small ferromagnetic moment. The neutron diffraction pattern at 4.2°K shows extra peaks which can be indexed on the basis of a primitive cell having the same dimensions as the body-centered chemical cell. The intensities of these peaks cannot be accounted for by any simple model for the magnetic structure. In fact, a systematic search of possible structures belonging to magnetic space groups with symmetries as low as classes m′m′2 and mm′2′ has failed to turn up any structure which is consistent with the diffraction data.