Structure of Nickel Chromite
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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 ...
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