The magnetic exchange interactions in chromium chalcogenide spinels

The chromium chalcogenide spinels, MCr2X4 (M = Zn, Cd, Hg; X = O, S, Se), have been the subject of considerable interest in recent years. In each case the crystal structure is that of a normal spinel with the chromium ions exclusively occupying the octahedral (B) sites, so that when diamagnetic ions are located at the tetrahedral (A) sites the only magnetic interactions present are those between B-site ions. Despite such apparently simple circumstances a rich variety of magnetic behaviour is exhibited. For the oxides the ground state spin configurations are antiferromagnetic whilst for the selenides ferromagnetic interactions dominate and several authors have drawn attention to the fact that the nature of the dominant interaction is a function of the nearest neighbour chromium - chromium separation. However, at least two of the compounds exhibit spiral structures and it has been proved difficult to account for the various spin configurations within a unified theory of the magnetic interactions involved. More recently, the possibility of formulating a simplified interpretation of the magnetic interactions has been provided by the discovery that the crystal struture of spinels does not always conform to the centrosymmetrical symmetry Fd3m that has been conventionally assumed. The deviation from this symmetry is associated with small < 111> displacements of the octahedrally coordinated metal ions and the structures so obtained are more correctly referred to the non-centrosymmetrical space group F43m. In the present study, therefore, extensive X-ray diffraction data have been collected from four chromium chalcogenide specimens and used to refine the corresponding structural parameters assuming F43m symmetry and also with conventional symmetry. The diffracted intensities from three of the compounds concerned cannot be satisfactorily accounted for on the basis of conventional symmetry and new locations have been found for the chromium ions in these cases. It is shown, however, that these displacements in chromium positions only partially resolve the difficulties in interpreting the magnetic behaviour. A re-examination of the magnetic data from different authors indicates much greater uncertainty in their measurements than they had claimed. By taking this into consideration it is shown that a unified theory of magnetic behaviour for the chromium chalcogenide spinels is a real possibility.