Local lattice distortions in chromium chalcogenide spinels at low temperatures

Structural changes in some chromium chalcogenide spinels in the surroundings of the magnetic ordering temperature have been investigated by means of X-ray powder diffraction. While the crystal symmetry was found to be unchanged, shifts of the anion parameter were detected for CdCr2Se4 by least squares structure refinement calculations. In all compounds unusual broadenings of the diffraction lines were found, indicating inhomogeneous lattice distortions. The reason for these distortions could not be conclusively explained. From the shift of the spinel anion parameter in CdCr2Se4 from 0.389 at 300 K to 0.392 at 77 K, it can be estimated that the changes of the interatomic distances arising therefrom may contribute essentially to the band edge shifts in these magnetic semiconductors