Exchangestriction in CdCr2S4 and CdCr2Se4

Abstract

The ferromagnetic semiconductors CdCr₂S₄ and CdCr₂Se₄ both show large (∼0.2 eV) shifts in their absorption edges as the temperature decreases below their Curie temperatures. In CdCr₂S₄, the absorption edge increases with decreasing temperature; while in CdCrSe₄, it decreases with decreasing temperature. It has been proposed that these shifts arise from a lattice expansion or contraction which is, in turn, produced by exchangestriction. Using x‐ray diffraction, we have measured the lattice parameters of these two materials from room‐temperature down to 4.2°K. CdCr₂S₄ is found to expand with decreasing temperature below its Curie point. The total fractional change is of the order of 10⁻³. CdCrSe₄ contracts, but its fractional change is even smaller than that of the sulfide. Therefore, if this small expansivity is to explain the absorption‐edge shift, it requires a deformation potential of the order of 100 eV, which is at least an order of magnitude larger than what one would reasonably expect. Therefore, it appears that exchangestriction is not responsible for the large absorption‐edge shifts in these materials.