Abstract
Results are presented of x-ray measurements of lattice constant versus temperature in the range 4°–500°K of the ferromagnetic compounds CoS2 and Co0.75Fe0.25S2. Earlier measurements in the range 4°–300° on CdCr2S4 and CdCr2Se4 are extended to the range 4°–700°K. For purposes of comparison, x-ray data are also presented on the nonmagnetic compounds FeS2 and CdIn2S4. The results of measurements of dTc/dP of CoS2 and Co0.75Fe0.25S2 are also reported. The magnitude of the volume exchangestriction of the ferromagnetic compounds is determined by using Grüneisen-Debye theory to fit the high-temperature lattice constant data, and extrapolating this fit into the temperature range below Tc. The volume exchangestriction is found to be anomalously small when compared to thermodynamic predictions based on values of dTc/dP and the magnetic energy determined from the high-temperature susceptibility. The proposed explanation involves atomic displacements internal to the unit cell which modify the magnetic exchange integral J. This phenomenon is related to the coupling between optical phonons and magnons.
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