Poisson's ratio limits and effects of hydrostatic pressure on the elastic behaviour of Sm1-xYxS alloys in the intermediate valence state

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Abstract
As cubic rare-earth chalcogenides in the intermediate valence state approach the valence transition (when B to 0 because C11 to 2C12 with C12 negative) crystal stability requires that Poisson's ratio should have a lower limit of -1.0 for a stress applied along any crystal symmetry axis. In agreement with this prediction, as the yttrium content in Sm1-xYxS solid solutions is reduced towards the critical concentration xc (=0.15) for the valence transition, the Poisson's ratios for stress applied along any symmetry axis do tend towards -1.0. To determine the effect of pressure on the elastic behaviour of an intermediate valence crystal in the Sm1-xYxS series, the elastic constants and their hydrostatic pressure derivatives of the intermediate valence state material Sm0.58Y0.42S have been measured by the ultrasonic pulse echo overlap technique.