Comment on "Mossbauer effect study of filled antimonide skutterudites"

In a recent paper [Phys. Rev. B 60, 7416 (1999)] G. J. Long et al. have introduced an "effective recoil mass" as an adjustable parameter to describe the linear temperature dependence of the Mossbauer second-order Doppler shift. It is argued on classical grounds that no physical meaning can be attributed to this concept, as the delta vs T slope in the high-temperature limit is a constant for each Mossbauer nuclide.