  We report on a study of the superconducting properties for a series of
polycrystalline BaPt(4-x)Au(x)Ge12 filled skutterudite compounds for x = 0,
0.5, 0.75, and 1. Muon spin rotation (muSR) spectroscopy as well as
magnetization, specific heat, and electrical resistivity measurements were
performed. The magnetic penetration depth \lambda, the coherence length \xi,
and the Ginzburg-Landau parameter \kappa are evaluated. The temperature
dependence of the superfluid density is well described by an s-wave
superconducting gap and this classical scenario is supported by the
field-independent \lambda. The gap-to-Tc ratio \Delta/kTc increases with the Au
content from 1.70 for x = 0 to 2.1(1) for x = 1. By combining muSR,
magnetization, and specific heat data, we find that BaPt(4-x)Au(x)Ge12
compounds are in between the dirty and clean limits with mean free paths of the
carriers l ~ \xi. Interestingly, resistivity data for BaPt4Ge12 indicate a much
higher upper critical field, which is probably due to defects or impurities
close to the surface of the crystallites.
