  Zero-field muon spin relaxation experiments have been carried out in the
Pr(Os_{1-x}Ru_x)_4Sb_12 and Pr_{1-y}La_yOs_4Sb_12 alloy systems to investigate
broken time-reversal symmetry (TRS) in the superconducting state, signaled by
the onset of a spontaneous static local magnetic field B_s. In both alloy
series B_s initially decreases linearly with solute concentration. Ru doping is
considerably more efficient than La doping, with a ~50% faster initial
decrease. The data suggest that broken TRS is suppressed for Ru concentration x
>~ 0.6, but persists for essentially all La concentrations. Our data support a
crystal-field excitonic Cooper pairing mechanism for TRS-breaking
superconductivity.
