sb bi o s se te or po or combinations of these atoms preferably the x atoms 16 are p as or sb most preferably the x atoms 16 are sb the transition metal atoms 14 may be mn tc re fe ru os co rh ir ni pd pt cu ag or au or combinations of these atoms preferably the transition metal atoms 14 are co rh or ir most preferably the transition metal atoms 14 are co the guest atoms 12 in the filled skutterudite 10 maybe rare earth atoms na k ca sr ba and combinations of these atoms according to the present invention a source or starting material of the guest atoms 12 is mischmetal mm alloy which is an alloy of mostly ce about 50 wt and la about 50 wt the mischmetal may be used alone or in combination with a rare earth metal or metals as the source or starting material for the guest atoms 12 these rare earth metals include those from the lanthanide series such as ce pr nd sm eu and gd as well as those from the actinides series such as th and u in a typical embodiment the composition of the filled skutterudite 10 is mm y co 4 sb 12 0 lt y u2266 1 in the filled skutterudite 10 u201c rattling u201d of the guest atoms 12 in the irregular dodecahedral cages formed by the adjacent tetrahedra of x atoms 16 reduces the lattice thermal conductivity of the material while minimally affecting carrier mobility by scattering phonons use of mischmetal as a source or starting material for the guest atoms 12 reduces the overall cost of the filled skutterudite 10 since mischmetal is relatively low in cost compared.