New ternary silicides (RE,Th,U) Os
2Si
2 have been synthesized from the elements. All the compounds (RE= Y,La,Ce,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm,Yb, Lu) were found to be isotypic and to crystallize with the ordered BaAl
4-type of structure (ThCr
2Si
2-type). Magnetic properties of these alloys — studied in the temperature range 1.5<T<1100 K — reveal a typical Van Vleck paramagnetism of free RE
3+ ions at temperatures higher than 300 K. The observed effective paramagnetic moment of CeOs
2Si
2, μ(eff)=0.98 BM, is compatible with a rather low concentration (15%) of Ce
3+. The effective moment of SmOs
2Si
2, μ(eff)=0.47 BM, is in reasonable agreement with a Hund's rule
ground level for free Sm
3+. For temperatures above 25 K, the magnetic susceptibility as a function of temperature corresponds to the Van Vleck behavior for free Sm
3+ (closely spaced multiplet,
,
. Magnetic ordering temperatures of REOs
2Si
2 silicides are generally below 42 K. (Pr,Nd,Ho,Er,Tm) Os
2Si
2 exhibit ferromagnetic ordering whereas (Sm,Gd, Tb,Dy) Os
2Si
2 show antiferromagnetic behavior. Above 1.8 K none of the samples was found to be superconducting.
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