MAGNETIC-PROPERTIES OF RCOSI (R-EQUIVALENT-TO-LA-SM, GD, TB) COMPOUNDS FROM SUSCEPTIBILITY MEASUREMENTS AND NEUTRON-DIFFRACTION STUDIES

Investigations by powder X-ray diffraction, susceptibility measurement s and neutron diffraction experiments on the ternary silicides RCoSi ( R = La-Sm, Gd, Tb) are reported. They crystallize in the tetragonal st ructure of the CeFeSi type (space group P4/nmm). This structure is clo sely related to the ThCr2Si2 structure and can be described as isolate d ''ThCr2Si2 blocks'' (''BaAl4 slab'') connected via R-R contacts. LaC oSi is a Pauli paramagnet while CeCoSi and PrCoSi are Curie-Weiss para magnets down to 1.6 K. The other compounds (Nd, Sm, Gd, Tb) are antife rromagnetic below T(N) = 7(1) K, 15(3) K, 175(3) K and 140(3) K for ne odymium, samarium, gadolinium and terbium respectively. The magnetic s tructures of NdCoSi and TbCoSi consist of ferromagnetic (001) R layers (with moments lying in the plane) antiferromagnetically coupled along the c-axis with the sequence + + - - (at 2 K, mu = 2.08(4) mu(B) and 8.1(1)mu(B)) for Nd and Tb respectively). In all compounds, no local m oment was detected on the cobalt site. The results are discussed and c ompared with those obtained on the closely related RCo2Si2, and isotyp ic RCoGe, RFeSi and RRuSi compounds.