CRYSTALLOGRAPHIC DATA AND MAGNETIC-PROPERTIES OF NEW CEFESI-TYPE RMNGE COMPOUNDS (R=LA-SM) STUDIED BY MAGNETIZATION AND NEUTRON-DIFFRACTIONMEASUREMENTS

Investigations made by powder X-ray diffraction, susceptibility measur ements and neutron diffraction experiments on new ternary RMnGe (R=La- Nd) are reported. They crystallize in the well-known tetragonal CeFeSi -type structure (space group P4/nmm), This structure, which is closely related to the ThCr2Si2-type structure, can be described as isolated ''ThCr2Si2 blocks'' connected via R-R contacts. All these compounds ar e antiferromagnetic below T-N similar to 410 K (not detected in the bu lk susceptibility measurements). At room temperature, the magnetic str uctures are characterized by a stacking of antiferromagnetic (001) Mn layers with Mn magnetic moments (about 3.6-3.2 mu(B), at 2 K) at 40 de grees (CeMnGe) or along the c axis. In all cases, this antiferromagnet ic arrangement of the Mn sublattice persists down to 2 K. At lower tem perature, the R sublattice orders in ferromagnetic (001) layers, with various ordering schemes along the stacking axis. At 150 K, PrMnGe exh ibits ferromagnetic behaviour followed by a ferro-antiferromagnetic tr ansition below 80 K simultaneously with a crystallographic phase trans ition (tetragonal to orthorhombic symmetry). Below 200 K, NdMnGe exhib its a ferromagnetic ordering of the Nd sublattice which persists until 2 K. Within the complete ordering temperature range, the Pr and Nd ma gnetic moments are in the basal plane with a magnitude of 2.45(3) and 3(1) mu(B), for Pr and Nd, respectively, at 2K. The magnetic structure s correspond to the sequence Mn(AF)R(+)R(+)Mn(AF)R(-or+). The Mn momen ts rotate at 50 degrees from the c axis in PrMnGe, while they are in q uadrature in NdMnGe. In CeMnGe, the Ce-sublattice orders antiferromagn etically below about 130 K (not detected in the magnetometric measurem ents), yielding the following ordering scheme: Mn(AF)Ce(+)Ce(+)Mn(AF)C e(-)Ce(-); at 2 K the Ce moment value is 2.2(1) mu(B). The Mn and Ce m oment directions are almost parallel, and make an angle of about 35 de grees with the c axis. The results are discussed and compared to those obtained previously on the corresponding ternary silicides and german ides of the CeFeSi- and ThCr2Si2-type structures.