NEUTRON-DIFFRACTION STUDY OF THE TBFESI2-TYPE NDMNCU0.5GE1.5-XSIX LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.5) SOLID-SOLUTION

Antiferromagnetic TbFeSi2-type NdMnCu0.5Ge1.5-xSix compounds have been investigated by bulk magnetometric measurements and neutron diffracti on. The Neel temperatures decrease from 370 K (x=0) to 200 K (x=1.5). The (010) Mn planes are antiferromagnetic in the germanium rich compou nds (x less than or equal to 0.3), and become ferromagnetic in the sil icon rich compounds (x>1.2). In the intermediate range (0.4<x<1.2), th e compounds display more complicated magnetic structures with ''mixed' ' Mn planes and/or helimagnetic arrangements. Whatever the concentrati on x, the Nd sublattice orders around T-t approximate to 70 K. The mag netic behaviour of the Mn sublattice is compared to that observed in t he RMnCu0.5Ge1.5 and closely related RMnSi, RMn2Ge2, and RMnSi2 compou nds. The various magnetic arrangements observed in NdMnCu0.5Ge1.5-xSix and RMnSi2 compounds are connected with modifications in the chemical bond in each series. (C) 1997 Elsevier Science S.A.