Magnetic properties of the lanthanoid nickel arsenides and phosphides LnNi(4)X(2) with ZrFe4Si2-type structure

The ternary compounds LnNi(4)As(2)(Ln:Tb --> Yb;Y) and LnNi(4)P(2) (Ln:Er,Yb;Y) have been prepared; they crystallize in the tetragonal ZrFe4Si2-type structure. Their magnetic properties have been studied with a SQUID magnetometer between 2 and 300 K. YNi4As2 shows a temperature-independent Pauli paramagnetism attributed to a non-magnetic contribution of the ''Ni4As2'' sublattice, while all the other LnNi(4)As(2), compounds, except YbNi4As2, present a Curie-Weiss behavior above 30 K. The experimental paramagnetic moments are close to those expected for Ln(3+)-free ions. At low temperature, only two compounds exhibit a magnetic ordering: TbNi4As2 behaves as a metamagnetic or spin-canted system, while DyNi4As2 is a weak ferromagnet. At last, YbNi4As2 shows a temperature-independent paramagnetism suggesting that ytterbium is in a non-magnetic divalent state. For the phosphides LnNi(4)P(2) (Ln:Er,Yb,Y), a non-magnetic contribution of the ''Ni4P2'' sublattice is found, as shown by the study of YNi4P2. ErNi4P2 exhibits a Curie-Weiss paramagnetism in the whole temperature range as well as YbNi4P2 for T > 20 K. This last result shows that contrary to YbNi4As2, the corresponding phosphide exhibits a trivalent state for ytterbium.