Neutron diffraction investigation of isostructural intermetallic compounds NpCo2Ge2 and NpCu2Ge2

Abstract NpCo 2 Ge 2 and NpCu 2 Ge 2 crystallize isostructurally in the tetragonal space group I 4/ mmm . Lattice parameters, atomic positions and interatomic distances have been determined from neutron diffraction measurements taken at 300 and 4.2 K. Unit cell volumes of NpCo 2 Ge 2 and NpCu 2 Ge 2 differ by about 7%; identical axial ratios a c = 0.40 are obtained. NpCo 2 Ge 2 undergoes a magnetic phase transition into an antiferromagnetic moment configuration of the neptunium ions with ferromagnetically ordered 00 l sheets coupled antiferromagnetically along the c axis. The moment alignment is perpendicular to c . The ordered moments are 1.6(2) μ B per neptunium ion at 4.2 K. For NpCu 2 Ge 2 no magnetic order is observed. The magnetic behaviour of NpCo 2 Ge 2 and NpCu 2 Ge 2 is discussed in context with previous results on isostructural NpCo 2 Si 2 and NpCu 2 Si 2 .