Crystal and magnetic structures of the ternary stannides U2Pd2+ySn1-y

U2Pd2.25Sn0.75 stannide, investigated by X-ray diffraction on a single crystal, adopts a structure deriving from the tetragonal U3Si2-type, where Pd excess atoms occupy one split position surrounding the replaced Sn site. All U2Pd2+ySn1-y compounds order antiferromagnetically but their magnetic structures are dependent on the chemical composition. Neutron powder diffraction yields for y = 0 and y = 0.35 non-collinear k = (0, 0, 0) and collinear k = (0, 0, 1/2) antiferromagnetic structures with magnetic moments at T = 1.5 K equal to 2.20(5) and 0.90(2) mu(B)/U respectively. The reduced U-magnetic moment observed for y = 0.35 can be correlated to an increase with y of the number of Pd atoms surrounding U ones and favouring the 5f(U)-4d(Pd) hybridization. The magnetic properties of U2Pd2.25Sn0.75 are sample-dependent and the influence of thermal treatment on these properties is discussed (two samples were investigated).