Gadolinium and neodymium citrates:: Evidence for weak ferromagnetic exchange between gadolinium(III) cations

A new lanthanide citrate motif of general formula [Ln(Hcit)(H2O)(2)center dot H2O](n), where Ln = Gd (1) and Nd (2) and HCit(3-)= C(OH)(COO-)(CH2COO-)(2), has been synthesized hydrothermally from Ln(2)O(3) and citric acid at 100 degrees C and characterized by elemental analysis, IR, TG-DTA, single-crystal X-ray diffraction, and magnetic measurements. The structures can be seen as "ladder chains" along the a axis, with dinuclear Ln(2)O(2) units serving as "steps" and R-COO groups as "uprights", which are connected by H bonds. The magnetic susceptibility between 2 and 300 K and the magnetization at 2 K, as a function of magnetic field between 0 and 5 T, were measured for both compounds, By modeling the magnetic behavior of the Gd compound with a dinuclear Hamiltonian Hs = g mu(B)(S-A + S-B)B-o - J(o)S(A)S(B) (S-A = S-B = 7/2), a ferromagnetic exchange interaction J(o) = 0.039 cm(-1) was evaluated between Gd ions situated at do = 4.321 A in dinuclear units bridged by two symmetry-related tridentate carboxylate oxygens. The EPR spectrum of the Gd compound is discussed. The temperature dependence of the susceptibility of the Nd compound is caused by the depopulation of the excited crystal-field levels when the temperature decreases. The magnetic-field dependence of the magnetization of 2 is attributed to the ground-state Kramers' doublet populated at 2 K. The g factor of this ground-state doublet is calculated from the data and compared with values for other compounds reported in the literature.