Magnetic properties of some neptunyl(+1) complexes

Abstract

A summary report on the magnetic properties of some neptunyl(+1) complexes (two formates; (NH4)[NpO2(O2CH)2](1), [NpO2(O2CH)(H2O)](2), and a phthalate, [(NpO2)2(O2C)2C6H4(H2O)3] (3)) is presented based on the magnetic and 237Np Mössbauer data obtained in our group for powder samples and the available single-crystal structural data. It is demonstrated that the remarkable correlations exist between the magnetic properties, the 237Np Mössbauer parameters and the structural features (dimensionality, neptunyl (O=Np=O) bonding network structure, etc.) among these uniquely low-dimensional neptunyl complexes. The magnetic data show that 1 with neptunyl (O=Np=O) 1D chain structure is paramagnetic (or only marginally ferromagnetic) down to 2 K, while 2 and 3 with similar 2D neptunyl sheet structure order ferromagnetically at Tc = 12 and 4.5 K, respectively. In addition, 3 having two kinds of Np sites exhibited a two-step magnetization behavior below Tc. Consistent with these magnetic data, 237Np Müssbauer spectra of 1 showed paramagnetic relaxation, while those of 2 and 3 showed a well resolved magnetic hyperfine splitting even above Tc's, indicating the existence of very slow paramagnetic relaxation. The all obeys the Curie-Weiss law in their paramagnetic temperature region up to 300 K. The values of the effective moments (and also of the saturation moments) of these systems are in the order, 1 > 2 > 3, and were smaller (and much smaller) even for 1 than those for free ion (Np5+(5f2)). These experimental findings on the present three neptunyl complexes are briefly discussed in connection with the reported magnetic properties of other neptunium compounds (pnictides, etc.) and also with a theoretical treatment based on an Ising-type behavior of individual neptunyl (O=Np=O)+ cations recently proposed by us.