Crystal structures and magnetic properties of NaK3(NpO2)4(SO4)4(H2O)2 and NaNpO2SO4H2O:: Cation-cation interactions in a neptunyl sulfate framework

Two novel neptunium(V) sulfates, NaK3(NpO2)(4)(SO4)(4)(H2O)(2) and NaNpO2SO4H2O, have been prepared by hydrothermal methods. The structure of each has been determined by single-crystal X-ray diffraction using Mo K alpha. radiation and a charge-coupled device area detector. Both structures are monoclinic with space group C2/c. NaK3(NpO2)(4)(SO4)(4)(H2O)(2) (R-1 = 0.0351) has unit cell parameters a = 18.0998(12), b = 5.7030(4), c = 23.1988(16), beta = 104.6330(10)degrees, V = 2317.0(3) angstrom(3), and Z = 16. NaNpO2SO4H2O (R-1 = 0.036) has unit cell parameters a = 18.3638(13), b = 5.6424(4), c = 23.5512(17), beta = 103.366(2)degrees, V = 2374.2(3) angstrom(3), and Z = 16. Both structures contain chains of Np(V)O-7 pentagonal bipyramids linked into a framework structure through the sulfate tetrahedra. The neptunyl polyhedra are involved in multiple cation-cation interactions. Magnetic susceptibility data, together with field-dependent studies, indicate the presence of ferromagnetic ordering at 6.5(5) K for NaK3(NpO2)(4)(SO4)(4)(H2O)(2) and 7.4(1) K for NaNpO2SO4H2O.