Crystal Structures and Magnetic Properties of NaK₃(NpO₂)₄(SO₄)₄(H₂O)₂and NaNpO₂SO₄H₂O: Cation−Cation Interactions in a Neptunyl Sulfate Framework

Abstract

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 Kα radiation and a charge-coupled device area detector. Both structures are monoclinic with space group C2/c. NaK3(NpO2)4(SO4)4(H2O)2 (R1 = 0.0351) has unit cell parameters a = 18.0998(12), b = 5.7030(4), c = 23.1988(16), β = 104.6330(10)°, V = 2317.0(3) Å3, and Z = 16. NaNpO2SO4H2O (R1 = 0.036) has unit cell parameters a = 18.3638(13), b = 5.6424(4), c = 23.5512(17), β = 103.366(2)°, V = 2374.2(3) Å3, and Z = 16. Both structures contain chains of Np(V)O7 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.