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, NaK 3 (NpO 2 ) 4 (SO 4 ) 4 (H 2 O) 2 and NaNpO 2 SO 4 H 2 O, 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 C 2/ c . NaK 3 (NpO 2 ) 4 (SO 4 ) 4 (H 2 O) 2 ( R 1 = 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) Å 3, and Z = 16. NaNpO 2 SO 4 H 2 O ( R 1 = 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) Å 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 NaK 3 (NpO 2 ) 4 (SO 4 ) 4 (H 2 O) 2 and 7.4(1) K for NaNpO 2 SO 4 H 2 O.