Synthesis and crystal structure of alkali metal uranium sulfides, Li₂US₃ and Na₂US₃

New mixed uranium sulfides, A₂US₃ (A=Li, Na), in which uranium is in a tetravalent state, have been synthesized. In the disordered state, the compounds are written as A(A_1/3,U_2/3)S₂ which have a hexagonal (R3m) structure the same as the lanthanide homologue, ALnS₂ (Ln=trivalent lanthanides). In the ordered state, the compounds take on a monoclinic (C2/m) structure in which the atom arrangement is very close to the above hexagonal structure. The partial ordering is realized by the coexistence of the two phases. The lattice parameters of hexagonal Li₂US₃ are a = 3.898 and c = 18.391 Å, while those of monoclinic Li₂US₃ are a = 6.747, b = 11.679, c = 6.537 Å and β = 110.2°. The lattice parameters of hexagonal Na₂US₃ are a = 4.036 and c = 19.780 Å. Those of monoclinic Na₂US₃ are a = 6.990, b = 12.105, c = 6.992 Å and β = 109.5°. The molar ratios of the hexagonal and monoclinic phases are 52.2:47.8 for Li₂US₃ and 68.0:32.0 for Na₂US₃, respectively. The atom parameters of uranium and sulfur were obtained by Rietveld calculation of the observed X-ray peaks. The atom separations are discussed in relation to the crystal radii of the component ions.