Crystal chemical studies of the 5f-series of elements. XVII. The crystal structure of neptunium metal

On the basis of powder diffraction patterns neptunium metal is found to be orthorhombic.The unit cell containing eight atoms has dimensions a1=4-723~-0-001, a2=4"887J:0"002, as= 6.663+0.003 A.The calculated density is ~----20.45±0.03g.cm.-3.The space group is Pmcn, and the atomic positions are:4 atoms in ±(~, Yl zl) (~, ½--Yl, z~-½) with y1=0.208~0"006,z~:0-036±0.006; and 4 atoms in ±(~, y2, z~) (¼, ½--y~, zg-~-½) with y~=0.842±0.006,z2~0"319±0"006 •The structure can be regarded as a heavily deformed cubic body-centered structure.The deformation is such that the coordination number is reduced from eight to four, the bond length being 2.60--2.64 A. The four short bonds are of covalent type and have a strength about halfway between a single and a double bond.It is concluded that there is at most one 5f-electron per atom in neptunium metal.Neptunium metal was first prepared by Sherman Fried in 1945 (Fried, 1945, p. 6; Fried & Davidson, 1949).The preparation, which was on the 50/~g.scale, was made by the reaction of barium vapor on NpF a at 1200 ° C. Fried made two density measurements and obtained values of 17.9 and 17.6 g.cm.-a.This writer obtained very weak X-ray diffraction patterns of Fried's preparation.The quality of the patterns was, however, too poor to justify attempts at interpretation.Preparations of neptunium metal on the 1 mg.scale were later made by Westrum & Eyring (1951).These workers determined the melting point as 640il ° C.They report the density to be 19.5±0.5 g.cm.-a.Individual density determinations ranged from 19.2 to 20.1 g.cm.-a, except for one metal sample which gave values of 16-9 and 17-4 g.cm.-a.X-ray diffraction examination of the preparations of Westrum & Eyring was made by D. Templeton who observed only the diffraction ~ lines of NpO.During recent months new preparations of neptunium metal on the 0.5 mg.scale have been carried out by James C. Wallman.The results presented in this paper have been obtained using Wallman's preparations.