X-ray diffraction and absorption spectrophotometric studies of AmI3 and AmOI

The anhydrous tri-iodides and the oxyiodide of americium were investigated by X-ray diffraction and absorption spectrophotometry. From the X-ray analysis of the tri-iodide, an orthorhombic form (PuBr3-type structure) was identified, which is isostructural with the lighter actinide (uranium through plutonium) tri-iodides. A hexagonal form (BiI3-type structure) of the tri-iodide was also found. The transition temperature for converting the orthorhombic form to the hexagonal form was established to be 400+-300C. Room temperature lattice parameters for the tri-iodide are (1) a0=0.428(4) nm, b0=1.394(1) nm and c0= 0.9974(7) nm for the orthorhombic form, and (2) a0=0.7637(4) nm and c0=2.091(2) nm for the hexagonal form. The room temperature parameters for the tetragonal oxyiodide (PbFCl-type structure) are a0=0.4010(4) nm and c0=0.9200(6) nm. From differences in the absorption spectra of the solids at 250C, it is possible to differentiate between these three materials by absorption spectrophotometry. (Auth.)