The crystal structure of high-pressure americium, phase III

The structure of high-pressure americium, phase III, which occurs at room temperature in the pressure range 10–15 GPa, has been determined. The material has a monoclinic lattice with a = 3.025±0.005, b = 11.887±0.019, c = 2.830±0.005 Å, and β = 106.11±0.14° at 12.5 GPa. The space group is P2₁/m with Z = 4. Intensity data were obtained from powder-pattern films taken at pressure with Mo Kα radiation, λ = 0.7107 Å. The structure, which can be described as a layer stacking intermediate between the lower-pressure f.c.c. and higher-pressure α-U structure types, was refined by least-squares calculations to R₁ = 0.07 with 12 observed reflections. Am–Am distances of nearest neighbors range from 2.83 to 3.21 Å.