Americium - Physical Properties

Physical Properties

In the periodic table, americium is located to the right of plutonium, to the left of curium, and below the lanthanide europium, with which it shares many similarities in physical and chemical properties. Americium is a highly radioactive element. When freshly prepared, it has a silvery-white metallic lustre, but then slowly tarnishes in air. With a density of 12 g/cm3, americium is lighter than both curium (13.52 g/cm3) and plutonium (19.8 g/cm3); but is heavier than europium (5.264 g/cm3)—mostly because of its higher atomic mass. Americium is relatively soft and easily deformable and has a significantly lower bulk modulus than the actinides before it: Th, Pa, U, Np and Pu. Its melting point of 1173 °C is significantly higher than that of plutonium (639 °C) and europium (826 °C), but lower than for curium (1340 °C).

At ambient conditions, americium is present in its most stable α form which has a hexagonal crystal symmetry, and a space group P6₃/mmc with lattice parameters a = 346.8 pm and c = 1124 pm, and four atoms per unit cell. The crystal consists of a double-hexagonal close packing with the layer sequence ABAC and so is isotypic with α-lanthanum and several actinides such as α-curium. The crystal structure of americium changes with pressure and temperature. When compressed at room temperature to 5 GPa, α-Am transforms to the β modification, which has a face-centered cubic (fcc) symmetry, space group Fm3m and lattice constant a = 489 pm. This fcc structure is equivalent to the closest packing with the sequence ABC. Upon further compression to 23 GPa, americium transforms to an orthorhombic γ-Am structure similar to that of α-uranium. There are no further transitions observed up to 52 GPa, except for an appearance of a monoclinic phase at pressures between 10 and 15 GPa. There is no consistency on the status of this phase in the literature, which also sometimes lists the α, β and γ phases as I, II and III. The β-γ transition is accompanied by a 6% decrease in the crystal volume; although theory also predicts a significant volume change for the α-β transition, it is not observed experimentally. The pressure of the α-β transition decreases with increasing temperature, and when α-americium is heated at ambient pressure, at 770 °C it changes into an fcc phase which is different from β-Am, and at 1075 °C it converts to a body-centered cubic structure. The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum, praseodymium and neodymium.

As with many other actinides, self-damage of the crystal lattice due to alpha-particle irradiation is intrinsic to americium. It is especially noticeable at low temperatures, where the mobility of the produced lattice defects is relatively low, by broadening of X-ray diffraction peaks. This effect makes somewhat uncertain the temperature of americium and some of its properties, such as electrical resistivity. So for americium-241, the resistivity at 4.2 K increases with time from about 2 µOhm·cm to 10 µOhm·cm after 40 hours, and saturates at about 16 µOhm·cm after 140 hours. This effect is less pronounced at room temperature, due to annihilation of radiation defects; also heating to room temperature the sample which was kept for hours at low temperatures restores its resistivity. In fresh samples, the resistivity gradually increases with temperature from about 2 µOhm·cm at liquid helium to 69 µOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium and protactinium, but is different from plutonium and curium which show a rapid rise up to 60 K followed by saturation. The room temperature value for americium is lower than that of neptunium, plutonium and curium, but higher than for uranium, thorium and protactinium.

Americium is paramagnetic in a wide temperature range, from that of liquid helium, to room temperature, and above. This behavior is markedly different from that of its neighbor curium which exhibit antiferromagnetic transition at 52 K. The thermal expansion coefficient of americium is slightly anisotropic and amounts to (7.5 ± 0.2)×10−6/°C along the shorter a axis and (6.2 ± 0.4)×10−6/°C for the longer c hexagonal axis. The enthalpy of dissolution of americium metal in hydrochloric acid at standard conditions is −620.6 ± 1.3 kJ/mol, from which the standard enthalpy change of formation (Δ_fH°) of aqueous Am3+ ion is −621.2 ± 2.0 kJ/mol−1. The standard potential Am3+/Am0 is 2.08 ± 0.01 V.