Low temperature lattice expansion of americium dioxide

The variation of the lattice parameter, of the thermal linear expansion, and of the coefficient of thermal linear expansion was determined for AmO ₂ in the range 38-300 K. On cooling, AmO ₂ contracts more strongly than the dioxides of lower actinides, but similarly strong contraction occurs for CeO ₂ and PrO _{2- x}; this confirms the observed analogy between Am and the lanthanides. The α self-irradiation causes lattice expansion and reversibly broadens the diffraction lines at low temperatures in both ²⁴¹AmO ₂ and ²⁴³AmO ₂. The former effect partly anneals at room temperature; the room temperature lattice parameter of previously cooled specimens decreases to a steady state value within some days. A much faster process of defect formation and annealing is responsible for the change in line width; the line width characteristic of a given temperature establishes within a few hours.