Elastic properties of the light actinides at high pressure

Using density-functional perturbation theory, we have calculated the elastic constants for the ground-state crystal structures of the light actinide metals both at their equilibrium volumes (for Th through Np) and as a function of pressure (for Th through U). As necessary, we take into account the effects of atomic relaxation and show that these can be neglected for α-U, but are crucial for α-Np. The elastic constants of Th and U are compared with experimental measurements near ambient conditions and good agreement is found. Studies of the Born stability criteria in pressure reveal that Th and Pa are mechanically unstable while U remains stable up to 85 GPa, as is observed in diamond anvil cell experiments.