Atomic clusters and phase transitions in the metastable beta-Ta phase between 4.2 and 293 K

Atomic clusters were identified in the ground state of the non-equilibrium Ta phase (Frank-Kasper sigma-structure type) at 15, 120 and 293K. The evolution of the clusters with temperature leads to two phase transformations at 65 and 150K which are related to the electrical and magnetic properties. The magnetic phase transition at 65 K is associated with the magnetic symmetry group transformation P4'(2) /mn'm (< 65 K) to P4(-/) (> 65 K). It is shown that beta-U is also a two-component composite containing similar clusters. The nature of the stabilisation of beta-Ta at the cathode is discussed.