The self-hosting structure of beta-Ta

Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) A, space group P42(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor approximately 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. Beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.