The local structure of β-tantalum was investigated by comparing experimental extended x-ray absorption fine structure (EXAFS) measurements with calculated spectra of proposed models. Four possible structure candidates were examined: a β-Uranium based structure, a distorted A15 structure, a bcc-Ta based superlattice structure with N interstitials and a simple hcp structure. The local structural measurements were found to be consistent with the β-Uranium based model containing 30 atoms per unit cell and having the space group P4 2 /mnm. The thermal effect analysis on x-ray diffraction and EXAFS spectra, which reveals that β-Ta is highly disordered, agrees with the low symmetry and anisotropic system of the β-U model.

In order to predict the stable and metastable phases of the bcc metals in the block of the Periodic Table defined by groups 5B to 6B and periods 4 to 6, as well as the structure dependence of their transport properties, we have performed full potential computations of the total energies per unit cell as a function of the a c ratio at constant experimental volume. In all cases, a metastable body centered tetragonal (bct) phase was predicted from the calculations. The total energy differences between the calculated stable and metastable phases ranged from 0.09 eV/cell (vanadium) to 0.39 eV/cell (tungsten). The trends in resistivity as a function of structure and atomic number are discussed in terms of a model of electron transport in metals. Theoretical calculations of 2 the electrical resistivity and other transport properties show that bct phases derived fromgroup 5B elements are more conductive than the corresponding bcc phases, while bct phases formed from group 6B elements are less conductive than the corresponding bcc phases. Special attention is paid to the phases of tantalum where we show that the frequently observed β phase is not a simple tetragonal distortion of bcc tantalum.