  Ta films with tetragonal crystalline structure (beta-phase), deposited by
magnetron sputtering on different substrates (steel, silicon and silicon
dioxide), have been studied. In all cases, very highly preferred (001)
orientation was observed in x-ray diffraction (XRD) measurements. All
diffraction data revealed two weak reflections corresponding to d-spacing of
0.5272 and 0.1777 nm. The presence of the two peaks, attributed to (001) and
(003) reflections, indicates that beta-Ta films exhibit a high preference for
the space group of P-421m over P42/mnm, previously proposed for beta-Ta.
Differences in relative intensities of (00l) reflections, calculated for single
crystal beta-Ta sigma-type Frank-Kasper structure and those measured in the
films, are attributed to defects in the films. Molecular dynamics simulations
performed on tantalum clusters with six different initial configurations using
the embedded-atom-method (EAM) potential revealed the stability of beta-Ta,
which might explain its growth on many substrates under various deposition
conditions.
