Structural characterization of the Ta-rich part of the Ta–Al system

The Ta-rich part of the Ta–Al system has been investigated. On the one hand, the accommodation of the non-stoichiometry in the σ phase has been studied by Rietveld refinement of X-ray powder data obtained from different samples on both sides of the ideal composition (Ta2Al, P42/mnm, tP30). On the other hand, the structure of the β phase has been determined ab initio from powder synchrotron data (analyzed composition Ta50.7Al49.3, refined composition Ta52.6(5)Al47.4(5) (Ta45.2(4)Al40.8(4)), stoichiometric composition Ta48Al38, mP86, P21/c, a=9.8707(1) Å, b=9.8766(1) Å, c=16.3539(2), β=116.478(1), RB=2.6%). This phase is shown to be closely related to the group of the topologically close packed phases. In addition, phase relations have been accurately determined in the Ta-rich end of the system.