Crystal chemistry of the solid solutions based on β-Mn

Binary solid solutions based on beta-Mn have been systematically investigated from a structural point of view. The site occupancies of the substitutional solute elements Al, Co, Fe, In, Ir, Ni, Os, Ru, Si, Sn and Zn on the two distinct sites of its crystal structure have been determined experimentally by Rietveld refinement of X-ray or neutron powder diffraction data. The atomic distribution is discussed in light of the changes of lattice parameters in the solution and compared between the different chemical systems. A systematic analysis of the volume change within the complete binary systems has been carried out. Anomalies are observed and explained for the time, by assuming that Mn presents two different atomic radii at its two different sites. This feature is unique in metallic solid solutions in the absence of any mixed valence. The study is complemented by a detailed DFT investigation of the different systems demonstrating an anomaly with the description of the Mn atom by traditional first-principles calculations. This problem leads to miscalculation of the lattice parameter of pure Mn and, as a consequence, of the site occupancies in the binary systems. The importance of the ordering of the solute element for the Calphad modeling is discussed and the use of a two sublattice model for the description of beta-Mn solid solutions in agreement with the presence of two different Wyckoff positions, instead of the conventional disordered solution model, is highly recommended.